How to choose a power supply - criteria and characteristics. Types of electrical circuits of a computer power supply

Circuitry of computer power supplies

Circuits for computers

R. ALEXANDROV, Maloyaroslavets, Kaluga region.
Radio, 2002, No. 5, 6, 8

UPSs of household computers are designed to operate from a single-phase alternating current network (110/230 V, 60 Hz ≈ imported, 127/220 V, 50 Hz ≈ domestic production). Since the 220 V, 50 Hz network is generally accepted in Russia, the problem of choosing a unit for the required mains voltage does not exist. You just need to make sure that the mains voltage switch on the unit (if there is one) is set to 220 or 230 V. The absence of a switch indicates that the unit is capable of operating in the mains voltage range indicated on its label without any switching. UPSs designed for 60 Hz operate flawlessly on a 50 Hz network.

The UPS is connected to AT format motherboards with two wire harnesses with sockets P8 and P9, shown in Fig. 1 (view from the nests). The wire colors indicated in brackets are standard, although not all UPS manufacturers strictly adhere to them. To properly orient the sockets when connecting to the motherboard plugs, there is a simple rule: the four black wires (GND circuit) going to both sockets must be located next to each other.

The main power circuits of ATX format motherboards are concentrated in the connector shown in Fig. 2. As in the previous case, view from the side of the socket sockets. UPSs of this format have a remote control input (PS-ON circuit), when connected to a common wire (COM ≈ "common" circuit, equivalent to GND), the unit connected to the network begins to operate. If the PS-ON≈COM circuit is open, there is no voltage at the UPS outputs, with the exception of the “standby” +5 V in the +5VSB circuit. In this mode, the power consumed from the network is very low.

ATX format UPSs are equipped with an additional output socket, shown in Fig. 3. The purpose of its circuits is as follows:

FanM ≈ output of the fan speed sensor cooling the UPS (two pulses per revolution);
FanC ≈ analog (0...12 V) input for controlling the rotation speed of this fan. If this input is disconnected from external circuits or a constant voltage of more than 10 V is applied to it, the fan performance is maximum;
3.3V Sense ≈ feedback signal input of the voltage stabilizer +3.3 V. It is connected with a separate wire directly to the power pins of the microcircuits on the system board, which allows you to compensate for the voltage drop on the supply wires. If there is no additional socket, this circuit can be routed to socket 11 of the main socket (see Fig. 2);
1394R ≈ minus of an 8...48 V voltage source isolated from the common wire to power the IEEE-1394 interface circuits;
1394V ≈ plus of the same source.

A UPS of any format must be equipped with several sockets to power disk drives and some other computer peripherals.

Each “computer” UPS produces a logical signal called R G. (Power Good) in AT blocks or PW-OK (Power OK) in ATX blocks, the high level of which indicates that all output voltages are within acceptable limits. On the “motherboard” of the computer, this signal is involved in generating the system reset signal. After turning on the UPS, the RG signal level. (PW-OK) remains low for some time, prohibiting the processor from operating until the transient processes in the power circuits are completed.

When the mains voltage is turned off or the UPS suddenly malfunctions, the logical level of the P.G. signal (PW-OK) changes before the unit’s output voltages drop below permissible values. This causes the processor to stop, prevents corruption of data stored in memory and other irreversible operations.

The interchangeability of a UPS can be assessed using the following criteria.

Number of output voltages to power an IBM PC AT format there must be at least four (+12 V, +5 V, -5 V and -12 V). The maximum and minimum output currents are regulated separately for each channel. Their usual values ​​for sources of various powers are given in table. 1 . ATX computers additionally require +3.3 V and some other voltages (they were mentioned above).

Please note that normal operation of the unit at a load less than the minimum is not guaranteed, and sometimes this mode is simply dangerous. Therefore, it is not recommended to connect the UPS without load to the network (for example, for testing).

The power of the power supply (total for all output voltages) in a household PC fully equipped with peripheral devices must be at least 200 W. It is practically necessary to have 230...250 W, and when installing additional hard drives and CD-ROM drives, more may be required. PC malfunctions, especially those that occur when the electric motors of the mentioned devices are turned on, are often associated with an overload of the power supply. Computers used as information network servers consume up to 350 W. Low-power UPSs (40...160 W) are used in specialized, for example, control computers with a limited set of peripherals.

The volume occupied by a UPS usually increases due to an increase in its length towards the front panel of the PC. The installation dimensions and mounting points of the unit in the computer case remain unchanged. Therefore, any (with rare exceptions) block can be installed in the place of the failed one.

The basis of most UPSs is a push-pull half-bridge inverter operating at a frequency of several tens of kilohertz. The inverter supply voltage (approximately 300 V) is rectified and smoothed mains voltage. The inverter itself consists of a control unit (pulse generator with an intermediate power amplification stage) and a powerful output stage. The latter is loaded onto a high-frequency power transformer. The output voltages are obtained using rectifiers connected to the secondary windings of this transformer. Voltage stabilization is carried out using pulse width modulation (PWM) of pulses generated by the inverter. Typically, only one output channel is covered by the stabilizing OS, usually +5 or +3.3 V. As a result, the voltages at other outputs do not depend on the network voltage, but remain subject to the influence of the load. Sometimes they are additionally stabilized using conventional stabilizer chips.

MAINS RECTIFIER

In most cases, this unit is performed according to a scheme similar to that shown in Fig. 4, the differences are only in the type of rectifier bridge VD1 and a greater or lesser number of protective and safety elements. Sometimes the bridge is assembled from individual diodes. When switch S1 is open, which corresponds to the unit being powered from a 220...230 V network, the rectifier is a bridge, the voltage at its output (capacitors C4, C5 connected in series) is close to the amplitude of the network. When powered from a network of 110... 127 V, by closing the contacts of the switch, they turn the device into a rectifier with doubling the voltage and obtain at its output a constant voltage that is twice the amplitude of the network voltage. Such switching is provided in UPSs, the stabilizers of which keep the output voltages within acceptable limits only when the mains voltage deviates by 20%. Units with more effective stabilization are able to operate at any mains voltage (usually from 90 to 260 V) without switching.

Resistors R1, R4 and R5 are designed to discharge the rectifier capacitors after it is disconnected from the network, and C4 and C5, in addition, equalize the voltages on capacitors C4 and C5. Thermistor R2 with a negative temperature coefficient limits the amplitude of the inrush current charging capacitors C4, C5 at the moment the unit is turned on. Then, as a result of self-heating, its resistance drops, and it practically does not affect the operation of the rectifier. Varistor R3 with a classification voltage greater than the maximum amplitude of the network protects against surges of the latter. Unfortunately, this varistor is useless if a unit with a closed switch S1 is accidentally turned on in a 220 V network. The serious consequences of this can be avoided by replacing resistors R4, R5 with varistors with a classification voltage of 180...220 V, the breakdown of which entails the combustion of the fuse-link FU1. Sometimes varistors are connected in parallel with the specified resistors or only one of them.

Capacitors C1 ≈ SZ and two-winding inductor L1 form a filter that protects the computer from interference from the network, and the network from interference created by the computer. Through capacitors C1 and SZ, the computer case is connected via alternating current to the network wires. Therefore, the voltage of touching an ungrounded computer can reach half the network voltage. This is not life-threatening, since the reactance of the capacitors is quite high, but it often leads to failure of the interface circuits when peripheral devices are connected to the computer.

POWERFUL INVERTER CASCADE

On rice. 5 shows part of the circuit diagram of the common GT-150W UPS. The pulses generated by the control unit are sent through transformer T1 to the bases of transistors VT1 and VT2, opening them alternately. Diodes VD4, VD5 protect transistors from reverse polarity voltage. Capacitors C6 and C7 correspond to C4 and C5 in the rectifier (see Fig. 4). The voltages of the secondary windings of transformer T2 are rectified to obtain output. One of the rectifiers (VD6, VD7 with filter L1C5) is shown in the diagram.

Most powerful UPS cascades differ from those considered only in the types of transistors, which can be, for example, field-effect ones or contain built-in protective diodes. There are several options for the design of basic circuits (for bipolar) or gate circuits (for field-effect transistors) with different numbers, ratings and circuits for connecting elements. For example, resistors R4, R6 can be connected directly to the bases of the corresponding transistors.

In steady state, the inverter control unit is supplied with the output voltage of the UPS, but at the moment of switching on it is absent. There are two main ways to obtain the supply voltage necessary to start the inverter. The first of them is implemented in the scheme under consideration (Fig. 5). Immediately after turning on the unit, the rectified mains voltage flows through the resistive divider R3 ≈ R6 into the base circuits of transistors VT1 and\/T2, opening them slightly, and diodes VD1 and VD2 prevent the base-emitter sections of the transistors from being shunted by windings II and III of transformer T1. At the same time, capacitors C4, C6 and C7 are charged, and the charging current of capacitor C4, flowing through winding I of transformer T2 and through part of winding II of transformer T1, induces a voltage in windings II and III of the latter that opens one of the transistors and closes the other. Which transistor will close and which will open depends on the asymmetry of the characteristics of the cascade elements.

As a result of the action of positive feedback, the process proceeds like an avalanche, and a pulse induced in winding II of transformer T2 through one of the diodes VD6, VD7, resistor R9 and diode VD3 charges the capacitor SZ to a voltage sufficient to start operation of the control unit. Subsequently, it is powered by the same circuit, and the voltage rectified by diodes VD6, VD7, after smoothing by the L1C5 filter, is supplied to the +12 V output of the UPS.

The version of the initial startup circuits used in the LPS-02-150XT UPS differs only in that the voltage to the divider, similar to R3 ≈ R6 (Fig. 5), is supplied from a separate half-wave rectifier of the mains voltage with a small-capacity filter capacitor. As a result, the inverter transistors open slightly before the main rectifier filter capacitors (C6, C7, see Fig. 5) are charged, which ensures a more reliable start.

The second method of powering the control unit during startup involves the presence of a special low-power step-down transformer with a rectifier, as shown in the diagram in Fig. 6 used in the PS-200B UPS.

The number of turns of the secondary winding of the transformer is selected so that the rectified voltage is slightly less than the output in the +12 V channel of the unit, but sufficient for the operation of the control unit. When the output voltage of the UPS reaches its nominal value, the diode VD5 opens, the diodes of the bridge VD1 ≈ VD4 remain closed during the entire period of alternating voltage and the control unit switches to power supply with the output voltage of the inverter, without consuming more energy from the “starting” transformer.

In high-power inverter stages triggered in this way, there is no need for an initial bias at the bases of the transistors and positive feedback. Therefore, resistors R3, R5 are not required, diodes VD1, VD2 are replaced with jumpers, and winding II of transformer T1 is made without a tap (see Fig. 5).

OUTPUT RECTIFIERS

In Fig. Figure 7 shows a typical diagram of a four-channel UPS rectifier unit. In order not to violate the symmetry of magnetization reversal of the magnetic circuit of a power transformer, rectifiers are built only using full-wave circuits, and bridge rectifiers, which are characterized by increased losses, are almost never used. The main feature of rectifiers in UPSs is smoothing filters, starting with inductance (choke). The voltage at the output of a rectifier with such a filter depends not only on the amplitude, but also on the duty cycle (the ratio of the duration to the repetition period) of the pulses arriving at the input. This makes it possible to stabilize the output voltage by changing the duty cycle of the input. Rectifiers with filters starting with a capacitor, used in many other cases, do not have this property. The process of changing the duty cycle of pulses is usually called PWM ≈ pulse width modulation (English PWM ≈ Pulse Width Modulation).

Since the amplitude of the pulses, proportional to the voltage in the supply network, at the inputs of all rectifiers in the block changes according to the same law, stabilizing one of the output voltages using PWM stabilizes all the others. To enhance this effect, filter chokes L1.1 ≈ L1.4 of all rectifiers are wound on a common magnetic core. The magnetic connection between them additionally synchronizes the processes occurring in the rectifiers.

For proper operation of a rectifier with an L-filter, it is necessary that its load current exceed a certain minimum value, depending on the inductance of the filter choke and the pulse frequency. This initial load is created by resistors R4 ≈ R7, connected in parallel with the output capacitors C5 ≈ C8. They also serve to speed up the discharge of capacitors after the UPS is turned off.

Sometimes a voltage of -5 V is obtained without a separate rectifier from a voltage of -12 V using an integrated stabilizer of the 7905 series. Domestic analogues are microcircuits KR1162EN5A, KR1179EN05. The current consumed by computer nodes along this circuit usually does not exceed several hundred milliamps.

In some cases, integrated stabilizers are installed in other UPS channels. This solution eliminates the influence of a changing load on the output voltages, but reduces the efficiency of the unit and for this reason is used only in relatively low-power channels. An example is the diagram of the PS-6220C UPS rectifier assembly shown in rice. 8. Diodes VD7 ≈ VD10 ≈ protective.

As in most other units, the +5 V voltage rectifier here contains Schottky barrier diodes (VD6 assembly), which are characterized by a lower forward voltage drop and reverse resistance recovery time than conventional diodes. Both of these factors are favorable for increasing efficiency. Unfortunately, the relatively low permissible reverse voltage does not allow the use of Schottky diodes in the +12 V channel. However, in the unit under consideration, this problem is solved by connecting two rectifiers in series: the missing 7 V is added to the 5 V by a rectifier on the Schottky diode assembly VD5.

To eliminate voltage surges that are dangerous for diodes and occur in the transformer windings at pulse fronts, damping circuits R1C1, R2C2, R3C3 and R4C4 are provided.

CONTROL UNIT

In most “computer” UPSs, this unit is built on the basis of the TL494CN PWM controller chip (domestic equivalent ≈ KR1114EU4) or its modifications. The main part of the diagram of such a node is shown in Fig. 9, it also shows the elements of the internal structure of the mentioned microcircuit.

The sawtooth voltage generator G1 serves as a master. Its frequency depends on the ratings of external elements R8 and SZ. The generated voltage is supplied to two comparators (A3 and A4), the output pulses of which are summed by the OR element D1. Next, the pulses through the NOR elements D5 and D6 are supplied to the output transistors of the microcircuit (V3, V4). Pulses from the output of element D1 also arrive at the counting input of trigger D2, and each of them changes the state of the trigger. Thus, if a log is applied to pin 13 of the microcircuit. 1 or, as in the case under consideration, it is left free, the pulses at the outputs of elements D5 and D6 alternate, which is necessary to control a push-pull inverter. If the TL494 chip is used in a single-ended voltage converter, pin 13 is connected to the common wire, as a result, trigger D2 is no longer involved in the operation, and pulses appear at all outputs simultaneously.

Element A1 is an error signal amplifier in the UPS output voltage stabilization circuit. This voltage (in this case ≈ +5 V) is supplied to one of the amplifier inputs through a resistive divider R1R2. At its second input ≈ the reference voltage obtained from the stabilizer A5 built into the chip using a resistive divider R3 ≈ R5. The voltage at output A1, proportional to the difference between the input ones, sets the operating threshold of comparator A4 and, consequently, the duty cycle of the pulses at its output. Since the output voltage of the UPS depends on the duty cycle (see above), in a closed system it is automatically maintained equal to the exemplary voltage, taking into account the division coefficient R1R2. The R7C2 chain is necessary for the stability of the stabilizer. The second amplifier (A2), in this case, from the switches by supplying the appropriate voltages to its inputs, does not participate in the operation.

The function of comparator A3 is to guarantee the presence of a pause between pulses at the output of element D1, even if the output voltage of amplifier A1 is outside the permissible limits. The minimum response threshold A3 (when connecting pin 4 to common) is set by the internal voltage source GV1. As the voltage at pin 4 increases, the minimum pause duration increases, therefore, the maximum output voltage of the UPS drops.

This property is used for smooth startup of the UPS. The fact is that at the initial moment of operation of the unit, the filter capacitors of its rectifiers are completely discharged, which is equivalent to shorting the outputs to the common wire. Starting the inverter immediately “at full power” will lead to a huge overload of the transistors of the powerful cascade and their possible failure. Circuit C1R6 ensures a smooth, overload-free start of the inverter.

At the first moment after switching on, capacitor C1 is discharged, and the voltage at pin 4 of DA1 is close to +5 V received from stabilizer A5. This guarantees a pause of the maximum possible duration, up to the complete absence of pulses at the output of the microcircuit. As capacitor C1 charges through resistor R6, the voltage at pin 4 decreases, and with it the duration of the pause. At the same time, the output voltage of the UPS increases. This continues until it approaches the exemplary one and stabilizing feedback comes into effect. Further charging of capacitor C1 does not affect the processes in the UPS. Since capacitor C1 must be completely discharged before each UPS is turned on, in many cases circuits for its forced discharge are provided (not shown in Fig. 9).

INTERMEDIATE CASCADE

The task of this cascade is to amplify the pulses before feeding them to powerful transistors. Sometimes the intermediate stage is missing as an independent unit, being part of the master oscillator microcircuit. The diagram of such a cascade used in the PS-200B UPS is shown in Fig. 10 . Matching transformer T1 here corresponds to the one of the same name in Fig. 5.

The APPIS UPS uses an intermediate stage according to the circuit shown in Fig. 11, which differs from the one discussed above by the presence of two matching transformers T1 and T2 ≈ separately for each power transistor. The polarity of the transformer windings is such that the intermediate stage transistor and the power transistor associated with it are in the open state at the same time. If special measures are not taken, after a few cycles of inverter operation, the accumulation of energy in the magnetic circuits of the transformers will lead to saturation of the latter and a significant decrease in the inductance of the windings.

Let's consider how this problem is solved, using the example of one of the “halves” of the intermediate stage with transformer T1. When the transistor of the microcircuit is open, winding Ia is connected to the power source and the common wire. A linearly increasing current flows through it. A positive voltage is induced in winding II, which enters the base circuit of the powerful transistor and opens it. When the transistor in the microcircuit is closed, the current in winding Ia will be interrupted. But the magnetic flux in the magnetic core of the transformer cannot change instantly, so a linearly decreasing current will appear in winding Ib, flowing through the opened diode VD1 from the common wire to the plus of the power source. Thus, the energy accumulated in the magnetic field during the pulse returns to the source during the pause. The voltage on winding II during the pause is negative, and the powerful transistor is closed. The second “half” of the cascade with transformer T2 operates in a similar way, but in antiphase.

The presence of pulsating magnetic fluxes with a constant component in magnetic circuits leads to the need to increase the mass and volume of transformers T1 and T2. In general, an intermediate stage with two transformers is not very successful, although it has become quite widespread.

If the power of the transistors of the TL494CN microcircuit is not enough to directly control the output stage of the inverter, use a circuit similar to that shown in Fig. 12, which shows the intermediate stage of the KYP-150W UPS. The halves of winding I of transformer T1 serve as collector loads of transistors VT1 and VT2, alternately opened by pulses coming from the DA1 microcircuit. Resistor R5 limits the collector current of the transistors to approximately 20 mA. Using diodes VD1, VD2 and capacitor C1 on the emitters of transistors VT1 and VT2, the voltage required for their reliable closing is +1.6 V. Diodes VD4 and VD5 dampen the oscillations that occur when switching transistors in the circuit formed by the inductance of winding I of transformer T1 and its own capacity. Diode VD3 closes if the voltage surge at the middle terminal of winding I exceeds the cascade supply voltage.

Another version of the intermediate stage circuit (UPS ESP-1003R) is shown in Fig. 13. In this case, the output transistors of the DA1 microcircuit are connected according to a circuit with a common collector. Capacitors C1 and C2 are boosting. Winding I of transformer T1 does not have a middle terminal. Depending on which of the transistors VT1, VT2 is currently open, the winding circuit is closed to the power source through resistor R7 or R8 connected to the collector of the closed transistor.

TROUBLESHOOTING

Before repairing the UPS, it must be removed from the computer system unit. To do this, disconnect the computer from the network by removing the plug from the outlet. Having opened the computer case, release all the UPS connectors and, by unscrewing the four screws on the back wall of the system unit, remove the UPS. Then remove the U-shaped cover of the UPS case by unscrewing the screws securing it. The printed circuit board can be removed by unscrewing the three self-tapping screws that secure it. A feature of many UPS boards is that the printed conductor of the common wire is divided into two parts, which are connected to each other only through the metal body of the unit. On the board removed from the case, these parts must be connected with an overhead conductor.

If the power supply was disconnected from the power supply less than half an hour ago, you need to find and discharge 220 or 470 uF x 250 V oxide capacitors on the board (these are the largest capacitors in the block). During the repair process, it is recommended to repeat this operation after each disconnection of the unit from the network, or to temporarily bypass the capacitors with 100...200 kOhm resistors with a power of at least 1 W.

First of all, they inspect the parts of the UPS and identify those that are clearly faulty, for example, those that are burnt or have cracks in the case. If the failure of the unit was caused by a fan malfunction, you should check the elements installed on the heat sinks: powerful transistors of the inverter and Schottky diode assemblies of the output rectifiers. When oxide capacitors “explode,” their electrolyte is sprayed throughout the unit. To avoid oxidation of metal live parts, it is necessary to wash off the electrolyte with a slightly alkaline solution (for example, diluting the “Fairy” product with water in a ratio of 1:50).

Having connected the unit to the network, first of all you should measure all its output voltages. If it turns out that in at least one of the output channels the voltage is close to the nominal value, the fault should be sought in the output circuits of the faulty channels. However, as practice shows, output circuits rarely fail.

In case of malfunction of all channels, the method for determining faults is as follows. Measure the voltage between the positive terminal of capacitor C4 and the negative terminal of C5 (see Fig. 4) or the collector of transistor VT1 and the emitter VT2 (see Fig. 5). If the measured value is significantly less than 310 V, you need to check and, if necessary, replace the diode bridge VD1 (see . Fig. 4) or its individual diodes. If the rectified voltage is normal, but the unit does not work, most likely, one or both transistors of the powerful inverter stage (VT1, VT2, see Fig. 5), which are subject to the greatest thermal overloads, have failed. If the transistors are working, all that remains is to check the TL494CN microcircuit and the associated circuits.

Failed transistors can be replaced with domestic or imported analogues that are suitable in terms of electrical parameters, overall and installation dimensions, guided by the data given in table. 2. Replacement diodes are selected according to the table. 3.

The rectifier diodes of the network rectifier (see Fig. 4) can be successfully replaced with domestic KD226G, KD226D. If the network rectifier has capacitors with a capacity of 220 μF, it is advisable to replace them with 470 μF; there is usually space for this on the board. To reduce interference, it is recommended to shunt each of the four rectifier diodes with a 1000 pF capacitor to a voltage of 400...450 V.

Transistors 2SC3039 can be replaced with domestic KT872A. But the PXPR1001 damping diode to replace the failed one is difficult to purchase even in big cities. In this situation, you can use three KD226G or KD226D diodes connected in series. It is possible to replace the failed diode and the powerful transistor protected by it by installing a transistor with a built-in damping diode, for example, 2SD2333, 2SD1876, 2SD1877 or 2SD1554. It should be noted that many UPSs released after 1998 have already undergone such a replacement.

To enlarge, click on the image (opens in a new window)

To increase the reliability of the IED, it is recommended to connect chokes with an inductance of 4 μH in parallel with resistors R7 and R8 (see Fig. 5). They can be wound with wire with a diameter of at least 0.15 mm in silk insulation on any ring magnetic cores. The number of turns is calculated using known formulas.

Many UPSs do not have a tuning resistor for adjusting the output voltage (R3, see Fig. 9); a constant one is installed instead. If adjustment is required, it can be done by temporarily installing a trim resistor, and then again replacing it with a constant of the found value.

To increase reliability, it is useful to replace the imported oxide capacitors installed in the filters of the most powerful + 12 V and +5 V rectifiers with K50-29 capacitors equivalent in capacity and voltage. It should be noted that on the boards of many UPSs, not all capacitors provided for by the circuit are installed (apparently, to save money), which negatively affects the characteristics of the unit. It is recommended to install the missing capacitors in their designated places.

When assembling the unit after repair, do not forget to remove the temporarily installed jumpers and resistors, and also connect the built-in fan to the corresponding connector.

LITERATURE
1. Kulichkov A. Switching power supplies for IBM PC. - M.: DMK, series "Repair and Service", 2000.
2. Guk M. IBM PC hardware. - St. Petersburg: Peter, 2000.
3. Kunevich A.. Sidorov I. Inductive elements on ferrites. - St. Petersburg: Lenizdat, 1997.
4. Nikulin S. Reliability of radio-electronic equipment elements. - M.: Energy, 1979.

A good laboratory power supply is quite expensive and not all radio amateurs can afford it.
Nevertheless, at home you can assemble a power supply with good characteristics, which can cope well with providing power to various amateur radio designs, and can also serve as a charger for various batteries.
Such power supplies are assembled by radio amateurs, usually from , which are available and cheap everywhere.

In this article, little attention is paid to the conversion of the ATX itself, since converting a computer power supply for a radio amateur of average qualification into a laboratory one, or for some other purpose, is usually not difficult, but beginning radio amateurs have many questions about this. Basically, what parts in the power supply need to be removed, what parts should be left, what should be added in order to turn such a power supply into an adjustable one, and so on.

Especially for such radio amateurs, in this article I want to talk in detail about converting ATX computer power supplies into regulated power supplies, which can be used both as a laboratory power supply and as a charger.

For the modification, we will need a working ATX power supply, which is made on a TL494 PWM controller or its analogues.
The power supply circuits on such controllers, in principle, do not differ much from each other and are all basically similar. The power of the power supply should not be less than that which you plan to remove from the converted unit in the future.

Let's look at a typical ATX power supply circuit with a power of 250 W. For Codegen power supplies, the circuit is almost no different from this one.

The circuits of all such power supplies consist of a high-voltage and low-voltage part. In the picture of the power supply printed circuit board (below) from the track side, the high-voltage part is separated from the low-voltage part by a wide empty strip (without tracks), and is located on the right (it is smaller in size). We will not touch it, but will work only with the low-voltage part.
This is my board and using its example I will show you an option for converting an ATX power supply.

The low-voltage part of the circuit we are considering consists of a TL494 PWM controller, an operational amplifier circuit that controls the output voltages of the power supply, and if they do not match, it gives a signal to the 4th leg of the PWM controller to turn off the power supply.
Instead of an operational amplifier, transistors can be installed on the power supply board, which in principle perform the same function.
Next comes the rectifier part, which consists of various output voltages, 12 volts, +5 volts, -5 volts, +3.3 volts, of which for our purposes only a +12 volt rectifier will be needed (yellow output wires).
The remaining rectifiers and accompanying parts will need to be removed, except for the “duty” rectifier, which we will need to power the PWM controller and cooler.
The duty rectifier provides two voltages. Typically this is 5 volts and the second voltage can be around 10-20 volts (usually around 12).
We will use a second rectifier to power the PWM. A fan (cooler) is also connected to it.
If this output voltage is significantly higher than 12 volts, then the fan will need to be connected to this source through an additional resistor, as will be later in the circuits under consideration.
In the diagram below, I marked the high-voltage part with a green line, the “standby” rectifiers with a blue line, and everything else that needs to be removed with red.

So, we unsolder everything that is marked in red, and in our 12 volt rectifier we change the standard electrolytes (16 volts) to higher voltage ones, which will correspond to the future output voltage of our power supply. It will also be necessary to unsolder the 12th leg of the PWM controller and the middle part of the winding of the matching transformer - resistor R25 and diode D73 (if they are in the circuit) in the circuit, and instead of them, solder a jumper into the board, which is drawn in the diagram with a blue line (you can simply close diode and resistor without soldering them). In some circuits this circuit may not exist.

Next, in the PWM harness on its first leg, we leave only one resistor, which goes to the +12 volt rectifier.
On the second and third legs of the PWM, we leave only the Master RC chain (in the diagram R48 C28).
On the fourth leg of the PWM we leave only one resistor (in the diagram it is designated as R49. Yes, in many other circuits between the 4th leg and the 13-14 legs of the PWM there is usually an electrolytic capacitor, we don’t touch it (if any) either, since it is intended for a soft start of the power supply. My board simply did not have it, so I installed it.
Its capacity in standard circuits is 1-10 μF.
Then we free the 13-14 legs from all connections, except for the connection with the capacitor, and also free the 15th and 16th legs of the PWM.

After all the operations performed, we should get the following.

This is what it looks like on my board (in the picture below).
Here I rewound the group stabilization choke with a 1.3-1.6 mm wire in one layer on the original core. It fit somewhere around 20 turns, but you don’t have to do this and leave the one that was there. Everything works well with him too.
I also installed another load resistor on the board, which consists of two 1.2 kOhm 3W resistors connected in parallel, the total resistance was 560 Ohms.
The native load resistor is designed for 12 volts of output voltage and has a resistance of 270 Ohms. My output voltage will be about 40 volts, so I installed such a resistor.
It must be calculated (at the maximum output voltage of the power supply at idle) for a load current of 50-60 mA. Since operating the power supply completely without load is not desirable, that’s why it is placed in the circuit.

View of the board from the parts side.

Now what will we need to add to the prepared board of our power supply in order to turn it into an regulated power supply;

First of all, in order not to burn the power transistors, we will need to solve the problem of load current stabilization and short circuit protection.
On forums for remaking similar units, I came across such an interesting thing - when experimenting with the current stabilization mode, on the forum pro-radio, forum member DWD I cited the following quote, I will quote it in full:

“I once told you that I couldn’t get the UPS to operate normally in current source mode with a low reference voltage at one of the inputs of the error amplifier of the PWM controller.
More than 50mV is normal, but less is not. In principle, 50mV is a guaranteed result, but in principle, you can get 25mV if you try. Anything less didn’t work. It does not work stably and is excited or confused by interference. This is when the signal voltage from the current sensor is positive.
But in the datasheet on the TL494 there is an option when negative voltage is removed from the current sensor.
I converted the circuit to this option and got an excellent result.
Here is a fragment of the diagram.

Actually, everything is standard, except for two points.
Firstly, is the best stability when stabilizing the load current with a negative signal from the current sensor an accident or a pattern?
The circuit works great with a reference voltage of 5mV!
With a positive signal from the current sensor, stable operation is obtained only at higher reference voltages (at least 25 mV).
With resistor values ​​of 10 Ohm and 10 KOhm, the current stabilized at 1.5 A up to the output short circuit.
I need more current, so I installed a 30 Ohm resistor. Stabilization was achieved at a level of 12...13A at a reference voltage of 15mV.
Secondly (and most interestingly), I don’t have a current sensor as such...
Its role is played by a fragment of a track on the board 3 cm long and 1 cm wide. The track is covered with a thin layer of solder.
If you use this track at a length of 2cm as a sensor, then the current will stabilize at the level of 12-13A, and if at a length of 2.5cm, then at the level of 10A."

Since this result turned out to be better than the standard one, we will go the same way.

First, you will need to unsolder the middle terminal of the secondary winding of the transformer (flexible braid) from the negative wire, or better without soldering it (if the signet allows) - cut the printed track on the board that connects it to the negative wire.
Next, you will need to solder a current sensor (shunt) between the track cut, which will connect the middle terminal of the winding to the negative wire.

It is best to take shunts from faulty (if you find them) pointer ampere-voltmeters (tseshek), or from Chinese pointer or digital instruments. They look something like this. A piece 1.5-2.0 cm long will be sufficient.

You can, of course, try to do it as I wrote above. DWD, that is, if the path from the braid to the common wire is long enough, then try to use it as a current sensor, but I didn’t do this, I came across a board of a different design, like this one, where the two wire jumpers that connected the output are indicated by a red arrow braids with a common wire, and printed tracks ran between them.

Therefore, after removing unnecessary parts from the board, I removed these jumpers and in their place soldered a current sensor from a faulty Chinese "tseshka".
Then I soldered the rewound inductor in place, installed the electrolyte and load resistor.
This is what my piece of board looks like, where I marked with a red arrow the installed current sensor (shunt) in place of the jumper wire.

Then you need to connect this shunt to the PWM using a separate wire. From the side of the braid - with the 15th PWM leg through a 10 Ohm resistor, and connect the 16th PWM leg to the common wire.
Using a 10 Ohm resistor, you can select the maximum output current of our power supply. On the diagram DWD The resistor is 30 ohms, but start with 10 ohms for now. Increasing the value of this resistor increases the maximum output current of the power supply.

As I said earlier, the output voltage of my power supply is about 40 volts. To do this, I rewound the transformer, but in principle you can not rewind it, but increase the output voltage in another way, but for me this method turned out to be more convenient.
I’ll tell you about all this a little later, but for now let’s continue and start installing the necessary additional parts on the board so that we have a working power supply or charger.

Let me remind you once again that if you did not have a capacitor on the board between the 4th and 13-14 legs of the PWM (as in my case), then it is advisable to add it to the circuit.
You will also need to install two variable resistors (3.3-47 kOhm) to adjust the output voltage (V) and current (I) and connect them to the circuit below. It is advisable to make the connection wires as short as possible.
Below I have given only part of the diagram that we need - such a diagram will be easier to understand.
In the diagram, newly installed parts are indicated in green.

Diagram of newly installed parts.

Let me give you a little explanation of the diagram;
- The topmost rectifier is the duty room.
- The values ​​of the variable resistors are shown as 3.3 and 10 kOhm - the values ​​are as found.
- The value of resistor R1 is indicated as 270 Ohms - it is selected according to the required current limitation. Start small and you may end up with a completely different value, for example 27 Ohms;
- I did not mark capacitor C3 as a newly installed part in the expectation that it might be present on the board;
- The orange line indicates elements that may have to be selected or added to the circuit during the process of setting up the power supply.

Next we deal with the remaining 12-volt rectifier.
Let's check what maximum voltage our power supply can produce.
To do this, we temporarily unsolder from the first leg of the PWM - a resistor that goes to the output of the rectifier (according to the diagram above at 24 kOhm), then you need to turn on the unit to the network, first connect it to the break of any network wire, and use a regular 75-95 incandescent lamp as a fuse Tue In this case, the power supply will give us the maximum voltage it is capable of.

Before connecting the power supply to the network, make sure that the electrolytic capacitors in the output rectifier are replaced with higher voltage ones!

All further switching on of the power supply should be carried out only with an incandescent lamp; it will protect the power supply from emergency situations in case of any errors. In this case, the lamp will simply light up, and the power transistors will remain intact.

Next we need to fix (limit) the maximum output voltage of our power supply.
To do this, we temporarily change the 24 kOhm resistor (according to the diagram above) from the first leg of the PWM to a tuning resistor, for example 100 kOhm, and set it to the maximum voltage we need. It is advisable to set it so that it is 10-15 percent less than the maximum voltage that our power supply is capable of delivering. Then solder a permanent resistor in place of the tuning resistor.

If you plan to use this power supply as a charger, then the standard diode assembly used in this rectifier can be left, since its reverse voltage is 40 volts and it is quite suitable for a charger.
Then the maximum output voltage of the future charger will need to be limited in the manner described above, around 15-16 volts. For a 12-volt battery charger, this is quite enough and there is no need to increase this threshold.
If you plan to use your converted power supply as an regulated power supply, where the output voltage will be more than 20 volts, then this assembly will no longer be suitable. It will need to be replaced with a higher voltage one with the appropriate load current.
I installed two assemblies on my board in parallel, 16 amperes and 200 volts each.
When designing a rectifier using such assemblies, the maximum output voltage of the future power supply can be from 16 to 30-32 volts. It all depends on the model of the power supply.
If, when checking the power supply for the maximum output voltage, the power supply produces a voltage less than planned, and someone needs more output voltage (40-50 volts for example), then instead of the diode assembly, you will need to assemble a diode bridge, unsolder the braid from its place and leave it hanging in the air, and connect the negative terminal of the diode bridge in place of the soldered braid.

Rectifier circuit with diode bridge.

With a diode bridge, the output voltage of the power supply will be twice as high.
Diodes KD213 (with any letter) are very suitable for a diode bridge, the output current with which can reach up to 10 amperes, KD2999A,B (up to 20 amperes) and KD2997A,B (up to 30 amperes). The last ones are best, of course.
They all look like this;

In this case, it will be necessary to think about attaching the diodes to the radiator and isolating them from each other.
But I took a different route - I simply rewound the transformer and did it as I said above. two diode assemblies in parallel, since there was space for this on the board. For me this path turned out to be easier.

Rewinding a transformer is not particularly difficult, and we’ll look at how to do it below.

First, we unsolder the transformer from the board and look at the board to see which pins the 12-volt windings are soldered to.

There are mainly two types. Just like in the photo.
Next you will need to disassemble the transformer. Of course, it will be easier to deal with smaller ones, but larger ones can also be dealt with.
To do this, you need to clean the core from visible varnish (glue) residues, take a small container, pour water into it, put the transformer there, put it on the stove, bring to a boil and “cook” our transformer for 20-30 minutes.

For smaller transformers this is quite enough (less is possible) and such a procedure will not harm the core and windings of the transformer at all.
Then, holding the transformer core with tweezers (you can do it right in the container), using a sharp knife we ​​try to disconnect the ferrite jumper from the W-shaped core.

This is done quite easily, since the varnish softens from this procedure.
Then, just as carefully, we try to free the frame from the W-shaped core. This is also quite easy to do.

Then we wind up the windings. First comes half of the primary winding, mostly about 20 turns. We wind it up and remember the direction of winding. The second end of this winding does not need to be unsoldered from the point of its connection with the other half of the primary, if this does not interfere with further work with the transformer.

Then we wind up all the secondary ones. Usually there are 4 turns of both halves of 12-volt windings at once, then 3+3 turns of 5-volt windings. We wind everything up, unsolder it from the terminals and wind a new winding.
The new winding will contain 10+10 turns. We wind it with a wire with a diameter of 1.2 - 1.5 mm, or a set of thinner wires (easier to wind) of the appropriate cross-section.
We solder the beginning of the winding to one of the terminals to which the 12-volt winding was soldered, we wind 10 turns, the direction of winding does not matter, we bring the tap to the “braid” and in the same direction as we started - we wind another 10 turns and the end solder to the remaining pin.
Next, we isolate the secondary and wind the second half of the primary onto it, which we wound earlier, in the same direction as it was wound earlier.
We assemble the transformer, solder it into the board and check the operation of the power supply.

If during the process of adjusting the voltage any extraneous noises, squeaks, or crackles occur, then to get rid of them, you will need to select the RC chain circled in the orange ellipse below in the figure.

In some cases, you can completely remove the resistor and select a capacitor, but in others you can’t do it without a resistor. You can try adding a capacitor, or the same RC circuit, between 3 and 15 PWM legs.
If this does not help, then you need to install additional capacitors (circled in orange), their ratings are approximately 0.01 uF. If this doesn’t help much, then install an additional 4.7 kOhm resistor from the second leg of the PWM to the middle terminal of the voltage regulator (not shown in the diagram).

Then you will need to load the power supply output, for example, with a 60-watt car lamp, and try to regulate the current with resistor “I”.
If the current adjustment limit is small, then you need to increase the value of the resistor that comes from the shunt (10 Ohms) and try to regulate the current again.
You should not install a tuning resistor instead of this one; change its value only by installing another resistor with a higher or lower value.

It may happen that when the current increases, the incandescent lamp in the network wire circuit will light up. Then you need to reduce the current, turn off the power supply and return the resistor value to the previous value.

Also, for voltage and current regulators, it is best to try to purchase SP5-35 regulators, which come with wire and rigid leads.

This is an analogue of multi-turn resistors (only one and a half turns), the axis of which is combined with a smooth and coarse regulator. At first it is regulated “Smoothly”, then when it reaches the limit, it begins to be regulated “Roughly”.
Adjustment with such resistors is very convenient, fast and accurate, much better than with a multi-turn. But if you can’t get them, then buy ordinary multi-turn ones, such as;

Well, it seems like I told you everything that I planned to complete on remaking the computer power supply, and I hope that everything is clear and intelligible.

If anyone has any questions about the design of the power supply, ask them on the forum.

Good luck with your design!

Repairing computer hardware yourself is quite a difficult task. At the same time, the user must know exactly which of all the components needs repair. It makes sense to repair a computer's power supply if it is (at a minimum) out of warranty, and also - the cost of replacement makes such repairs truly worthwhile. High-quality repairs in a service center can reach the cost of “budget” power supplies. Usually, the user can do some things himself... Provided that he has the skills to work with electrical equipment (220 Volts) and well understands the danger of mistakes in such work.

Recommendations for self-repair of computer power supplies:

1. Connection to a 220 V network of any power supply must be made through a “fast” fuse with a current of no more than 2A.
2. The first start after repair work is carried out in series with an incandescent lamp. A short circuit at the input of the device will be indicated by the glow of the lamp. Such a power supply cannot be included in the network.
3. In the process of both diagnostics and repair, it is necessary to discharge all electrolytic containers (after each switch on/off). You need to wait 3-5 minutes, or use a 220V electric lamp - the flash will indicate that the discharge has actually been produced.
4. All repair operations are carried out with the power supply completely disconnected from the network.

It is advisable that there are no grounded objects near the workplace (such as heating radiators, pipes, etc.)

Actually, we will not “get into” the high-voltage part of the power supply circuit. Self-repair comes down to: searching for “ring” cracks; replacing power diodes (if necessary); replacing “bad” capacitors (if necessary).

In any case, repairing a computer power supply begins with removing it from the PC. Of course, this is worth doing if you are 100% sure that it is the power supply that needs to be repaired.

The body of the power supply itself is disassembled by unscrewing the self-tapping screws (screws) securing the two halves to each other. A Phillips screwdriver is used.

Note: by disassembling the power supply yourself, you damage the manufacturer's seal - which entails the loss of further warranty for this device.

Directly how the power supply is repaired and the main malfunctions are described below. Most often, failures that occur can be detected and eliminated quite simply:

• Check if the “standby” voltage (+5V SB) is present. This is the purple wire of the 24-pin (main) power supply connector. Between “black” and “purple” there should be a voltage of +5 Volts. You can check its presence before disassembling the unit case; in this case, the power supply itself must be connected to the network.

• We disassembled the power supply - look at the board. Faulty (swollen) electrolytic capacitors are common. This can be determined visually; most often it is electrolytic capacitors of not very large capacity (470-220 µF or less) that are susceptible to defects. Such a capacitor must be unsoldered from the board (to do this, it will have to be removed), and a new one must be of the same capacity and designed for the same (or higher) voltage. Attention: observe the polarity of the leads! On imported ones, the “stripe” indicates “minus”.

• The next malfunction is the failure of low-voltage diodes (12 or 5V). They can be structurally designed as assemblies of two diodes (flat housing with three terminals), or they can be installed separately.

• Checking/replacing diodes is a little more difficult than with capacitors. To check, you need to unsolder one terminal of each diode (you can also unsolder the entire part). Everyone knows how a working diode “rings”. With a direct connection, the tester will show a value (close to “0”), with a reverse connection it shows nothing (the tester itself is turned on in the “diode” mode):

• As a replacement, it is recommended to install Schottky diodes that have a similar (or higher) declared current/voltage.

• When repairing the power supply yourself, unscrew the screws of the board itself and remove it (make sure once again that the unit must be de-energized). Looking carefully at the installation, you can quickly notice the defects of “ring cracks”:

They need to be “soldered”, then everything must be assembled and turned on (perhaps everything will work).

Separately, it is necessary to say about “standby” food. As a rule, repairing the power supply by simply replacing burnt-out transistors will not yield results - the transistors burn out again, and the same ones. The transformer may also be the culprit of the breakdown. This is a scarce item that is difficult to buy and find. In rare cases, the reason for the absence of 5V “standby” voltage may be a change in the operating frequency, for which the “frequency-setting” parts are responsible: a resistor and a capacitor (not electrolytic).

Note: to desolder a part installed on the heat sink, first dismantle (unscrew) its fastening. Installation is done in the reverse order (first fastening, then soldering). Try not to disturb the insulation of the part from the heat sink (usually mica is used).

Starting the power supply: check for +5V SB. If it is there, let’s try to start the power supply (connect the “lime” wire, PS-ON, to the “black” wire, common).

At this point, the user’s capabilities for independent repairs are, one might say, exhausted.

Attention! Do not attempt to repair the power supply yourself unless you have experience in electrical engineering! After each shutdown, it is necessary to discharge the high-voltage capacitors (wait 3-5 minutes)!

Read more: “swollen” capacitors and their replacement

We hope that from the photograph it is clear which capacitors are “swollen” and which are not.

If there are several identical ones on the board (or a set of parallel connected ones), of which at least one is defective, it is better to change everything. Companies that produce reliable products: Nichicon, Rubycon. But you are unlikely to find such ones. For budget ones, we can recommend Teapo, Samsung.

When installing, it is necessary to observe polarity (operating voltage must be the same or greater than that indicated on the one being replaced).

In the photo there is a 16 Volt capacitor, 470 MicroFarad (Rubycon, the most expensive series).

Soldering technology

When installing and dismantling parts on a computer power supply board, it is recommended to use a 40-watt soldering iron. In some cases, for bulky parts (“powerful” leads), you can use a 60-watt soldering iron (but no more).

The simplest solder (such as POS-60) is suitable in this case. It is better to take it in the form of a thin wire.

Flux – not used (it is enough to have regular rosin available).

Dismantling the part:

• Heat with a soldering iron until the solder completely melts;
• Using a desoldering device (made of plastic), quickly pump out the liquid solder:

• Repeat steps 1 and 2.

A correctly soldered part easily comes out of the board on its own (no need to “press” the lead with a soldering iron).

If the capacitor is being dismantled, you can first “bite off” the protruding terminal with side cutters.

If the power element is unsoldered, you must completely unscrew the fastening screw.

Replacing the fuse

In the circuit of any power supply, the fuse goes immediately after the power outlet (in series with one of the 220 V phases). The fuses themselves, as parts, vary in current strength (that is, how many amperes it will withstand at maximum). Also, fuses are divided into “F”-type (“fast”), “T”-type (“thermal”).

If the fuse needs to be replaced, you must find out what rating (current) it was designed for. Also, it is advisable to know the “type”.

Replacement with a fuse with a higher rating is not allowed. Replacing F with T does the same.

Note: If you know the required “current” but not the “type”, you can install a new type “F” fuse.

Exactly. And so that there are no questions about why it burns out more often, it will still be easier to find out reliable data (both denomination and type).

If the fuse is in a glass cylindrical case, then in any case it is designed for 220V power supply. The use of other types of construction is not permitted.

What is used (devices and materials)

When repairing a computer power supply , You won’t need any “non-standard” devices or equipment:

But what is in fig. – implies that you at least know how to handle: a soldering iron, a tester (pliers, side cutters...). For professional repairs, there should have been an oscilloscope (3 MHz bandwidth is sufficient). That's just the price... (like 2-3 new power supplies).

We hope the information provided here will be useful for performing “initial” repairs. More complex operations (repairing a transformer, working with high-voltage wiring, restoring generation) can be done by professionals (who have experience specifically in repairing power supplies).

A switching power supply is not a very “simple” device; in some cases, restoration of viability is carried out by complete replacement of parts (of one or another unit). More complex, “independent” repairs do not have to be “successful” in every case...

Diode characteristics

The diode itself, as a separate element, can be one of three types: a simple diode (p-n junction), a microwave diode, and a Schottky diode (quantum). We are only interested in the last of them.

The job of a diode is to pass current in one direction (and not pass it in the other). If the voltage drop in direct connection on conventional diodes is 1 or 2 volts, then on Schottky diodes it is close to zero. The voltages obtained in a computer power supply are low (12 Volts and 5), which is why only Schottky ones are used.

You can see what the voltage drop across the diode is. The tester must be in “diode” mode (as mentioned above). If it “shows” from 0.015 to 0.7, then everything is correct. Such values ​​are typical for a Schottky diode (less is a “breakdown”).

Inside the power supply circuits, a pair of diodes is used, turning them on counter:

For positive voltage, “assemblies” are used (three-terminal, with 2 diodes in them). Single diodes (round body) - usually used to produce negative voltages. When replacing single diodes (even if one “flies”), it is recommended to replace them in “pairs”.

What is the best way to choose a replacement? If on the “rectangular” plastic case (3-pin) the brand is written:

Then, with “round” ones it will be more difficult. The stripe on the body only means “direction”.

If we know the brand of diodes, we look for the same ones, or look at the parameters (voltage, current), and look for an analogue (with the same or slightly higher value).

If we don’t know, well, you need to “download” the circuit diagram of your power supply and take a look. By the way, in the SC they also do this (but thinking and guessing what the current strength is is not a very rewarding task). Not forgetting that computer power supplies contain only Schottky diodes.

Note: installing diode assemblies/diodes with obviously high current and voltage parameters is not recommended (let’s say: it was 50 Volts 12 A, but they install 50 Volts 20 A). There is no need to do this, because: there may be a different case. In addition, there are “additional” parameters (which in a more “powerful” case differ “not for the better”).

Typical example (assemblies, low-power power supply): 12CTQ040 (40V, 12A); 10CTQ150 (150V, 10A).

Example of single diodes: 90SQ045 (45V, 9A); SR350 (50V, 3A).

Replacing the power supply fan

How to choose a new fan for PSU? It, that is, the fan, must be: with a hydraulic bearing, three-pin (3 wires in the cable), and of suitable dimensions (12cm/8cm).

It is also important that the power supply uses a low-speed “vent”, usually 1200-1400 (for 12 cm) and 1600-2000 (for 8).

When the power supply starts, not all the voltage is supplied to the fan (not 12 Volts), but, let’s say, 3-5 Volts. It is important that the fan is able to “start” at such voltages (otherwise, it will not spin up after switching on). Check the “starting voltage” of the fan, be careful.

Method of connecting the fan to the power supply:

1. Two wires (black, red) are soldered to the power supply board.
2. Two wires (black, red) are connected with a 2-pin connector to the board connector.
3. Three wires (black, red + yellow) are connected to the board using a 3-pin connector.

In the first two cases, the yellow wire - the tachometer - can be removed from the power supply housing for monitoring by the motherboard itself.

Pay attention to such a parameter as the height of the fan. If you take more than you need, the PSU case “will not close.”

When replacing, it is important that the performance of the new fan (in “liters per minute”) is at least the same as that of the old fan. Perhaps this parameter is the main one (it is usually indicated in the product description).

Thus, you can immediately “mod” the power supply by installing an equally productive, but quieter propeller (a hydraulic bearing in budget power supplies is not often included “by default”).

That's probably all that can be said about fans. Choose.

Equivalent load

The power supply, when started by “wiring”, started. Don't rush to install it on your computer. Let's try to test the power supply on an equivalent load.

The following resistors are taken:

They are called “PEV” (the brand of copper wire they are made from). You can take it at 25 watts, or at 10 (at 7.5):

The main thing here is to make a circuit of them (connecting: in parallel, in series) to get a “powerful” resistance (3 Ohms and 5-6 Ohms).

We will connect a 5-ohm load to the “12V” line, a 3-ohm load to the “5V” line. To connect to the power supply, use a Molex connector (yellow wire is 12 V):

Note: when creating an "equivalent", take into account the power that falls on each resistor (it should not exceed the value for which it is designed).

Knowing the voltage across the resistor, the power is found according to the law: voltage squared / resistance.

Example: 4 resistors of 20 Ohms - “in parallel”, the power of each is 7.5 Watts (will be used for testing the “12-volt” line).

You can also use 12V halogen light bulbs (for example: two 10 Watts in parallel).

So, having connected the equivalent load to the Molex connector, we try to turn on the power supply (“lime”/“black”, ATX connector). The cord “220 Volt” must also be “standard”.

If it turns on, wait 10 seconds. Is the block going into defense? The fan must rotate, all voltages must be in the required range (deviation of no more than 5-6% is allowed).

Actually, in such a mode that is “gentle” for it, any power supply should work for as long as desired.

A more powerful “equivalent” can be made. That is, the resistance in Ohms will be even lower. The main thing is not to “overdo it” (for each power supply, the maximum current is indicated):

The current through the load is equal to the voltage divided by its resistance (in ohms). Well, you already know this...

When testing, the “load” will be included in only two lines (“plus 5”, “plus 12”). This is, in general, enough. Other voltages (“minuses”) can be measured with a voltmeter (on a 24-pin plug).

Note: if you want to “test” the “+12” line with a current strength higher than 6A, do not use Molex connectors! 4-pin processor power connector (+12 V) – holds up to 10 Amps. If necessary, the load is “spread” between two connectors (processor, Molex).

Note 2: When making any connections, use a wire of sufficient cross-section (per 1 mm2 - current 10 A).

At the equivalent load, heat will be generated (thermal power is equal to electrical power). Take care of cooling (air flow). During the testing process, the first 2-3 minutes - it is better to monitor whether one of the resistors overheats.

The photo shows a “serious” approach to creating an “equivalent”.

Power supply repair

Linear and switching power supplies

Let's start with the basics. The power supply in a computer performs three functions. First, alternating current from the household power supply must be converted to direct current. The second task of the power supply is to reduce the voltage of 110-230 V, which is excessive for computer electronics, to the standard values ​​​​required by power converters of individual PC components - 12 V, 5 V and 3.3 V (as well as negative voltages, which we will talk about a little later) . Finally, the power supply plays the role of a voltage stabilizer.

There are two main types of power supplies that perform the above functions - linear and switching. The simplest linear power supply is based on a transformer, on which the alternating current voltage is reduced to the required value, and then the current is rectified by a diode bridge.

However, the power supply is also required to stabilize the output voltage, which is caused by both voltage instability in the household network and a voltage drop in response to an increase in current in the load.

To compensate for the voltage drop, in a linear power supply the transformer parameters are calculated to provide excess power. Then, at high current, the required voltage will be observed in the load. However, the increased voltage that will occur without any means of compensation at low current in the payload is also unacceptable. Excess voltage is eliminated by including a non-useful load in the circuit. In the simplest case, this is a resistor or transistor connected through a Zener diode. In a more advanced version, the transistor is controlled by a microcircuit with a comparator. Be that as it may, excess power is simply dissipated as heat, which negatively affects the efficiency of the device.

In the switching power supply circuit, one more variable appears, on which the output voltage depends, in addition to the two already existing: input voltage and load resistance. There is a switch in series with the load (which in the case we are interested in is a transistor), controlled by a microcontroller in pulse width modulation (PWM) mode. The higher the duration of the open states of the transistor in relation to their period (this parameter is called duty cycle, in Russian terminology the inverse value is used - duty cycle), the higher the output voltage. Due to the presence of a switch, a switching power supply is also called Switched-Mode Power Supply (SMPS).

No current flows through a closed transistor, and the resistance of an open transistor is ideally negligible. In reality, an open transistor has resistance and dissipates some of the power as heat. In addition, the transition between transistor states is not perfectly discrete. And yet, the efficiency of a pulsed current source can exceed 90%, while the efficiency of a linear power supply with a stabilizer reaches 50% at best.

Another advantage of switching power supplies is the radical reduction in the size and weight of the transformer compared to linear power supplies of the same power. It is known that the higher the frequency of alternating current in the primary winding of a transformer, the smaller the required core size and the number of winding turns. Therefore, the key transistor in the circuit is placed not after, but before the transformer and, in addition to voltage stabilization, is used to produce high-frequency alternating current (for computer power supplies this is from 30 to 100 kHz and higher, and as a rule - about 60 kHz). A transformer operating at a power supply frequency of 50-60 Hz would be tens of times more massive for the power required by a standard computer.

Linear power supplies today are used mainly in the case of low-power applications, where the relatively complex electronics required for a switching power supply constitute a more sensitive cost item compared to a transformer. These are, for example, 9 V power supplies, which are used for guitar effects pedals, and once for game consoles, etc. But chargers for smartphones are already entirely pulsed - here the costs are justified. Due to the significantly lower amplitude of voltage ripple at the output, linear power supplies are also used in those areas where this quality is in demand.

⇡ General diagram of an ATX power supply

A desktop computer's power supply is a switching power supply, the input of which is supplied with household voltage with parameters of 110/230 V, 50-60 Hz, and the output has a number of DC lines, the main ones of which are rated 12, 5 and 3.3 V In addition, the power supply provides a voltage of -12 V, and sometimes also a voltage of -5 V, required for the ISA bus. But the latter was at some point excluded from the ATX standard due to the end of support for the ISA itself.

In the simplified diagram of a standard switching power supply presented above, four main stages can be distinguished. In the same order, we consider the components of power supplies in the reviews, namely:

1. EMI filter - electromagnetic interference (RFI filter);
2. primary circuit - input rectifier (rectifier), key transistors (switcher), creating high-frequency alternating current on the primary winding of the transformer;
3. main transformer;
4. secondary circuit - current rectifiers from the secondary winding of the transformer (rectifiers), smoothing filters at the output (filtering).

⇡ EMI filter

The filter at the power supply input is used to suppress two types of electromagnetic interference: differential (differential-mode) - when the interference current flows in different directions in the power lines, and common-mode (common-mode) - when the current flows in one direction.

Differential noise is suppressed by capacitor CX (the large yellow film capacitor in the photo above) connected in parallel with the load. Sometimes a choke is additionally attached to each wire, which performs the same function (not on the diagram).

The common mode filter is formed by CY capacitors (blue drop-shaped ceramic capacitors in the photo), connecting the power lines to ground at a common point, etc. a common-mode choke (LF1 in the diagram), the current in the two windings of which flows in the same direction, which creates resistance for common-mode interference.

In cheap models, a minimum set of filter parts is installed; in more expensive ones, the described circuits form repeating (in whole or in part) links. In the past, it was not uncommon to see power supplies without any EMI filter at all. Now this is rather a curious exception, although if you buy a very cheap power supply, you can still run into such a surprise. As a result, not only and not so much the computer itself will suffer, but other equipment connected to the household network - switching power supplies are a powerful source of interference.

In the filter area of ​​a good power supply, you can find several parts that protect the device itself or its owner from damage. There is almost always a simple fuse for short circuit protection (F1 in the diagram). Note that when the fuse trips, the protected object is no longer the power supply. If a short circuit occurs, it means that the key transistors have already broken through, and it is important to at least prevent the electrical wiring from catching fire. If a fuse in the power supply suddenly burns out, then replacing it with a new one is most likely pointless.

Separate protection is provided against short-term surges using a varistor (MOV - Metal Oxide Varistor). But there are no means of protection against prolonged voltage increases in computer power supplies. This function is performed by external stabilizers with their own transformer inside.

The capacitor in the PFC circuit after the rectifier can retain a significant charge after being disconnected from power. To prevent a careless person who sticks his finger into the power connector from receiving an electric shock, a high-value discharge resistor (bleeder resistor) is installed between the wires. In a more sophisticated version - together with a control circuit that prevents charge from leaking when the device is operating.

By the way, the presence of a filter in the PC power supply (and the power supply of a monitor and almost any computer equipment also has one) means that buying a separate “surge filter” instead of a regular extension cord is, in general, pointless. Everything is the same inside him. The only condition in any case is normal three-pin wiring with grounding. Otherwise, the CY capacitors connected to ground simply will not be able to perform their function.

⇡ Input rectifier

After the filter, the alternating current is converted into direct current using a diode bridge - usually in the form of an assembly in a common housing. A separate radiator for cooling the bridge is highly welcome. A bridge assembled from four discrete diodes is an attribute of cheap power supplies. You can also ask what current the bridge is designed for to determine whether it matches the power of the power supply itself. Although, as a rule, there is a good margin for this parameter.

⇡ Active PFC block

In an AC circuit with a linear load (such as an incandescent light bulb or an electric stove), the current flow follows the same sine wave as the voltage. But this is not the case with devices that have an input rectifier, such as switching power supplies. The power supply passes current in short pulses, approximately coinciding in time with the peaks of the voltage sine wave (that is, the maximum instantaneous voltage) when the smoothing capacitor of the rectifier is recharged.

The distorted current signal is decomposed into several harmonic oscillations in the sum of a sinusoid of a given amplitude (the ideal signal that would occur with a linear load).

The power used to perform useful work (which, in fact, is heating the PC components) is indicated in the characteristics of the power supply and is called active. The remaining power generated by harmonic oscillations of the current is called reactive. It does not produce useful work, but heats the wires and creates a load on transformers and other power equipment.

The vector sum of reactive and active power is called apparent power. And the ratio of active power to total power is called power factor - not to be confused with efficiency!

A switching power supply initially has a fairly low power factor - about 0.7. For a private consumer, reactive power is not a problem (fortunately, it is not taken into account by electricity meters), unless he uses a UPS. The uninterruptible power supply carries the full power of the load. At the scale of an office or city network, excess reactive power created by switching power supplies already significantly reduces the quality of power supply and causes costs, so it is being actively combated.

In particular, the vast majority of computer power supplies are equipped with active power factor correction (Active PFC) circuits. A unit with an active PFC is easily identified by a single large capacitor and inductor installed after the rectifier. In essence, Active PFC is another pulse converter that maintains a constant charge on the capacitor with a voltage of about 400 V. In this case, current from the supply network is consumed in short pulses, the width of which is selected so that the signal is approximated by a sine wave - which is required to simulate a linear load . To synchronize the current consumption signal with the voltage sinusoid, the PFC controller has special logic.

The active PFC circuit contains one or two key transistors and a powerful diode, which are placed on the same heatsink with the key transistors of the main power supply converter. As a rule, the PWM controller of the main converter key and the Active PFC key are one chip (PWM/PFC Combo).

The power factor of switching power supplies with active PFC reaches 0.95 and higher. In addition, they have one additional advantage - they do not require a 110/230 V mains switch and a corresponding voltage doubler inside the power supply. Most PFC circuits handle voltages from 85 to 265 V. In addition, the sensitivity of the power supply to short-term voltage dips is reduced.

By the way, in addition to active PFC correction, there is also a passive one, which involves installing a high-inductance inductor in series with the load. Its efficiency is low, and you are unlikely to find this in a modern power supply.

⇡ Main converter

The general principle of operation for all pulse power supplies of an isolated topology (with a transformer) is the same: a key transistor (or transistors) creates alternating current on the primary winding of the transformer, and the PWM controller controls the duty cycle of their switching. Specific circuits, however, differ both in the number of key transistors and other elements, and in qualitative characteristics: efficiency, signal shape, noise, etc. But here too much depends on the specific implementation for this to be worth focusing on. For those interested, we provide a set of diagrams and a table that will allow you to identify them in specific devices based on the composition of the parts.

	Transistors	Diodes	Capacitors	Transformer primary legs
Single-Transistor Forward	1	1	1	4
	2	2	0	2
	2	0	2	2
	4	0	0	2
	2	0	0	3

In addition to the listed topologies, in expensive power supplies there are resonant versions of Half Bridge, which are easily identified by an additional large inductor (or two) and a capacitor forming an oscillatory circuit.

Single-Transistor Forward

⇡ Secondary circuit

The secondary circuit is everything that comes after the secondary winding of the transformer. In most modern power supplies, the transformer has two windings: 12 V is removed from one of them, and 5 V from the other. The current is first rectified using an assembly of two Schottky diodes - one or more per bus (on the highest loaded bus - 12 V - in powerful power supplies there are four assemblies). More efficient in terms of efficiency are synchronous rectifiers, which use field-effect transistors instead of diodes. But this is the prerogative of truly advanced and expensive power supplies that claim the 80 PLUS Platinum certificate.

The 3.3V rail is typically driven from the same winding as the 5V rail, only the voltage is stepped down using a saturable inductor (Mag Amp). A special winding on a transformer for a voltage of 3.3 V is an exotic option. Of the negative voltages in the current ATX standard, only -12 V remains, which is removed from the secondary winding under the 12 V bus through separate low-current diodes.

PWM control of the converter key changes the voltage on the primary winding of the transformer, and therefore on all secondary windings at once. At the same time, the computer's current consumption is by no means evenly distributed between the power supply buses. In modern hardware, the most loaded bus is 12-V.

To separately stabilize voltages on different buses, additional measures are required. The classic method involves using a group stabilization choke. Three main buses are passed through its windings, and as a result, if the current increases on one bus, the voltage drops on the others. Let's say the current on the 12 V bus has increased, and in order to prevent a voltage drop, the PWM controller has reduced the duty cycle of the key transistors. As a result, the voltage on the 5 V bus could go beyond the permissible limits, but was suppressed by the group stabilization choke.

The voltage on the 3.3 V bus is additionally regulated by another saturable inductor.

A more advanced version provides separate stabilization of the 5 and 12 V buses due to saturable chokes, but now this design has given way to DC-DC converters in expensive high-quality power supplies. In the latter case, the transformer has a single secondary winding with a voltage of 12 V, and the voltages of 5 V and 3.3 V are obtained thanks to DC-DC converters. This method is most favorable for voltage stability.

Output filter

The final stage on each bus is a filter that smoothes out voltage ripple caused by the key transistors. In addition, the pulsations of the input rectifier, whose frequency is equal to twice the frequency of the supply network, penetrate to one degree or another into the secondary circuit of the power supply.

The ripple filter includes a choke and large capacitors. High-quality power supplies are characterized by a capacitance of at least 2,000 uF, but manufacturers of cheap models have reserves for savings when they install capacitors, for example, of half the nominal value, which inevitably affects the ripple amplitude.

⇡ Standby power supply +5VSB

A description of the components of the power supply would be incomplete without mentioning the 5 V standby voltage source, which makes the PC sleep mode possible and ensures the operation of all devices that must be turned on at all times. The “duty room” is powered by a separate pulse converter with a low-power transformer. In some power supplies, there is also a third transformer, which is used in the feedback circuit to isolate the PWM controller from the primary circuit of the main converter. In other cases, this function is performed by optocouplers (an LED and a phototransistor in one package).

⇡ Methodology for testing power supplies

One of the main parameters of the power supply is voltage stability, which is reflected in the so-called. cross-load characteristic. KNH is a diagram in which the current or power on the 12 V bus is plotted on one axis, and the total current or power on the 3.3 and 5 V buses is plotted on the other. At the intersection points for different values ​​of both variables, the voltage deviation from the nominal value is determined one tire or another. Accordingly, we publish two different KNHs - for the 12 V bus and for the 5/3.3 V bus.

The color of the dot indicates the percentage of deviation:

• green: ≤ 1%;
• light green: ≤ 2%;
• yellow: ≤ 3%;
• orange: ≤ 4%;
• red: ≤ 5%.
• white: > 5% (not allowed by ATX standard).

To obtain KNH, a custom-made power supply test bench is used, which creates a load by dissipating heat on powerful field-effect transistors.

Another equally important test is determining the ripple amplitude at the power supply output. The ATX standard allows ripple within 120 mV for a 12 V bus and 50 mV for a 5 V bus. A distinction is made between high-frequency ripple (at double the frequency of the main converter switch) and low-frequency (at double the frequency of the supply network).

We measure this parameter using a Hantek DSO-6022BE USB oscilloscope at the maximum load on the power supply specified by the specifications. In the oscillogram below, the green graph corresponds to the 12 V bus, the yellow graph corresponds to 5 V. It can be seen that the ripples are within normal limits, and even with a margin.

For comparison, we present a picture of ripples at the output of the power supply of an old computer. This block wasn't great to begin with, but it certainly hasn't improved over time. Judging by the magnitude of the low-frequency ripple (note that the voltage sweep division is increased to 50 mV to fit the oscillations on the screen), the smoothing capacitor at the input has already become unusable. High-frequency ripple on the 5 V bus is on the verge of permissible 50 mV.

The following test determines the efficiency of the unit at a load from 10 to 100% of rated power (by comparing the output power with the input power measured using a household wattmeter). For comparison, the graph shows the criteria for the various 80 PLUS categories. However, this does not cause much interest these days. The graph shows the results of the top-end Corsair PSU in comparison with the very cheap Antec, and the difference is not that great.

A more pressing issue for the user is the noise from the built-in fan. It is impossible to directly measure it close to the roaring power supply testing stand, so we measure the rotation speed of the impeller with a laser tachometer - also at power from 10 to 100%. The graph below shows that when the load on this power supply is low, the 135mm fan remains at low speed and is hardly audible at all. At maximum load the noise can already be discerned, but the level is still quite acceptable.

Computer power supply (PSU) is an independent pulse electronic device designed to convert AC voltage into a series of DC voltages (+3.3 / +5 / +12 and -12) to power the motherboard, video card, hard drive and other computer units.

Before you start repairing the computer power supply, you need to make sure that it is faulty, since the inability to start the computer may be due to other reasons.

Photo of the appearance of a classic ATX power supply for a stationary computer (desktop).

Where is the power supply located in the system unit and how to disassemble it

To gain access to the computer's power supply, you must first remove the left side wall from the system unit by unscrewing two screws on the rear wall on the side where the connectors are located.

To remove the power supply from the system unit case, you need to unscrew the four screws marked in the photo. To conduct an external inspection of the power supply, it is enough to disconnect from the computer units only those wires that interfere with the installation of the power supply on the edge of the system unit case.

Having placed the power supply on the corner of the system unit, you need to unscrew the four screws located on top, in the pink photo. Often one or two screws are hidden under a sticker, and to find the screw you need to peel it off or pierce it with the tip of a screwdriver. There are also stickers on the sides that make it difficult to remove the cover; they need to be cut along the line of mating parts of the power supply housing.

After the cover from the power supply unit is removed, be sure to remove all dust with a vacuum cleaner. It is one of the main reasons for the failure of radio components, since by covering them with a thick layer, it reduces the heat transfer from the parts, they overheat and, working in difficult conditions, fail faster.

For reliable operation of the computer, it is necessary to remove dust from the system unit and power supply, and also check the operation of coolers at least once a year.

Block diagram of the power supply unit of an ATX computer

A computer power supply is a rather complex electronic device and repairing it requires deep knowledge of radio engineering and the availability of expensive equipment, but, nevertheless, 80% of failures can be eliminated independently, having the skills of soldering, working with a screwdriver and knowing the block diagram of the power source.

Almost all computer power supplies are made according to the block diagram below. I have shown the electronic components in the diagram only those that most often fail and are available for non-professionals to replace on their own. When repairing an ATX power supply, you will definitely need color coding of the wires coming out of it.

The supply voltage is supplied via a power cord through a plug-in connection to the power supply board. The first element of protection is fuse Pr1, usually rated at 5 A. But depending on the power of the source, it may have a different rating. Capacitors C1-C4 and inductor L1 form a filter that serves to suppress common-mode and differential noise that arises from the operation of the power supply itself and can come from the network.

Surge filters assembled according to this scheme are required to be installed in all products in which the power supply is made without a power transformer, in televisions, VCRs, printers, scanners, etc. Maximum efficiency of the filter is possible only when connected to a network with a ground wire. Unfortunately, cheap Chinese computer power supplies often do not have filter elements.

Here is an example of this: the capacitors are not installed, and instead of the inductor, jumpers are soldered. If you are repairing a power supply and find that filter elements are missing, it is advisable to install them.

Here is a photo of a high-quality computer power supply, as you can see, filter capacitors and a noise suppression choke are installed on the board.

To protect the power supply circuit from supply voltage surges, expensive models install varistors (Z1-Z3), pictured on the right side in blue. Their operating principle is simple. At normal network voltage, the resistance of the varistor is very high and does not affect the operation of the circuit. If the voltage in the network increases above the permissible level, the resistance of the varistor sharply decreases, which leads to the fuse blowing, and not to the failure of expensive electronics.

To repair a failed unit due to overvoltage, it will be enough to simply replace the varistor and fuse. If you don’t have a varistor at hand, then you can only get by by replacing the fuse; the computer will work normally. But at the first opportunity, in order not to take risks, you need to install a varistor in the board.

Some models of power supplies provide the ability to switch to operate at a supply voltage of 115 V; in this case, the contacts of switch SW1 must be closed.

For a smooth charge of electrolytic capacitors C5-C6, connected immediately after the rectifier bridge VD1-VD4, an RT thermistor with a negative TCR is sometimes installed. In a cold state, the resistance of the thermistor is a few ohms; when current passes through it, the thermistor heats up and its resistance decreases by 20-50 times.

To be able to turn on the computer remotely, the power supply has an independent, additional low-power power source that is always on, even if the computer is turned off, but the electrical plug is not removed from the socket. It generates a voltage of +5 B_SB and is built according to the circuit of a transformer self-oscillating blocking oscillator on a single transistor, powered from a rectified voltage by diodes VD1-VD4. This is one of the most unreliable components of the power supply and is difficult to repair.

The voltages required for the operation of the motherboard and other devices of the system unit, when leaving the voltage generation unit, are filtered from interference by chokes and electrolytic capacitors and then supplied to consumption sources through wires with connectors. The cooler, which cools the power supply itself, is powered, in older power supply models from a voltage of minus 12 V, in modern ones from a voltage of +12 V.

ATX computer power supply repair

Attention! To avoid damaging the computer, undocking and connecting the connectors of the power supply and other components inside the system unit must be performed only after completely disconnecting the computer from the power supply (unplug the plug from the socket or turn off the switch in the “Pilot”).

The first thing that needs to be done is to check the presence of voltage in the outlet and the serviceability of the “Pilot” type extension cord by the glow of its switch key. Next, you need to check that the computer’s power cord is securely inserted into the “Pilot” and the system unit and that the switch (if any) on the back wall of the system unit is turned on.

How to find a power supply fault by pressing the “Start” button

If power is supplied to the computer, then in the next step you need to look at the power supply cooler (visible behind the grille on the back wall of the system unit) and press the “Start” button of the computer. If the cooler blades move even a little, it means that the filter, fuse, diode bridge and capacitors on the left side of the block diagram are working, as well as the independent low-power power supply +5 B_SB.

In some PSU models, the cooler is on the flat side and to see it, you need to remove the left side wall of the system unit.

Turning at a small angle and stopping the cooler impeller when you press the “Start” button indicates that output voltages momentarily appear at the output of the power supply unit, after which the protection is triggered, stopping the operation of the power supply unit. The protection is configured in such a way that if the current value for one of the output voltages exceeds a specified threshold, then all voltages are turned off.

The cause of an overload is usually a short circuit in the low-voltage circuits of the power supply itself or in one of the computer units. A short circuit usually occurs when there is a breakdown in semiconductor devices or insulation in capacitors.

To determine the node in which a short circuit has occurred, you need to disconnect all power supply connectors from the computer units, leaving only those connected to the motherboard. Then connect the computer to the power supply and press the “Start” button. If the cooler in the power supply was spinning, it means that one of the disconnected nodes is faulty. To determine the faulty node, you need to connect them in series to the power supply.

If the power supply connected only to the motherboard does not work, you should continue troubleshooting and determine which of these devices is faulty.

Checking the computer's power supply
measuring the resistance value of output circuits

When repairing a power supply, some types of its malfunction can be determined by measuring with an ohmmeter the resistance value between the common black GND wire and the remaining contacts of the output connectors.

Before starting measurements, the power supply must be disconnected from the power supply, and all its connectors must be disconnected from the system unit components. The multimeter or tester must be turned on in resistance measurement mode and select a limit of 200 Ohms. Connect the common wire of the device to the connector contact to which the black wire goes. The end of the second probe touches the contacts in turn, in accordance with the table.

The table shows generalized data obtained as a result of measuring the resistance value of the output circuits of 20 serviceable power supply units of computers of different capacities, manufacturers and years of manufacture.

To be able to connect a power supply for testing without load, load resistors are installed inside the unit at some outputs, the value of which depends on the power of the power supply and the manufacturer’s decision. Therefore, the measured resistance can fluctuate over a wide range, but should not be below the permissible value.

If a load resistor is not installed in the circuit, then the ohmmeter readings will vary from a small value to infinity. This is due to the charging of the filter electrolytic capacitor from the ohmmeter and indicates that the capacitor is working. If you swap the probes, a similar picture will be observed. If the resistance is high and does not change, then the capacitor may be broken.

A resistance less than the permissible value indicates the presence of a short circuit, which may be caused by an insulation breakdown in an electrolytic capacitor or a rectifying diode. To determine the faulty part, you will have to open the power supply and unsolder one end of the filter choke of this circuit from the circuit. Next, check the resistance before and after the throttle. If after it, then there is a short circuit in the capacitor, wires, between the tracks of the printed circuit board, and if before it, then the rectifier diode is broken.

Troubleshooting the power supply by external inspection

Initially, you should carefully inspect all the parts, paying special attention to the integrity of the geometry of the electrolytic capacitors. As a rule, due to severe temperature conditions, electrolytic capacitors fail most often. About 50% of power supply failures are due to faulty capacitors. Often, swelling of capacitors is a consequence of poor performance of the cooler. The cooler bearings run out of lubrication and the speed drops. The cooling efficiency of the power supply parts decreases and they overheat. Therefore, at the first sign of a malfunction of the power supply cooler, additional acoustic noise usually appears; you need to clean the cooler from dust and lubricate it.

If the capacitor body is swollen or traces of leaked electrolyte are visible, then the failure of the capacitor is obvious and it should be replaced with a serviceable one. The capacitor swells in the event of an insulation breakdown. But it happens that there are no external signs of failure, but the level of output voltage ripple is greater. In such cases, the capacitor is faulty due to lack of contact between its terminal and the plate inside it, as they say, the capacitor is broken. You can check the capacitor for open circuit using any tester in resistance measurement mode. The technology for testing capacitors is presented in the website article “Measuring Resistance”.

Next, the remaining elements, fuse, resistors and semiconductor devices are inspected. Inside the fuse, a thin metal wire should run along the center, sometimes with a thickening in the middle. If the wire is not visible, then most likely it has burned out. To accurately check the fuse, you need to test it with an ohmmeter. If the fuse is blown, it must be replaced with a new one or repaired. Before making a replacement, to check the power supply, you can not solder the blown fuse from the board, but solder a copper wire with a diameter of 0.18 mm to its terminals. If the wiring does not burn out when you turn on the power supply to the network, then it makes sense to replace the fuse with a working one.

How to check the serviceability of the power supply by closing the PG and GND contacts

If the motherboard can only be checked by connecting it to a known-good power supply, then the power supply can be checked separately using a load block or started by connecting the +5 V PG and GND contacts to each other.

From the power supply to the motherboard, supply voltages are supplied using a 20 or 24 pin connector and a 4 or 6 pin connector. For reliability, the connectors have latches. In order to remove the connectors from the motherboard, you need to press the latch upward with your finger at the same time, applying quite a lot of force, rocking from side to side, and pull out the mating part.

Next, you need to short-circuit the two terminals in the connector removed from the motherboard with each other, using a piece of wire or perhaps a metal paper clip. The wires are located on the latch side. In the photographs, the location of the jumper is indicated in yellow.

If the connector has 20 contacts 14 (green wire, in some power supplies it may be gray, POWER ON) and output 15 (black wire, GND).

If the connector has 24 contacts, then you need to connect the output 16 (green green, in some power supplies the wire may be gray, POWER ON) and output 17 (black GND wire).

If the impeller in the power supply cooler rotates, then the ATX power supply can be considered operational, and, therefore, the reason for the computer not working is in other units. But such a check does not guarantee stable operation of the computer as a whole, since deviations in output voltages may be greater than permissible.

Checking the computer's power supply
measuring voltages and ripple levels

After repairing the power supply or in case of unstable operation of the computer, in order to be completely sure that the power supply is in good working order, it is necessary to connect it to the load block and measure the level of output voltages and the ripple range. The deviation of voltage values ​​and ripple ranges at the output of the power supply should not exceed the values ​​​​given in the table.

You can do without a load block by measuring the voltage and ripple level directly at the terminals of the power supply connectors in a running computer.

Table of output voltages and ripple range of ATX power supply
Output voltage, V	+3,3	+5,0	+12,0	-12,0	+5.0 SB	+5.0 PG	GND
Wire color	orange	red	yellow	blue	violet	grey	black
Permissible deviation, %	±5	±5	±5	±10	±5	–	–
Permissible minimum voltage	+3,14	+4,75	+11,40	-10,80	+4,75	+3,00	–
Permissible maximum voltage	+3,46	+5,25	+12,60	-13,20	+5,25	+6,00	–
Ripple range no more than, mV	50	50	120	120	120	120	–

When measuring voltages with a multimeter, the “negative” end of the probe is connected to the black wire (common), and the “positive” end to the desired connector contacts.

Voltage +5 V SB (Stand-by), purple wire – produces an independent low-power power supply built into the power supply unit, made on one field-effect transistor and transformer. This voltage ensures the computer operates in standby mode and serves only to start the power supply. When the computer is running, the presence or absence of +5 V SB voltage does not matter. Thanks to +5 V SB, the computer can be started by pressing the “Start” button on the system unit or remotely, for example, from an uninterruptible power supply unit in the event of a prolonged absence of 220 V supply voltage.

Voltage +5 V PG (Power Good) - appears on the gray wire of the power supply unit after 0.1-0.5 seconds if it is in good condition after self-testing and serves as an enabling signal for the operation of the motherboard.

A voltage of minus 12 V (blue wire) is only needed to power the RS-232 interface, which is absent in modern computers. Therefore, power supplies of the latest models may not have this voltage.

How to replace a fuse in a computer's power supply

Typically, computer power supplies are equipped with a tubular glass fuse designed for a protection current of 6.3 A. For reliability and compactness, the fuse is soldered directly into the printed circuit board. For this purpose, special fuses are used that have terminals for sealing. The fuse is usually installed in a horizontal position next to the surge protector and is easy to spot by its appearance.

But sometimes there are power supplies in which the fuse is installed in a vertical position and a heat-shrinkable tube is put on it, as in the photo above. As a result, it is difficult to detect. But the inscription on the printed circuit board next to the fuse helps: F1 - this is how the fuse is designated on electrical circuits. Next to the fuse, the current for which it is rated may also be indicated; on the presented board, a current of 6.3 A is indicated.

When repairing the power supply and checking the vertically installed fuse using a multimeter, it was discovered that it was broken. After desoldering the fuse and removing the heat shrink tubing, it became obvious that it had blown. The inside of the glass tube was completely covered with a black coating from the burnt wire.

Fuses with wire leads are rare, but they can be successfully replaced with ordinary 6.3 ampere fuses by soldering single-core pieces of copper wire with a diameter of 0.5-0.7 mm to the ends of the cups.

All that remains is to solder the prepared fuse into the printed circuit board of the power supply and check its functionality.

If, when the power supply is turned on, the fuse burns out again, it means that there is a failure of other radio elements, usually a breakdown of the transitions in the key transistors. Repairing a power supply with such a fault requires high qualifications and is not economically feasible. Replacing a fuse designed for a higher protection current than 6.3 A will not lead to a positive result. The fuse will still blow.

Searching for faulty electrolytic capacitors in the power supply

Very often, a power supply failure, and as a result, unstable operation of the computer as a whole, occurs due to swelling of the electrolytic capacitor housings. To protect against explosion, notches are made at the end of electrolytic capacitors. As the pressure inside the capacitor increases, the housing swells or ruptures at the notch, and by this sign it is easy to find a failed capacitor. The main reason for the failure of capacitors is their overheating due to a malfunction of the cooler or exceeding the permissible voltage.

The photo shows that the capacitor on the left side has a flat end, while the end on the right is swollen, with traces of leaking electrolyte. This capacitor has failed and must be replaced. In the power supply, electrolytic capacitors on the +5 V power bus usually fail, since they are installed with a small voltage margin, only 6.3 V. I have encountered cases when all the capacitors in the power supply on the +5 V circuit were swollen.

When replacing capacitors on a 5 V power supply circuit, I recommend installing capacitors that are designed for a voltage of at least 10 V. The higher the voltage the capacitor is designed for, the better, the main thing is that the dimensions fit into the installation location. If a capacitor with a higher voltage does not fit due to its size, you can install a capacitor with a smaller capacity, but designed for a higher voltage. All the same, the capacitance of the capacitors installed at the factory has a larger reserve and such a replacement will not worsen the performance of the power supply and the computer as a whole.

There is no point in replacing electrolytic capacitors in the power supply if they are all swollen. This means that the output voltage stabilization circuit has failed, and a voltage exceeding the permissible value was applied to the capacitors. Such a power supply can be repaired only with professional education and measuring instruments, but such repairs are not economically feasible.

The main thing when repairing a power supply is not to forget that electrolytic capacitors have polarity. On the negative terminal side of the capacitor body there is a marking in the form of a wide light vertical stripe, as shown in the photo above. On the printed circuit board, the hole for the negative terminal of the capacitor is located in the marking area of ​​the white (black) semicircle, or the hole for the positive terminal is indicated by a “+” sign.

Checking the group stabilization choke BP ATX

If you suddenly smell something burning from the computer system unit, then one of the reasons may be overheating of the group stabilization choke in the power supply unit or a burnt winding of one of the coolers. The computer usually continues to work normally. If, after opening the system unit and inspecting it, all coolers rotate, then the throttle is faulty. The computer must be turned off immediately and repaired.

The photo shows a computer power supply with the cover removed, in the center of which you can see the inductor, covered with green insulation, burnt on top. When I connected this power supply to the load and applied supply voltage to it, after a couple of minutes a thin stream of smoke came out of the inductor. The check showed that all output voltages within the tolerance and the ripple range do not exceed the permissible value.

The current of all voltages supplying the computer passes through the inductor and it is obvious that there has been a violation of the insulation of the wires of the windings as a result of which they short-circuited among themselves.

The windings can be rewinded onto the same core, but as a result of strong heating, the magnetodielectric of the core may lose its quality factor; as a result, due to high Foucault currents, it will heat up even with intact windings. Therefore, I recommend installing a new throttle. If there is no analogue, then you need to count the turns of the windings, winding them on the burnt inductor, and wind them with an insulated wire of the same cross-section on a new core. In this case, the direction of the windings must be observed.

Checking other power supply elements

Resistors and simple capacitors should not have any darkening or deposits. The cases of semiconductor devices must be intact, without chips or cracks. When making repairs yourself, it is advisable to replace only the elements shown in the block diagram. If the paint on the resistor has darkened, or the transistor has fallen apart, then there is no point in changing them, since most likely this is a consequence of the failure of other elements that cannot be detected without instruments. A darkened resistor body does not always indicate a malfunction. It is quite possible that only the paint has darkened, but the resistance of the resistor is normal.