How to check the voltage on a transistor. A device for testing high-power IGBT and MOSFET transistors (n-channel). Checking the transistor with a dial tester

Field-effect transistors are semiconductor devices in which the control of transient processes, as well as the magnitude of the output current, is carried out by changing the magnitude of the electric field. There are two types of these devices: with (in turn divided into transistors with a built-in channel and with an induction channel) and with a controlled transition. Due to their unique characteristics, field-effect transistors are widely used in electronic equipment: power supplies, televisions, computers, etc.

When repairing such equipment, probably every novice radio amateur is faced with the following question: how to test a field-effect transistor? Most often, checking such elements can be encountered when repairing switching power supplies. In this article we will tell you in detail how to do this correctly.

Howcheck the field effect transistor with an ohmmeter

First of all, in order to start checking the field-effect transistor, you need to understand its “pinout”, that is, the location of the pins. Today, there are many different designs of such elements; accordingly, the location of the electrodes is different. You can often find semiconductor transistors with labeled contacts. For marking, use the Latin letters G, D, S. If there is no signature, then you need to use reference literature.

So, having dealt with the markings of the contacts, let's look at how to check the field-effect transistor. The next step is to take the necessary safety measures, because field devices are very sensitive to static voltage, and to prevent failure of such an element, it is necessary to organize grounding. To remove accumulated static charge, you usually wear an antistatic grounding strap on your wrist.

We should also not forget that field-effect transistors must be stored with their terminals closed. Having removed the static voltage, you can proceed to the verification procedure. To do this you will need a simple ohmmeter. For a working element, the resistance between all terminals should tend to infinity, but there are some exceptions. Now we will look at how to test an n-type field-effect transistor.

We apply the positive probe of the device to the gate electrode (G), and the negative probe to the source contact (S). At this moment, the shutter capacitance begins to charge and the element opens. When measuring the resistance between source and drain (D), the ohmmeter will show some resistance value. This value is different in different types of transistors. If you short-circuit the terminals of the transistor, the resistance between the drain and source will again tend to infinity. If this does not happen, then the transistor is faulty.

If you ask how to test a P-type field effect transistor, then the answer is simple: we repeat the above procedure, only we change the polarity. We should also not forget that modern high-power field-effect transistors have a built-in diode between the source and drain; accordingly, it “rings” only in one direction.

Checking a field-effect transistor with a multimeter

If you have a multimeter, you can check the field-effect transistor. To do this, set the diodes to “testing” mode and enter the field element into saturation mode. If the transistor is N-type, then we touch the drain with the negative probe, and the gate with the positive probe. In this case, a working transistor opens. We transfer the positive probe, without removing the negative one, to the source, and the multimeter shows some resistance value. After this, we lock the transistor: without lifting the probe from the source, touch the gate with the negative one and return it to the drain. The transistor is locked, and the resistance tends to infinity.

Many radio amateurs ask: “How to test a field-effect transistor without desoldering?” Let us answer right away that there is no 100% method. To do this, use a multimeter with an HFE socket, but this method often fails and can waste a lot of time.

Radio amateurs know that they often have to spend a lot of time searching for faults that arise in electronic circuits for various reasons. If the circuit is assembled independently, then the final stage of the work will be to check its functionality. And you need to start with selecting known-good electronic components. Semiconductor devices are widely used in amateur radio designs. Checking a transistor, how to ring a transistor with a multimeter - these are important questions.

Types of transistors

As electronics develops, more and more varieties of this type of semiconductor devices appear. The emergence of each new group is due to increasing requirements for the operation of electronic devices and their technical characteristics.

Bipolar devices

Bipolar semiconductor transistors are the most commonly found elements of electronic circuits. Even if we consider the construction of various large microcircuits, we can see a huge number of representatives of semiconductors of this type.

The definition of “bipolar” comes from the types of electric current carriers that are present in them. This current is determined by the movement of negative and positive charges in the body of the semiconductor.

Each area of the three-layer structure has its own metal terminal, with the help of which the device is connected to other elements of the electronic circuit. These pins have their own names: emitter, base, collector. The emitter and collector are the outer regions. The inner area is the base.

Bipolar transistors form two groups depending on the type of semiconductor. They are designated “p - n - p” and “n - p - n”. The areas of contact between semiconductors of different types are called “p - n” junctions.

The base area is the thinnest. Its thickness determines the frequency properties of the device, that is, the maximum frequency of the radio signal at which the transistor can operate as an amplifying element. The collector area has a maximum area, since at high currents it is necessary to remove excess thermal energy using an external radiator to prevent overheating of the device.

In the diagrams, the emitter pin is indicated by an arrow., which determines the direction of the main current through the device. The main current is in the collector - emitter section (or emitter - collector, depending on the direction of the arrow). But it occurs only if control current flows in the base circuit. The ratio of these currents determines the amplifying properties of the transistor. Thus, a bipolar transistor is a current device.

Field effect transistors

Transistors of this type differ significantly from bipolar devices. If the latter are devices controlled by a weak base current of a certain polarity, then field devices require the presence of a control voltage (electric field) for current to flow through the semiconductor.

The electrodes have names: gate, source, drain. And the voltage that opens the “n” type or “p” type channel is applied to the gate area and determines the intensity of the current with the correct polarity. These devices are also called unipolar.

Checking with a multimeter

Transistors are active elements of an electronic circuit. Their serviceability determines its correct operation. How to check a transistor with a tester - this question is important. If you know the principles of its operation, this task is not difficult.

Bipolar type devices

Their circuit can be simplified in the form of two semiconductor diodes connected towards each other. For “p - n - p” conductivity devices, the cathodes will be connected, and for the “n - p - n” structure, the anodes of the diodes will have a common point. In any case, the connection point will be the base electrode terminal, and the other two terminals will be the emitter and collector, respectively.

For the “p - n - p” structure in the diagram, the emitter arrow is directed towards the base terminal. Accordingly, for conductivity “n - p - n” the emitter arrow will change its direction to the opposite. To determine the state of a semiconductor transistor, information about its type and, accordingly, the marking of its electrodes is of great importance. This information can be found from numerous reference books or from communication on thematic forums.

For bipolar “p - n - p” conductivity devices, the open state will correspond to the connection of the “negative” (black) probe of the tester to the base terminal. The “positive” (red) tip is alternately connected to the collector and emitter. This will be a direct connection of “p - n” transitions.

In this case, the resistance of each will be in the range (600−1200) Ohms. The exact value depends on the manufacturer of the electronic components. The resistance of the collector junction will be slightly lower than that of the emitter junction.

Since a bipolar transistor is presented in the form of a back-to-back connection of two semiconductor diodes with one-way conductivity, when changing the polarity of the resistance tester probes, the “p - n” junctions of normally operating transistors will ideally tend to infinity.

The same picture should be observed when measuring the resistance between the emitter and collector terminals. Moreover, this large value does not depend on changing the polarity of the measuring probes. All this applies to working transistors.

The process of checking the serviceability (or malfunction) of a bipolar semiconductor element using a multimeter comes down to the following:

determination of the type of device and its terminal diagram;
checking the resistance of its “p - n” junctions in the forward direction;
changing the polarity of the probes and determining the transition resistances with such a connection;
checking the collector-emitter resistance in both directions.

Determining the health of devices of the “n - p - n” structure differs only in that in order to directly switch on the transitions, it is necessary to connect the red “positive” wire of the multimeter to the base terminal, and alternately connect the black (negative) wire to the emitter and collector terminals. The picture with the resistance values for this conductivity should be repeated.

Signs of a faulty bipolar transistor include the following:

"continuity" of "p - n" transitions shows too low resistance values;
The “p - n” transition does not “ring” in both directions.

In the first case, we can talk about an electrical breakdown of the junction, or even a short circuit.

The second case shows an internal break in the structure of the device.

In both cases, this instance cannot be used to work in the circuit.

Field effect transistors

To check the functionality of this element, we use the same multimeter as for the bipolar device. It must be remembered that field workers can be n-channel and p-channel.

To check an element of the first type, you must perform the following steps:

To determine the resistance of a closed device with an n-channel, touch the “source” terminal with the red wire, and the “drain” terminal with the black wire.

The field device is opened by applying a positive potential to its “gate” (red wire).

To check the open state of the transistor, the resistance of the drain-source section is measured again (black wire - drain, red - source). The resistance of the slightly open n-channel decreases slightly compared to the first measurement.

Closing the device is achieved by applying a negative potential to its “gate” (black wire of the multimeter). After this, the resistance of the drain-source section will return to its original value.

When checking a p-channel device, repeat all previous steps, changing the polarity of the tester's measuring probes.

Before testing field devices, it is necessary to take measures to protect against the effects of static charges, which can introduce significant difficulties into the testing process, or even completely damage the product being tested. Such proven measures include simply touching the central heating radiator with your hand. Experts use a bracelet that has antistatic properties.

When testing high-power transistors of this type, it is often possible to determine the presence of resistance when the semiconductor channel is completely closed. This means that a protective diode built into the device body is connected between the “source” and “drain”. Changing the polarity of the tester leads helps to verify this.

Checking devices in the circuit

How to test a transistor with a multimeter without desoldering it, how to test a field-effect transistor - these questions arise among radio amateurs quite often. Removing a semiconductor device from a circuit requires great care and experience. It is necessary to have in your arsenal a low-voltage soldering iron with a thin tip and a bracelet that protects against static discharges. The conductors of the printed circuit board can overheat during operation, or even accidentally short-circuit with each other.

Although, if you have experience in such work, the task is completely solvable. Of course, you need to be able to read electrical diagrams and imagine the operation of each of its components.

Assessing the performance of low- and medium-power bipolar transistors differs little from checking these elements “on the table”, when all the terminals of the device are in a position accessible for testing.

It is more difficult to check directly in the circuit of high-power devices used in the circuits of output stages of amplifiers and switching power supplies. These circuits contain elements that protect transistors from reaching the maximum permissible modes. When checking the states of “p - n” transitions in these cases, you can get absolutely incorrect results. The way out is to solder the base output.

Checking field devices can give results that are far from the real state of affairs. The reason is the presence in the circuits of a large number of elements for correcting the operation of transistors, including low-resistance inductors.

There are still a large number of different types of transistors, to assess the condition of which it is necessary to use various special probes. But this is a topic for a separate article.

Modern electronic multimeters have specialized connectors for testing various radio components, including transistors.

This is convenient, however, the check is not entirely correct. Experienced radio amateurs remember how to check a transistor with a tester with a dial indicator. The testing technique on digital devices has not changed. To accurately determine the condition of a semiconductor device, each element is tested separately.

Classic question: how to test a bipolar transistor with a multimeter

This popular explorer performs two tasks:

Signal amplification mode. Receiving a command to the control pins, the device duplicates the signal shape at the working contacts, only with a larger amplitude;
key mode. Like a water faucet, a semiconductor opens or closes the path of electrical current at the command of a control signal.

Semiconductor chips are connected in a package, forming p-n junctions. The same technology is used in diodes. In essence, a bipolar transistor consists of two diodes connected at one point by terminals of the same name.
To understand how to test a transistor with a multimeter, consider the difference between pnp and npn structures.

The so-called “straight” (see photo)

With reverse transition, as shown in the photo

Of course, if you solder the diodes as shown in the schematic diagram, the transistor will not work. But from the point of view of checking the serviceability, you can imagine that you have ordinary diodes in one housing.

That is, by placing in front of you a diagram of semiconductor junctions, you can easily determine not only the serviceability of the part as a whole, but also localize a specific faulty p-n junction. This will help to understand the cause of the breakdown, because the semiconductor does not work autonomously, but as part of an electrical circuit.

How to test a bipolar transistor with a multimeter - video.

A reasonable question arises: How to determine the marking of the transistor pins without a catalog? This practice is useful not only for checking radio components. When assembling a circuit board, ignorance of the transistor design will lead to its burnout.

Before assembling any circuit or starting to repair an electronic device, you need to make sure that the elements that will be installed in the circuit are in good condition. Even if these elements are new, you need to be sure of their functionality. Such common elements of electronic circuits as transistors are also subject to mandatory testing.

To check all parameters of transistors, there are complex instruments. But in some cases it is enough to carry out a simple test and determine the suitability of the transistor. For such a check, it is enough to have a multimeter.

Various types of transistors are used in technology - bipolar, field-effect, compound, multi-emitter, phototransistors and the like. In this case, we will consider the most common and simple ones - bipolar transistors.

Such a transistor has 2 p-n junctions. It can be thought of as a plate with alternating layers of different types of conductivity. If hole conductivity (p) predominates in the outer regions of a semiconductor device, and electronic conductivity (n) predominates in the middle, then the device is called a pnp transistor. If it’s the other way around, then the device is called an n-p-n transistor. For different types of bipolar transistors, the polarity of the power sources that are connected to it in the circuits changes.

The presence of two transitions in a transistor allows us to present its equivalent circuit in a simplified form as a series connection of two diodes.

In this case, for a p-n-p device, the cathodes of the diodes are connected to each other in the equivalent circuit, and for an n-p-n device, the anodes of the diodes are connected.

In accordance with these equivalent circuits, the bipolar transistor is checked for serviceability with a multimeter.

The procedure for checking the device - follow the instructions

The measurement process consists of the following steps:

checking the operation of the measuring device;
determining the type of transistor;
measurement of direct resistances of emitter and collector junctions;
measurement of the reverse resistance of the emitter and collector junctions;
assessing the health of the transistor.

Before checking the bipolar transistor with a multimeter, you need to make sure that the measuring device is working properly. To do this, you first need to check the battery charge indicator of the multimeter and, if necessary, replace the battery. When checking transistors, the polarity of the connection will be important. It should be taken into account that the multimeter has a negative pole at the “COM” terminal, and a positive pole at the “VΩmA” terminal. To be certain, it is advisable to connect a black probe to the “COM” terminal, and a red one to the “VΩmA” terminal.

In order to connect multimeter probes of the correct polarity to the terminals of the transistor, it is necessary to determine the type of device and the marking of its terminals. To do this, you need to consult a reference book or find a description of the transistor on the Internet.

At the next stage of testing, the multimeter operation switch is set to the resistance measurement position. The measurement limit of “2k” is selected.

Before checking the PNP transistor with a multimeter, you need to connect the negative probe to the base of the device. This will allow you to measure the direct resistance of the p-n-p radio element junctions. The positive probe is connected in turn to the emitter and collector. If the resistance of the transitions is 500-1200 Ohms, then these transitions are working properly.

When checking the reverse resistance of junctions, a positive probe is connected to the base of the transistor, and the negative probe is connected in turn to the emitter and collector.

If these transitions are working properly, then in both cases a large resistance is recorded.

Checking the NPN transistor with a multimeter follows the same method, but the polarity of the connected probes is reversed. Based on the measurement results, the serviceability of the transistor is determined:

if the measured forward and reverse junction resistances are large, this means that there is a break in the device;
if the measured forward and reverse junction resistances are small, this means that there is a breakdown in the device.

In both cases the transistor is faulty.

Gain Estimation

The characteristics of transistors usually have a large spread in value. Sometimes when assembling a circuit it is necessary to use transistors that have a current gain of similar magnitude. A multimeter allows you to select such transistors. To do this, it has an “hFE” switching mode and a special connector for connecting the outputs of 2 types of transistors.

By connecting the leads of a transistor of the appropriate type into the connector, you can see the value of parameter h21 on the screen.

conclusions:

Using a multimeter you can determine the health of bipolar transistors.
To carry out correct measurements of the forward and reverse resistance of transistor junctions, you need to know the type of transistor and the marking of its terminals.
Using a multimeter, you can select transistors with the desired gain.

Video on how to test a transistor with a multimeter

Transistors have to be checked quite often. Even if you have in your hands a obviously new one that has never been soldered, then before installing it in the circuit it is better to check it. There are often cases when transistors purchased on the radio market turned out to be unusable, and not even just one copy, but a whole batch of 50 - 100 pieces. Most often this happens with powerful transistors of domestic production, less often with imported ones.

Sometimes design descriptions provide some requirements for transistors, for example, a recommended transmission ratio. For these purposes, there are various transistor testers, quite complex in design and measuring almost all the parameters that are given in reference books. But more often you have to check transistors according to the “pass or fail” principle. It is these verification methods that will be discussed in this article.

Often in a home laboratory you find used transistors on hand, once obtained from some old circuit boards. In this case, 100% “input control” is necessary: it is much easier to immediately identify an unusable transistor than to later look for it in a non-working design.

Although many authors of modern books and articles strongly discourage the use of parts of unknown origin, quite often this recommendation has to be violated. After all, it’s not always possible to go to a store and buy the part you need. Due to such circumstances, it is necessary to check each transistor, resistor, capacitor or diode. Next, we will mainly talk about checking transistors.

Testing transistors in amateur conditions is usually carried out using an old analog avometer.

Checking transistors with a multimeter

Most modern radio amateurs are familiar with a universal device called a multimeter. With its help, it is possible to measure direct and alternating voltages and currents, as well as the resistance of conductors to direct current. One of the resistance measurement limits is intended for “continuity testing” of semiconductors. As a rule, a symbol of a diode and a sounding speaker is drawn near the switch in this position.

Before checking transistors or diodes, you should make sure that the device itself is working properly. First of all, look at the battery charge indicator; if necessary, replace the battery immediately. When you turn on the multimeter in the “dialing” mode of semiconductors, a unit should appear in the most significant digit on the indicator screen.

Then check the serviceability by connecting them together: zeros will appear on the indicator and a beep will sound. This is not a vain warning, since wire breaks in Chinese probes are quite common, and this should not be forgotten.

For radio amateurs and professional electronics engineers of the older generation, this gesture (checking the probes) is performed mechanically, because when using a dial tester, each time you switched to the resistance measurement mode, you had to set the arrow to the zero scale division.

After these checks have been completed, you can begin checking semiconductors - diodes and transistors. Pay attention to the polarity of the voltage on the probes. The negative pole is on the socket labeled “COM” (common), and the socket labeled VΩmA is positive. In order not to forget about this during the measurement process, a red probe should be inserted into this socket.

Figure 1. Multimeter

This remark is not as idle as it might seem at first glance. The fact is that with dial avometers (AmperVoltOhmmeter) in resistance measurement mode, the positive pole of the measuring voltage is located on the socket marked “minus” or “common”, well, exactly the opposite, compared to a digital multimeter. Although digital multimeters are now more commonly used, pointer testers are still used and in some cases allow one to obtain more reliable results. This will be discussed below.

Figure 2. Pointer avometer

What does the multimeter show in the “dialing” mode?

Diode check

The simplest semiconductor element is one that contains only one P-N junction. The main property of a diode is one-way conductivity. Therefore, if the positive pole of the multimeter (red probe) is connected to the anode of the diode, then numbers will appear on the indicator showing the forward voltage at the P-N junction in millivolts.

Figure 3.

For silicon diodes this will be about 650 - 800 mV, and for germanium diodes it will be about 180 - 300, as shown in Figures 4 and 5. Thus, from the readings of the device, you can determine the semiconductor material from which the diode is made. It should be noted that these figures depend not only on the specific diode or transistor, but also on the temperature, with an increase of 1 degree, the forward voltage drops by approximately 2 millivolts. This parameter is called the voltage temperature coefficient.

Figure 4.

Figure 5.

If, after this check, the multimeter probes are connected in reverse polarity, then the device indicator will show a unit in the most significant digit. Such results will occur if the diode turns out to be serviceable. This is actually the whole technique for testing semiconductors: in the forward direction the resistance is negligible, but in the reverse direction it is almost infinite.

If the diode is “broken” (the anode and cathode are short-circuited), then most likely an audible signal will be heard, and in both directions. If the diode is “broken,” no matter how you change the polarity of connecting the probes, the indicator will still show one.

Transistor testing

Unlike diodes, transistors have two P-N junctions, and have P-N-P and N-P-N structures, the latter being much more common. In terms of testing with a multimeter, a transistor can be considered as two diodes connected back-to-back in series, as shown in Figure 6. Therefore, testing transistors comes down to “testing” the base-collector and base-emitter transitions in the forward and reverse directions.

Consequently, everything that was said just above about checking the diode is completely true for studying transistor transitions. Even the multimeter readings will be the same as for the diode.

Figure 6.

Figure 7 shows the polarity of turning on the device in the forward direction to “test” the base-emitter transition of transistors of the N-P-N structure: the positive probe of the multimeter is connected to the base terminal. To measure the base-collector transition, the negative terminal of the device should be connected to the collector terminal. In this case, the number on the display was obtained by testing the base-emitter transition of the KT3102A transistor.

Figure 7.

If the transistor turns out to be of a P-N-P structure, then the negative (black) probe of the device should be connected to the base of the transistor.

At the same time, you should “ring” the collector-emitter section. A working transistor has almost infinite resistance, which is symbolized by one in the most significant digit of the indicator.

Sometimes it happens that the collector-emitter junction is broken, as evidenced by the sound signal of the multimeter, although the base-emitter and base-collector junctions “ring” as if normal!

It is performed in the same way as with a digital multimeter, but one should not forget that the polarity in the ohmmeter mode is reverse compared to the DC voltage measurement mode. In order not to forget this during the measurement process, the red probe of the device should be plugged into the socket with the “-” sign, as shown in Figure 2.

Avometers, unlike digital multimeters, do not have a semiconductor dialing mode, so in this regard their readings differ markedly depending on the specific model. Here you already have to rely on your own experience accumulated in the process of working with the device. Figure 8 shows the measurement results using the TL4-M tester.

Figure 8.

The figure shows that measurements are carried out at the limit of *1Ω. In this case, it is better to focus on the readings not on the scale for measuring resistance, but on the upper uniform scale. It can be seen that the arrow is in the area of number 4. If measurements are made at the limit of *1000Ω, then the arrow will be between numbers 8 and 9.

Compared to a digital multimeter, an avometer allows you to more accurately determine the resistance of the base-emitter section if this section is shunted with a low-resistance resistor (R2_32), as shown in Figure 9. This is a fragment of the output stage circuit of an ALTO amplifier.

Figure 9.

All attempts to measure the resistance of the base-emitter section using a multimeter lead to the sound of the speaker (short circuit), since a resistance of 22Ω is perceived by the multimeter as a short circuit. An analog tester at the measurement limit of *1Ω shows some difference when measuring the base-emitter junction in the opposite direction.

Another pleasant nuance when using a dial tester can be found if you take measurements at the limit of *1000Ω. When connecting the probes, of course, observing the polarity (for a transistor of the N-P-N structure, the positive terminal of the device is on the collector, minus on the emitter), the arrow of the device will not move, remaining at the infinity scale mark.

If you now wet your index finger, as if to check the heating of the iron, and close the base and collector terminals with this finger, the arrow of the device will move, indicating a decrease in the resistance of the emitter-collector section (the transistor will open slightly). In some cases, this technique allows you to check the transistor without removing it from the circuit.

This method is most effective when testing composite transistors, for example KT 972, KT973, etc. We should not just forget that composite transistors often have protective diodes connected parallel to the collector-emitter junction, and in reverse polarity. If the transistor is an N-P-N structure, then the cathode of the protective diode is connected to its collector. An inductive load, for example, relay windings, can be connected to such transistors. The internal structure of a compound transistor is shown in Figure 10.

Figure 10.