Voltage measuring device. How to measure voltage with a multimeter. Method for measuring the effective value of voltage using μ
Electrical voltage refers to the work done by an electric field to move a charge of 1 C (coulomb) from one point of a conductor to another.
How does tension arise?
All substances consist of atoms, which are a positively charged nucleus around which smaller negative electrons circle at high speed. In general, atoms are neutral because the number of electrons matches the number of protons in the nucleus.
However, if a certain number of electrons are taken away from the atoms, they will tend to attract the same number, forming a positive field around themselves. If you add electrons, then an excess of them will appear, and a negative field will appear. Potentials are formed - positive and negative.
When they interact, mutual attraction will arise.
The greater the difference - the potential difference - the stronger the electrons from the material with their excess content will be drawn to the material with their deficiency. The stronger the electric field and its voltage will be.
If you connect potentials with different charges of conductors, then electric will arise - a directed movement of charge carriers, seeking to eliminate the difference in potentials. To move charges along a conductor, the electric field forces perform work, which is characterized by the concept of electric voltage. 
What is it measured in?
Temperatures;
Types of voltage
Constant pressure
The voltage in the electrical network is constant when there is always a positive potential on one side and a negative potential on the other. Electric in this case has one direction and is constant.
The voltage in a direct current circuit is defined as the potential difference at its ends.
When connecting a load to a DC circuit, it is important not to mix up the contacts, otherwise the device may fail. A classic example of a constant voltage source is batteries. Networks are used when there is no need to transmit energy over long distances: in all types of transport - from motorcycles to spacecraft, in military equipment, electric power and telecommunications, for emergency power supply, in industry (electrolysis, smelting in electric arc furnaces, etc.) .
AC voltage
If you periodically change the polarity of the potentials, or move them in space, then the electric one will rush in the opposite direction. The number of such changes in direction over a certain time is shown by a characteristic called frequency. For example, standard 50 means that the polarity of the voltage in the network changes 50 times per second.
Voltage in AC electrical networks is a time function.
The law of sinusoidal oscillations is most often used.
This happens due to what occurs in the coil of asynchronous motors due to the rotation of an electromagnet around it. If you expand the rotation in time, you get a sinusoid.
Consists of four wires - three phase and one neutral. the voltage between the neutral and phase wires is 220 V and is called phase. Between phase voltages also exist, called linear and equal to 380 V (potential difference between two phase wires). Depending on the type of connection in a three-phase network, you can get either phase voltage or linear voltage.
Current measurement. To measure the current in the circuit, ammeter 2 (Fig. 332, a) or milliammeter is connected to the electrical circuit in series with receiver 3 of electrical energy.
To ensure that turning on the ammeter does not affect the operation of electrical installations and does not create large energy losses, ammeters are made with low internal resistance. Therefore, practically its resistance can be considered equal to zero and the voltage drop it causes can be neglected. The ammeter can only be connected in series with the load. If the ammeter is connected directly to source 1, then a very large current will flow through the coil of the device (the resistance of the ammeter is low) and it will burn out.
To expand the measurement limits of ammeters intended for operation in DC circuits, they are included in the circuit parallel to shunt 4 (Fig. 332,b). In this case, only part I A of the measured current I passes through the device, inversely proportional to its resistance R A. B O Most of this current passes through the shunt. The device measures the voltage drop across the shunt, which depends on the current passing through the shunt, i.e. it is used as a millivoltmeter. The instrument scale is graduated in amperes. Knowing the resistance of the device R A and the shunt R w, it is possible to determine the measured current from the current I A recorded by the device:
I = I A (R A +R w)/R w = I A n (105)
where n = I/I A = (R A + R w)/R w - shunt coefficient. It is usually chosen to be equal to or a multiple of 10. The shunt resistance required to measure a current I n times greater than the device current I A,
R w = R A /(n-1) (106)
Structurally, shunts are either mounted in the device body (shunts for currents up to 50 A), or installed outside it and connected to the device with wires. If the device is intended for constant operation with a shunt, then its scale is calibrated immediately in the values of the measured current, taking into account the shunt coefficient, and no calculations are required to determine the current. In the case of using external (separate from devices) shunts, they are indicated by the rated current for which they are designed and the rated voltage at the terminals (calibrated shunts). According to standards, this voltage can be 45, 75, 100 and 150 mV. Shunts are selected for devices so that at the rated voltage at the shunt terminals, the device needle deflects to the full scale. Therefore, the nominal voltages of the device and the shunt must be the same. There are also individual shunts designed to work with a specific device. Shunts are divided into five accuracy classes (0.02; 0.05; 0.1; 0.2; 0.5). The class designation corresponds to the permissible error in percentage.
To ensure that an increase in the temperature of the shunt when current passes through it does not affect the readings of the device, the shunts are made from materials with high resistivity and a low temperature coefficient (constantan, manganin, nickel, etc.). To reduce the effect of temperature on the ammeter readings, in some cases an additional resistor made of constant tan or other similar material is connected in series with the device coil.
Voltage measurement. To measure the voltage U acting between any two points of the electrical circuit, voltmeter 2 (Fig. 332, c) is connected to these points, i.e. parallel to the source 1 of electrical energy or receiver 3.
To ensure that turning on the voltmeter does not affect the operation of electrical installations and does not create large energy losses, voltmeters are made with high resistance. Therefore, it is practically possible to neglect the current passing through the voltmeter.
To expand the measurement limits of voltmeters, an additional resistor 4 (R d) is connected in series with the winding of the device (Fig. 332,d). In this case, the device accounts for only part U v of the measured voltage U, proportional to the resistance of the device R v .
Knowing the resistance of the additional resistor and the voltmeter, you can determine the voltage acting in the circuit from the voltage value U v recorded by the voltmeter:
U = (R v+R d)/R v*U v= nU v (107)
The value n = U/U v =(R v +R d)/R v shows how many times the measured voltage U is greater than the voltage U v attributable to the device, i.e. how many times the voltage measurement limit of a voltmeter increases when using an additional resistor.
The resistance of the additional resistor required to measure voltage U, n times greater than the voltage of the device Uv, is determined by the formula R d = (n- 1) R v .
An additional resistor can be built into the device and at the same time used to reduce the influence of ambient temperature on the device readings. For this purpose, the resistor is made of a material having a low temperature coefficient, and its resistance significantly exceeds the resistance of the coil, as a result of which the total resistance of the device becomes almost independent of temperature changes. In terms of accuracy, additional resistors are divided into the same accuracy classes as shunts.
Voltage dividers. To expand the measurement limits of voltmeters, voltage dividers are also used. They allow you to reduce the voltage to be measured to a value corresponding to the rated voltage of a given voltmeter (the maximum voltage on its scale). The ratio of the input voltage of the divider U 1 to the output voltage U 2 (Fig. 333, a) is called division factor. At idle, U 1 /U 2 = (R 1 +R 2)/R2 = 1 + R 1 /R 2. In voltage dividers, this ratio can be chosen to be 10, 100, 500, etc., depending on which
A voltmeter is connected to the divider terminals (Fig. 333b). The voltage divider introduces a small error into the measurements only if the voltmeter's resistance R v is sufficiently high (the current passing through the divider is small) and the resistance of the source to which the divider is connected is small.
Instrument transformers. To switch on electrical measuring instruments in alternating current circuits, instrument transformers are used, ensuring the safety of operating personnel when performing electrical measurements in high-voltage circuits. The inclusion of electrical measuring instruments in these circuits without such transformers is prohibited by safety regulations. In addition, instrument transformers expand the measurement limits of instruments, i.e., they make it possible to measure large currents and voltages using simple instruments designed to measure small currents and voltages.
Instrument transformers are divided into voltage transformers and current transformers. Voltage transformer 1 (Fig. 334, a) is used to connect voltmeters and other devices that must respond to voltage. It is performed like a regular two-winding step-down transformer: the primary winding is connected to two points between which the voltage needs to be measured, and the secondary winding is connected to a voltmeter 2.
In the diagrams, the voltage measuring transformer is depicted as a regular transformer (in Fig. 334, a shown in a circle).
Since the resistance of the voltmeter winding connected to the voltage transformer is high, the transformer practically operates in no-load mode, and can we assume with a reasonable degree of accuracy that the voltages U 1 and U 2 on the primary and secondary windings will be directly proportional to the number of turns? 1 and? 2 of both windings of the transformer, i.e.
U 1 / U 2 = ? 1 /? 2 = n (108)
Thus, choosing the appropriate number of turns? 1 and? 2 transformer windings, high voltages can be measured by applying small voltages to the electrical measuring instrument.
The voltage U 1 can be determined by multiplying the measured secondary voltage U 2 by the transformer transformation ratio n.
Voltmeters designed for continuous operation with voltage transformers are calibrated at the factory taking into account the transformation ratio, and the values of the measured voltage can be directly read off the instrument scale.
To prevent the risk of electric shock to operating personnel in the event of damage to the transformer insulation, one terminal of its secondary winding and the steel casing of the transformer must be grounded.
Current transformer 3 (Fig. 334,b) is used to connect ammeters and other devices that must respond to alternating current flowing through the circuit. It is performed in the form
a conventional two-winding step-up transformer; The primary winding is connected in series to the measured current circuit, and an ammeter 4 is connected to the secondary winding.
The circuit designation of measuring current transformers is shown in Fig. 334, b in a circle.
Since the resistance of the winding of an ammeter connected to a current transformer is usually small, the transformer practically operates in short circuit mode, and with a reasonable degree of accuracy we can assume that the currents I 1 and I 2 passing through its windings will be inversely proportional to the number of turns? 1 and? 2 of these windings, i.e.
I 1 /I 2 = ? 1 /? 2 = n (109)
Therefore, choosing the number of turns accordingly? 1 and? 2 transformer windings, you can measure large currents I 1 by passing small currents I 2 through the electrical measuring device. The current I 1 can be determined by multiplying the measured secondary current I 2 by the value n.
Ammeters intended for continuous operation in conjunction with current transformers are calibrated at the factory taking into account the transformation ratio, and the values of the measured current I 1 can be directly read off the instrument scale.
To prevent the danger of electric shock to operating personnel in the event of damage to the transformer insulation, one of the terminals of the secondary winding and the transformer casing are grounded.
On e. p.s. so-called feed-through current transformers are used (Fig. 335). In such a transformer, the magnetic circuit 3 and the secondary winding 2 are mounted on a bushing insulator 4, which serves to introduce high voltage into the body, and the role of the primary winding of the transformer is played by a copper rod 1 passing inside the insulator.
The operating conditions of current transformers differ from ordinary ones. For example, opening the secondary winding of a current transformer while the primary winding is turned on is unacceptable, since this will cause a significant increase in the magnetic flux and, as a consequence, the temperature of the core and winding of the transformer, i.e., its failure. In addition, a large e can be induced in the open-circuit secondary winding of the transformer. d.s., dangerous for personnel performing measurements.
When switching on devices through measuring transformers, errors of two types arise: an error in the transformation ratio and an angular error (with changes in voltage or current, the ratios U 1 / U 2 and I 1 / I 2 change slightly and the phase shift angle between the primary and secondary voltages and currents deviates from 180°). These errors increase when the transformer load exceeds the rated load. The angular error affects the measurement results
with instruments whose readings depend on the phase angle between voltage and current (for example, wattmeters, electricity meters, etc.). Depending on the permissible errors, instrument transformers are divided into accuracy classes. The accuracy class (0.2; 0.5; 1, etc.) corresponds to the largest permissible error in the transformation ratio as a percentage of its nominal value.
Remember one rule when measuring: when measuring current, connect in series with the load, and when measuring other quantities - in parallel.
The figure below shows how to correctly connect the probes and the load in order to measure the current:
We do not touch the black probe, which is plugged into the COM socket, but transfer the red one to the socket where mA or xA is written, where instead of x is the maximum current value that the device can measure. In my case, this is 20 Amperes, since 20 A is written next to the socket. Depending on what current value you are going to measure, we stick the red probe there. If you don’t know approximately how much current will flow in the circuit, then put in the xA socket:
Let's check how it all works in action.In our case, the load is the computer fan. Our power supply has a built-in indication to show the current strength, and as you know from the physics course, the current strength is measured in Amperes. We set it to 12 Volts, turn the knob on the multimeter to measure DC current. We set the measurement limit on the cartoon to 20 Amps. We assemble as per the diagram above and look at the readings on the cartoon. It coincided exactly with the built-in ammeter on the .
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To measure current AC voltage We place the multimeter knob on the icon for measuring the current strength of alternating voltage - “A~” and take measurements in exactly the same way.
How to measure DC voltage with a multimeter
Let's take a battery like this
As we can see, it says a current of 550 mAh, which it can supply to the load for an hour, that is, milliamps per hour, as well as the voltage that our battery has - 1.2 Volts. Voltage is understandable, but what is “current for an hour”? Let's say our load, a light bulb, consumes a current of 550 mA. This means the light bulb will shine for one hour. Or let's take a light bulb that shines weaker, and let it consume 55 mA, which means it can work for 10 hours.
We divide the 550 mA value that is written on the battery by the value that is written on the load and get the time during which all this will work until the battery runs out. In short, anyone who is good at mathematics will not have any difficulty understanding this miracle :-)
Let's measure the voltage on the battery, set one multimeter probe to positive and the other to negative, that is, connect parallel, and voila!
In this case, the voltage on the battery is 1.28 Volts. The value on a new battery should always be higher than what is written on the label.
Let's measure the voltage on the power supply. We set it to 10 Volts and measure it.
Red is a plus, black is a minus. Everything matches, the voltage is 10.09 Volts. We'll write off 0.09 Volts as an error.
If we confuse the multimeter probes or the unit probes, then nothing bad will happen. The multimeter will show us the same value, but with a minus sign.
Keep in mind, this does not work on such multimeters
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In order to accurately determine the polarity without a multimeter, you can resort to several tips that are described in the article.
How to measure AC voltage with a multimeter
We set the limit for measuring alternating voltage on the cartoon and measure the voltage in the socket. It doesn't matter how you insert the probes. There is no plus or minus. There is a phase and a zero. Roughly speaking, one wire in a socket does not pose a danger - it is a zero, while the other can greatly ruin your well-being or even your health - this is a phase.
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In theory, the outlet should have 220 Volts. But mine shows 215. There’s nothing wrong with that. The voltage in the socket “plays”. You are unlikely to see exactly 220 Volts when measuring the voltage in the sockets of your home :-)
The basic unit of measurement for electrical voltage is the volt. Depending on the magnitude, voltage can be measured in volts(IN), kilovolts(1 kV = 1000 V), millivolts(1 mV = 0.001 V), microvolts(1 µV = 0.001 mV = 0.000001 V). In practice, most often you have to deal with volts and millivolts.
There are two main types of stress - permanent And variable. Batteries and accumulators serve as a source of constant voltage. The source of alternating voltage can be, for example, the voltage in the electrical network of an apartment or house.
To measure voltage use voltmeter. There are voltmeters switches(analog) and digital.
Today, pointer voltmeters are inferior to digital ones, since the latter are more convenient to use. If, when measuring with a pointer voltmeter, the voltage readings have to be calculated on a scale, then with a digital one, the measurement result is immediately displayed on the indicator. And in terms of dimensions, a pointer instrument is inferior to a digital one.
But this does not mean that pointer instruments are not used at all. There are some processes that cannot be seen with a digital instrument, so switches are more used in industrial enterprises, laboratories, repair shops, etc.
On electrical circuit diagrams, a voltmeter is indicated by a circle with a capital Latin letter “ V" inside. Next to the symbol of the voltmeter its letter designation is indicated “ P.U." and the serial number in the diagram. For example. If there are two voltmeters in the circuit, then next to the first one they write “ PU 1", and about the second " PU 2».
When measuring direct voltage, the diagram indicates the polarity of the voltmeter connection, but if alternating voltage is measured, the polarity of the connection is not indicated.
The voltage is measured between two points circuits: in electronic circuits between positive And minus poles, in electrical circuits between phase And zero. Voltmeter connected parallel to the voltage source or parallel to the chain section- a resistor, lamp or other load on which the voltage needs to be measured:
Let's consider connecting a voltmeter: in the upper diagram, the voltage is measured across the lamp HL1 and simultaneously on the power source GB1. In the diagram below, the voltage is measured across the lamp HL1 and resistor R1.
Before measuring the voltage, determine it view and approximate size. The fact is that the measuring part of voltmeters is designed for only one type of voltage, and this results in different measurement results. A voltmeter for measuring direct voltage does not see alternating voltage, but a voltmeter for alternating voltage, on the contrary, can measure direct voltage, but its readings will not be accurate.
It is also necessary to know the approximate value of the measured voltage, since voltmeters operate in a strictly defined voltage range, and if you make a mistake with the choice of range or value, the device can be damaged. For example. The measurement range of a voltmeter is 0...100 Volts, which means that voltage can only be measured within these limits, since if a voltage is measured above 100 Volts, the device will fail.
In addition to devices that measure only one parameter (voltage, current, resistance, capacitance, frequency), there are multifunctional ones that measure all these parameters in one device. Such a device is called tester(mostly pointer measuring instruments) or digital multimeter.
We won’t dwell on the tester, that’s the topic of another article, but let’s move straight to the digital multimeter. For the most part, multimeters can measure two types of voltage within the range of 0...1000 Volts. For ease of measurement, both voltages are divided into two sectors, and within the sectors into subranges: constant voltage has five subranges, alternating voltage has two.
Each subrange has its own maximum measurement limit, which is indicated by a digital value: 200m, 2V, 20V, 200V, 600V. For example. At the “200V” limit, voltage is measured in the range of 0...200 Volts.
Now the measurement process itself.
1. DC voltage measurement.
First we decide on view measured voltage (DC or AC) and move the switch to the desired sector. For example, let's take a AA battery, the constant voltage of which is 1.5 Volts. We select the constant voltage sector, and in it the measurement limit is “2V”, the measurement range of which is 0...2 Volts.
The test leads must be inserted into the sockets as shown in the figure below:
red the dipstick is usually called positive, and it is inserted into the socket, opposite which there are icons of the measured parameters: “VΩmA”;
black the dipstick is called minus or general and it is inserted into the socket opposite which there is a “COM” icon. All measurements are made relative to this probe.
We touch the positive pole of the battery with the positive probe, and the negative pole with the negative one. The measurement result of 1.59 Volts is immediately visible on the multimeter indicator. As you can see, everything is very simple.
Now there's another nuance. If the probes on the battery are swapped, a minus sign will appear in front of the one, indicating that the polarity of the multimeter connection is reversed. The minus sign can be very convenient in the process of setting up electronic circuits, when you need to determine the positive or negative buses on the board.
Well, now let’s consider the option when the voltage value is unknown. We will use a AA battery as a voltage source.
Let’s say we don’t know the battery voltage, and in order not to burn the device, we start measuring from the maximum limit “600V”, which corresponds to the measurement range of 0...600 Volts. Using the multimeter probes, we touch the poles of the battery and on the indicator we see the measurement result equal to “ 001 " These numbers indicate that there is no voltage or its value is too small, or the measurement range is too large.
Let's go lower. We move the switch to the “200V” position, which corresponds to the range of 0...200 Volts, and touch the battery poles with the probes. The indicator showed readings equal to “ 01,5 " In principle, these readings are already enough to say that the voltage of the AA battery is 1.5 Volts.
However, the zero in front suggests going even lower and measuring the voltage more accurately. We go down to the “20V” limit, which corresponds to the range of 0...20 Volts, and take the measurement again. The indicator showed “ 1,58 " Now we can say with accuracy that the voltage of the AA battery is 1.58 Volts.
In this way, without knowing the voltage value, they find it, gradually decreasing from a high measurement limit to a low one.
There are also situations when, when taking measurements, the unit "" is displayed in the left corner of the indicator. 1 " A unit indicates that the measured voltage or current is higher than the selected measurement limit. For example. If you measure a voltage of 3 Volts at the “2V” limit, then a unit will appear on the indicator, since the measurement range of this limit is only 0…2 Volts.
There remains one more limit “200m” with a measurement range of 0...200 mV. This limit is intended to measure very small voltages (millivolts), which are sometimes encountered when setting up some amateur radio design.
2. AC voltage measurement.
The process of measuring alternating voltage is no different from measuring direct voltage. The only difference is that for alternating voltage the polarity of the probes is not required.
The AC voltage sector is divided into two subranges 200V And 600V.
At the “200V” limit, you can measure, for example, the output voltage of the secondary windings of step-down transformers, or any other voltage in the range of 0...200 Volts. At the “600V” limit, you can measure voltages of 220 V, 380 V, 440 V or any other voltage in the range of 0...600 Volts.
As an example, let's measure the voltage of a 220 Volt home network.
We move the switch to the “600V” position and insert the multimeter probes into the socket. The measurement result of 229 Volts immediately appeared on the indicator. As you can see, everything is very simple.
And one moment.
Before measuring high voltages, ALWAYS double check that the insulation of the probes and wires of the voltmeter or multimeter is in good condition. and also additionally check the selected measurement limit. And only after all these operations take measurements. This way you will protect yourself and the device from unexpected surprises.
And if anything remains unclear, then watch the video, which shows how to measure voltage and current using a multimeter.
General information. The need to measure voltage in practice arises very often. In electrical and radio circuits and devices, the voltage of direct and alternating (sinusoidal and pulsed) current is most often measured.
DC voltage (Fig. 3.5, A) is expressed as . The sources of such voltage are DC generators and chemical power sources.
Rice. 3.5. Voltage timing diagrams: direct (a), alternating sinusoidal (b) and alternating pulse (c) current
AC sinusoidal current voltage (Fig. 3.5, b) is expressed as 
and is characterized by root-mean-square and amplitude values:
The sources of such voltage are low- and high-frequency generators and the electrical network.
AC pulse current voltage (Fig. 3.5 V) is characterized by amplitude and average (constant component) voltage values. The source of such voltage is pulse generators with signals of different shapes.
The basic unit of measurement for voltage is the volt (V).
In the practice of electrical measurements, submultiple and multiple units are widely used:
Kilovolt (1 kV - V);
Millivolt (1mV - V);
Microvolt (1 µV - V).
International designations of voltage units are given in Appendix 1.
In the catalog classification, electronic voltmeters are designated as follows: B1 - exemplary, B2 - direct current, VZ - alternating sinusoidal current, B4 - alternating pulse current, B5 - phase-sensitive, B6 - selective, B7 - universal.
On the scales of analog indicators and on the front panels (on limit switches) of domestic and foreign electronic and electromechanical voltmeters, the following designations are used: V - voltmeters, kV - kilovoltmeters, mV - millivolt meters, V - microvoltmeters.
DC voltage measurement. To measure DC voltage, electromechanical voltmeters and multimeters, electronic analog and digital voltmeters, and electronic oscilloscopes are used.
Electromechanical voltmeters Direct evaluation of the measured value constitutes a large class of analog-type devices and has the following advantages:
Ability to work without connecting to a power source;
Small overall dimensions;
Lower price (compared to electronic ones);
Simplicity of design and ease of operation.
Most often, when performing electrical measurements in high-current circuits, voltmeters based on electromagnetic and electrodynamic systems are used, and in low-current circuits, a magnetoelectric system is used. Since all of the above systems are themselves current meters (ammeters), to create voltmeters based on them it is necessary to increase the internal resistance of the device, i.e. connect an additional resistor in series with the measuring mechanism (Fig. 3.6, A).
The voltmeter is connected to the circuit under test in parallel (Fig. 3.6, b), and its input impedance must be large enough.
To expand the measuring range of the voltmeter, an additional resistor is also used, which is connected to the device in series (Fig. 3.6, V).
The resistance value of the additional resistor is determined by the formula:
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Rice. 3.6. Scheme for creating a voltmeter based on an ammeter ( A), connecting the voltmeter to the load ( 6 ), connecting an additional resistor to a voltmeter ( V)
(3.8)
Where is a number showing how many times the measurement limit of the voltmeter expands:
where is the original measurement limit;
— new measurement limit.
Additional resistors placed inside the device body are called internal, while those connected to the device from the outside are called external. Voltmeters can be multi-range. There is a direct relationship between the measurement limit and the internal resistance of a multi-limit voltmeter: the larger the measurement limit, the greater the resistance of the voltmeter.
Electromechanical voltmeters have the following disadvantages:
Limited voltage measurement range (even in multi-range voltmeters);
Low input resistance, therefore, high internal power consumption from the circuit under study.
These disadvantages of electromechanical voltmeters determine the preferred use of electronic voltmeters for measuring voltage in electronics.
Electronic analogue DC voltmeters built according to the scheme shown in Fig. 3.7. The input device consists of an emitter follower (to increase the input resistance) and an attenuator - a voltage divider.
The advantages of electronic analog voltmeters compared to analog ones are obvious:
Rice. 3.7. Block diagram of an electronic analogue DC voltmeter
Wide voltage measurement range;
Large input resistance, therefore, low intrinsic power consumption from the circuit under study;
High sensitivity due to the presence of an amplifier at the input of the device;
Impossibility of overloads.
However, electronic analog voltmeters have a number of disadvantages:
Availability of power sources, mostly stabilized;
The reduced relative error is larger than that of electromechanical voltmeters (2.5-6%);
Large weight and dimensions, higher price.
Currently, analog electronic DC voltmeters are not widely used, since their parameters are noticeably inferior to digital voltmeters.
AC voltage measurement.
To measure AC voltage, electromechanical voltmeters and multimeters, electronic analog and digital voltmeters, and electronic oscilloscopes are used.
Let's consider inexpensive and fairly accurate electromechanical voltmeters. It is advisable to do this in frequency ranges.
At industrial frequencies of 50, 100, 400 and 1000 Hz, voltmeters of electromagnetic, electrodynamic, ferrodynamic, rectifier, electrostatic and thermoelectric systems are widely used.
At low frequencies (up to 15-20 kHz), voltmeters of rectifier, electrostatic and thermoelectric systems are used.
At high frequencies (up to a few - tens of megahertz) devices of electrostatic and thermoelectric systems are used.
For electrical measurements, universal instruments - multimeters - are widely used.
Multimeters(testers, ampere-volt-ohmmeters, combined devices) allow you to measure many parameters: direct and alternating current strength, direct and alternating current voltage, resistor resistance, capacitor capacity (not all devices), some static parameters of low-power transistors (, , And ).
Multimeters are available with analog and digital reading.
The widespread use of multimeters is explained by the following advantages:
Multifunctionality, i.e. Possibility of use as ammeters, voltmeters, ohmmeters, faradometers, meters of parameters of low-power transistors:
Wide range of measured parameters due to the presence of several measurement limits for each parameter;
Possibility of use as portable devices, since there is no mains power supply;
Small weight and dimensions;
Versatility (the ability to measure alternating and direct currents and voltages),
Multimeters also have a number of disadvantages:
Narrow frequency range of applicability;
Large own power consumption from the studied 1 circuit;
Large reduced errors for analog (1.5, 2.5 and 4) and digital multimeters;
Inconsistency of internal resistance at different limits 4 of current and voltage measurements.
According to the domestic catalog classification, multimeters are designated Ts43 and then the model number, for example, Ts4352.
To determine the internal resistance of an analog multimeter at the included measurement limit, the specific resistance can be given in the device passport 1. For example, in the passport of the Ts4341 tester, the resistivity = 16.7 kOhm/V, the measurement limits for DC voltage are 1.5 - 3 - 6 - 15 V.
In this case, the resistance of the multimeter at the limit of 6 V DC is determined by the formula:
The device passport may contain the information necessary to calculate resistance according to Ohm's law.
If the tester is used as a voltmeter, then its input resistance is determined by the formula:
where is the selected measurement limit;
The current value in the selected limit (indicated on the back panel of the device or in its passport).
If the tester is used as an ammeter, then its input resistance is determined by the formula:
Where is the selected measurement limit;
— voltage value shown on the back panel of the device or in its data sheet.
For example, the passport of the Ts4341 tester shows a voltage drop across the device equal to 0.3 V in the range of 0.06 - 0.6 - 6 - 60 - 600 mA DC, and a voltage drop of 1.3 V in the range: 0. 3 - 3 - 30 - 300 mA AC. The input impedance of the multimeter in the 3 mA AC limit will be
Electronic analogue AC voltmeters are built according to one of the block diagrams (Fig. 3.8), which differ in the sequence of arrangement of the main blocks - the amplifier and the converter (detector) of alternating current voltage into direct current voltage. The properties of these voltmeters largely depend on the chosen circuit.
Rice. 3.8. Block diagrams of electronic analog voltmeters of alternating current type U-D ( A) and type D-U (b)
Voltmeters of the first group - the amplifier-detector type (A-D) - have high sensitivity, which is associated with the presence of an additional amplifier. Therefore, all micro- and millivoltmeters are built according to the V-D circuit. However, the frequency range of such voltmeters is not wide (up to several megahertz), since the creation of a broadband AC amplifier is associated with certain difficulties. Voltmeters of the U-D type are classified as non-universal (VZ subgroup), i.e. can only measure AC voltage.
Voltmeters of the second group - the detector-amplifier (D-A) type - have a wide frequency range (up to several gigahertz) and low sensitivity. Voltmeters of this type are universal (subgroup B7), i.e. measure voltage not only of alternating current, but also of direct current; can measure voltage at a significant level, since it is not difficult to provide high gain using CNTs.
In both types of voltmeters, an important function is performed by converters of AC voltage into DC voltage - detectors, which, based on the function of converting input voltage to output voltage, can be classified into three types: amplitude, rms and rms rectified values.
The properties of the device largely depend on the type of detector. Volt meters with an amplitude value detector are the highest frequency ones; voltmeters with an RMS value detector allow you to measure AC voltage of any shape; voltmeters with an average-rectified value detector are suitable for measuring the voltage of only a harmonic signal and are the simplest, most reliable and inexpensive.
Amplitude value detector is a device whose output voltage corresponds to the amplitude value of the measured signal, which is ensured by storing the voltage on the capacitor.
In order for the real load circuit of any detector to effectively filter the useful signal and suppress unwanted high-frequency harmonics, the following condition must be met:
Or , (3.12)
where is the capacitance of the output filter;
— detector load resistance.
The second condition for good detector operation:
Figure 3.9 shows the block diagram and timing diagrams of the output voltage of the amplitude value detector with a diode connected in parallel and the input closed. A detector with a closed input has a capacitor connected in series, which does not allow the DC component to pass through. Let's consider the operation of such a detector when a sinusoidal voltage is applied to its input .
Rice. 3.9. Block diagram of an amplitude value detector with a diode connected in parallel and a closed input (A) and voltage timing diagrams (b) When a positive half-wave of a sine wave arrives, the capacitor WITH is charged through a VD diode, which has low resistance when open.
The charge time constant of the capacitor is small, and the capacitor quickly charges to its maximum value . When the polarity of the input signal changes, the diode is closed and the capacitor is slowly discharged through the load resistance, which is selected large - 50-100 MOhm.
Thus, the discharge constant turns out to be significantly greater than the period of the sinusoidal signal. As a result, the capacitor remains charged to a voltage close to 
.
The change in voltage across the load resistor is determined by the difference in the amplitudes of the input voltage and the voltage across the capacitor 
.As a result, the output voltage will pulsate with double the amplitude of the measured voltage (see Fig. 3.9, b).
This is confirmed by the following mathematical calculations:
at , , at , at .
To isolate the constant component of the signal, the detector output is connected to a capacitive filter, which suppresses all other current harmonics.
Based on the foregoing, the conclusion follows: the shorter the period of the signal under study (the higher its frequency), the more accurately the equality is satisfied , which explains the high-frequency properties of the detector. When using voltmeters with an amplitude value detector, it should be borne in mind that these devices are most often calibrated in the root-mean-square values of the sinusoidal signal, i.e., the readings of the device indicator are equal to the quotient of the amplitude value divided by the amplitude factor of the sinusoid:
where is the amplitude factor.
RMS detector(Fig. 3.10) converts AC voltage to DC voltage, proportional to the square of the root mean square value of the measured voltage. Therefore, measuring the rms voltage involves performing three operations: squaring the instantaneous value of the signal, averaging its value, and taking the root of the averaging result (the last operation is ensured by calibrating the voltmeter scale). The squaring of the instantaneous signal value is usually carried out by a diode cell by using the quadratic portion of its characteristic.
Rice. 3.10. RMS detector: A - diode cell; b— CVC of the diode
In the diode cell VD, R1(see Fig. 3.10, A) a constant voltage is applied to the diode VD in such a way that it remains closed as long as the measured voltage () across the resistor R2 will not exceed the value .
The initial section of the diode's current-voltage characteristic is short (see Fig. 3.10, b), Therefore, the quadratic part is artificially lengthened by the piecewise linear approximation method by using several diode cells.
When designing RMS voltmeters, difficulties arise in providing a wide frequency range. Despite this, such voltmeters are the most popular, since they can measure voltage of any complex shape.
Rectified average detector converts AC voltage to DC voltage proportional to the average rectified voltage value. The output current of a measuring device with such a detector is similar to the output current of the rectifier system.
AC voltages operating in electronic devices can change over time according to various laws. For example, the voltage at the output of the master oscillator of a connected radio transmitter varies according to a sinusoidal law, at the output of an oscilloscope sweep generator the pulses have a sawtooth shape, and the synchronizing pulses of a complete television signal are rectangular.
In practice, it is necessary to carry out measurements in various sections of circuits, the voltages in which may differ in value and shape. Measuring non-sinusoidal voltage has its own characteristics that must be taken into account in order to avoid errors.
It is very important to choose the right type of device and the method of converting the voltmeter readings into the value of the required parameter of the measured voltage. To do this, it is necessary to clearly understand how AC voltages are assessed and compared and how the shape of the voltage affects the values of the coefficients that relate individual voltage parameters.
The criterion for assessing an alternating current voltage of any form is the connection with the corresponding direct current voltage for the same thermal effect (rms value U), defined by the expression
(3.14)
where is the signal repetition period;
- a function that describes the law of change in the instantaneous voltage value. It is not always possible for the operator to have a voltmeter at his disposal, with which he can measure the desired voltage parameter. In this case, the required voltage parameter is measured indirectly using an existing voltmeter, using crest and shape coefficients. Let's consider an example of calculating the necessary parameters of a sinusoidal voltage.
It is necessary to determine the amplitude () and the mean-rectified () values of the sinusoidal voltage with a voltmeter, calibrated in the root-mean-square values of the sinusoidal voltage, if the device showed .
We perform the calculation as follows. Since the voltmeter is calibrated in rms values , then in Appendix 3 for this device, the reading of 10 V corresponds to a direct reading on the rms value scale, i.e.
Alternating voltage is characterized by average, amplitude) (maximum) and root mean square values.
Average value(constant component) for a period of alternating voltage:
(3.15)
Maximum value is the largest instantaneous value of alternating voltage during the signal period:
Average rectified value - this is the average voltage at the output of a full-wave rectifier having an alternating voltage at the input :
(3.17)
The ratio of the root mean square, average and maximum values of the alternating current voltage depends on its shape and is generally determined by two coefficients:
(amplitude factor), (3.18)
(form factor). (3.19)
The values of these coefficients for stresses of different shapes and their ratios are given in Table. 3.1
Table 3.1
Values and for voltages of different shapes
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Note, - duty cycle: .
In a number of devices, voltage is assessed not in absolute units (V, mV, µV), but in a relative logarithmic unit - decibel (dB, or dB). To simplify the transition from absolute units to relative units and, conversely, most analog voltmeters (stand-alone and built into other devices: generators, multimeters, nonlinear distortion meters) have a decibel scale along with the usual one. This scale is distinguished by a clearly defined nonlinearity, which, if necessary, allows you to obtain the result immediately in decibels, without appropriate calculations and the use of conversion tables. Most often, for such devices, the zero decibel scale corresponds to an input voltage of 0.775 V.
Voltage greater than the conventional zero level is characterized by positive decibels, less than this level - negative. On the limit switch, each measurement subrange differs in level from the neighboring one by 10 dB, which corresponds to a voltage factor of 3.16. The readings taken on the decibel scale are algebraically added to the readings on the measurement limit switch, and are not multiplied, as in the case of absolute voltage readings.
For example, the limit switch is set to “- 10 dB”, while the indicator arrow is set to “- 0.5 dB”. The total level will be: ---- 10 + (- 0.5) = - 10.5 dB, And the basis for converting voltage from absolute values to relative values is the formula
(3.20)
Where = 0.775V.
Since bel is a large unit, in practice a fractional (tenth) part of bel is used - decibel.
Pulse and digital voltmeters. When measuring pulse voltages with small amplitude, preliminary pulse amplification is used. The block diagram of an analog pulse voltmeter (Fig. 3.11) consists of a remote probe with an emitter follower, an attenuator, a broadband preamplifier, an amplitude value detector, a direct current amplifier (DCA) and an electromechanical indicator. Voltmeters implemented according to this scheme directly measure voltages of 1 mV - 3 V with an error of ± (4 - 10)%, a pulse duration of 1 - 200 μs and a duty cycle of 100 ... 2500.
Rice. 3.11.t Block diagram of a pulse voltmeter
To measure small voltages over a wide range of durations (from nanoseconds to milliseconds), voltmeters operating on the basis of the autocompensation method are used.
Electronic digital voltmeters have significant advantages over analog ones:
High measurement speed;
Eliminating the possibility of subjective operator error;
Small reduced error.
Due to these advantages, digital electronic voltmeters are widely used for measurement purposes. Figure 3.12 shows a simplified block diagram of a digital voltmeter.
Rice. 3.12. Simplified block diagram of a digital voltmeter
Input device designed to create a large input resistance, select measurement limits, reduce interference, and automatically determine the polarity of the measured DC voltage. In AC voltmeters, the input device also includes an AC-to-DC voltage converter.
From the output of the input device, the measured voltage is supplied to analog-to-digital converter(ADC), in which the voltage is converted into a digital (discrete) signal in the form of an electrical code or pulses, the number of which is proportional to the measured voltage. The result appears on the scoreboard digital indicator. The operation of all blocks is controlled control device.
Digital voltmeters, depending on the type of ADC, are divided into four groups: pulse-code, time-pulse, pulse-frequency, spatial coding.
Currently widely used digital time-pulse voltmeters , converters of which perform intermediate conversion of the measured voltage into a proportional time interval filled with pulses with a known repetition frequency. As a result of this transformation, the discrete signal of measuring information at the input of the ADC has the form of a packet of counting pulses, the number of which is proportional to the measured voltage.
The error of time-pulse voltmeters is determined by the sampling error of the measured signal, the instability of the counting pulse frequency, the presence of a sensitivity threshold of the comparison circuit, and the nonlinearity of the converted voltage at the input of the comparison circuit.
There are several options for circuit design solutions when constructing time-pulse voltmeters. Let's consider the operating principle of a pulse voltmeter with a linearly varying voltage generator (GLIN).
Figure 3.13 shows a block diagram of a digital time-pulse voltmeter with GLIN and timing diagrams explaining its operation.
The discrete signal of measuring information at the output of the converter has the form of a packet of counting pulses, the number of which is proportional to the value of the input voltage . From the output of GLIN, a voltage linearly increasing in time is supplied to inputs 1 of comparison devices. Input 2 of comparison device II is connected to the housing.
At the moment of equality, a pulse appears at the input of comparison device II and at its output, which is fed to the single input of the trigger (T), causing the appearance of a signal at its output. The trigger returns to its original position by a pulse coming from the output of comparison device II. This signal appears at the moment of equality of the linearly increasing voltage and the measured one. The signal thus generated with a duration (where — conversion factor) is supplied to input 1 of the AND logical multiplication circuit, and input 2 receives a signal from the counting pulse generator (CPG). The pulses follow with a frequency. A pulse signal appears when there are pulses at both inputs, i.e. Counting pulses pass when there is a signal at the trigger output.
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Rice. 3.13. Structural scheme (A) and time charts (b) digital time-pulse voltmeter with GLIN
The pulse counter counts the number of passed pulses (taking into account the conversion factor). The measurement result is displayed on the digital indicator (DI) board. The given formula does not take into account the discreteness error due to the discrepancy between the appearance of counting pulses and the beginning and end of the interval
In addition, a large error is introduced by the nonlinearity factor of the conversion coefficient . As a result, digital time-pulse voltmeters with GLIN are the least accurate among digital voltmeters.
Double Integration Digital Voltmeters differ from time-pulse voltmeters in the principle of operation. In them, during the measurement cycle, two time intervals are formed - and . In the first interval, integration of the measured voltage is ensured , in the second - the reference voltage. The measurement cycle time is pre-set as a multiple of the period of the noise acting at the input, which leads to improved noise immunity of the voltmeter.
Figure 3.14 shows a block diagram of a digital voltmeter with double integration and timing diagrams explaining its operation.
Rice. 3.14. Structural scheme (A) and timing diagrams (6) double integration digital voltmeter
At (at the moment the measurement begins), the control device generates a calibrated pulse with a duration
, (3.21) moves the switch to position 2 and the reference voltage source (VS) is supplied to the integrator; the reference negative voltage becomes equal to zero, the comparison device produces a signal sent to the trigger and returns the latter to its original state. At the output of the trigger, the generated voltage pulse
; ; 
(3.25)
From the obtained relationships it follows that the error in the measurement result depends only on the level of the reference voltage, and not on several parameters (as in a pulse code voltmeter), but there is also a discreteness error here.
The advantages of a voltmeter with double integration are high noise immunity and a higher accuracy class (0.005-0.02%) compared to voltmeters with GLIN.
Digital voltmeters with built-in microprocessor are combined and belong to the voltmeters of the highest accuracy class. The principle of their operation is based on the methods of bit-by-bit balancing and time-pulse integrating transformation.
The microprocessor and additional converters included in the circuit of such a voltmeter expand the capabilities of the device, making it universal in measuring a large number of parameters. Such voltmeters measure DC and AC voltage, current strength, resistor resistance, oscillation frequency and other parameters. When used together with an oscilloscope, they can measure time parameters: period, pulse duration, etc. The presence of a microprocessor in the voltmeter circuit allows for automatic correction of measurement errors, fault diagnostics, and automatic calibration.
Figure 3.15 shows a block diagram of a digital voltmeter with a built-in microprocessor.
 |
Rice. 3.15. Block diagram of a digital voltmeter with a built-in microprocessor
Using appropriate converters, the signal normalization unit converts the input measured parameters (97 pages) to a unified signal arriving at the input of the ADC, which performs the conversion using the double integration method. The selection of the voltmeter operating mode for a given type of measurement is carried out by the ADC control unit with a display. The same block provides the required configuration of the measurement system.
The microprocessor is the basis of the control unit and is connected to other units through shift registers. The microprocessor is controlled using the keyboard located on the control panel. Management can also be carried out through a standard interface of a connected communication channel. Read-only memory (ROM) stores the microprocessor operating program, which is implemented using random access memory (RAM).
Built-in highly stable and accurate resistive reference voltage dividers, a differential amplifier (DA) and a number of external elements (attenuator, mode selector, reference voltage unit ) perform direct measurements. All blocks are synchronized by signals from the clock generator.
The inclusion of a microprocessor and a number of additional converters in the voltmeter circuit allows for automatic error correction, automatic calibration and fault diagnostics.
The main parameters of digital voltmeters are conversion accuracy, conversion time, limits for changing the input value, and sensitivity.
Conversion Accuracy is determined by the level quantization error, characterized by the number of bits in the output code.
The error of a digital voltmeter has two components. The first component (multiplicative) depends on the measured value, the second component (additive) does not depend on the measured value.
This representation is associated with the discrete principle of measuring an analog quantity, since during the quantization process an absolute error arises due to a finite number of quantization levels. The absolute error of voltage measurement is expressed as
signs) or (signs), (3.27)
where is the actual relative measurement error;
— the value of the measured voltage;
— final value at the selected measuring limit;
T signs - the value determined by the unit of the least significant digit of the CI (additive discreteness error). The main actual relative measurement error can be presented in another form:
(3.2)
Where a, b - constant numbers characterizing the accuracy class of the device.
First term of error (A) does not depend on the instrument readings, and the second (b) increases when decreasing .
Conversion time is the time it takes to complete one conversion of an analog value to a digital code.
Limits of change of input value — These are the ranges of transformation of the input value, which are completely determined by the number of digits and the “weight” of the smallest digit.
Sensitivity(resolution) is the smallest change in the value of the input quantity discernible by the converter.
The main metrological characteristics of voltmeters that you need to know to correctly select a device include the following characteristics:
Parameter of the measured voltage (rms, amplitude);
Voltage measurement range;
Frequency range;
Permissible measurement error;
Input impedance() .
These characteristics are given in the technical description and passport of the device.
