Voltmeters, millivoltmeters, and microvoltmeters of various systems are used to measure voltage. These devices are connected in parallel to the load, so their resistance should be as high as possible (about two orders of magnitude greater than the resistance of any circuit element).

Figure 6 Figure 7

To expand the measurement limits of the voltmeter (in k times) in DC circuits with voltages up to 500V, additional resistances are usually used R d , connected in series with a voltmeter.

From the relation
let's define
,

Where U max is the highest voltage value that can be measured by a voltmeter with additional resistance;

U int - the limiting (nominal) value of the voltmeter scale in the absence of Rd.

The value of the actually measured voltage U is determined from the relationship:

;
,

where U in - voltmeter reading.

In alternating current circuits, voltage transformers are used to change the measurement limits of the voltmeter.

Power measurement. Power measurement in direct and single-phase current circuits

The power in DC circuits consumed by this section of the electrical circuit is equal to:

and can be measured with an ammeter and voltmeter.

In addition to the inconvenience of simultaneous reading of two instruments, power measurement by this method is carried out with inevitable error. It is more convenient to measure power in DC circuits with a wattmeter.

It is impossible to measure active power in an alternating current circuit with an ammeter and voltmeter, because the power of such a circuit also depends on cosφ:

Therefore, in AC circuits, active power is measured only with a wattmeter.

Figure 8

Fixed winding 1-1 (current) is connected in series, and movable winding 2-2 (voltage winding) is connected in parallel with the load.

To correctly turn on the wattmeter, one of the current winding terminals and one of the voltage winding terminals are marked with an asterisk (*). These terminals, called generator terminals, must be turned on from the power supply side by combining them together. In this case, the wattmeter will show the power coming from the network (generator) to the electrical energy receiver.

Measurement of active power in three-phase current circuits

When measuring the power of three-phase current, various circuits for connecting wattmeters are used depending on:

wiring systems (three- or four-wire);

load (uniform or uneven);

load connection diagrams (star or delta).

a) power measurement under symmetrical load; three- or four-wire wiring system:

Figure 9 Figure 10

In this case, the power of the entire circuit can be measured with one wattmeter (Figures 9,10), which will show the power of one phase P=3P f =3U f I f cosφ

b) with an asymmetric load, the power of a three-phase consumer can be measured with three wattmeters:

Figure 11

The total power of the consumer is equal to:

c) power measurement using the two-wattmeter method:

Figure 12

It is used in 3-wire three-phase current systems with symmetrical and asymmetrical loads and any method of connecting consumers. In this case, the current windings of the wattmeters are connected to phases A and B (for example), and the parallel windings to linear voltages U AC and U BC (or A and C  U AB and U CA), (Fig. 12).

Total power P=P 1 +P 2.

To measure alternating voltage, analog electromechanical devices (electromagnetic, electrodynamic, rarely inductive), analog electronic devices (including rectifier systems) and digital measuring instruments are used. Compensators, oscilloscopes, recorders and virtual instruments can also be used for measurements.

When measuring alternating voltage, one should distinguish between instantaneous, amplitude, average and effective values ​​of the desired voltage.

Sinusoidal alternating voltage can be represented in the form of the following relationships:

Where u(t)- instantaneous voltage value, V; U m - amplitude voltage value, V; (U - average voltage value, V T - period

(T = 1//) desired sinusoidal voltage, s; U- effective voltage value, V.

The instantaneous value of the alternating current can be displayed on an electronic oscilloscope or using an analog recorder (chart recorder).

Average, amplitude and effective values ​​of alternating voltages are measured by pointer or digital devices for direct assessment or alternating voltage compensators. Instruments for measuring average and amplitude values ​​are used relatively rarely. Most devices are calibrated in effective voltage values. For these reasons, the quantitative values ​​of stresses given in the textbook are given, as a rule, in effective values ​​(see expression (23.25)).

When measuring variable quantities, the shape of the desired voltages is of great importance, which can be sinusoidal, rectangular, triangular, etc. The passports for devices always indicate what voltages the device is designed to measure (for example, to measure sinusoidal or rectangular voltages). In this case, it is always indicated which AC voltage parameter is being measured (amplitude value, average value or effective value of the measured voltage). As already noted, for the most part calibration of devices is used in the effective values ​​of the desired alternating voltages. Because of this, all further considered variable voltages are given in effective values.

To expand the measurement limits of alternating voltage voltmeters, additional resistances, instrument transformers and additional capacitances (with electrostatic system devices) are used.

The use of additional resistances to expand the measurement limits has already been discussed in subsection 23.2 in relation to DC voltmeters and therefore is not considered in this subsection. Voltage and current measuring transformers are also not considered. Information on transformers is given in the literature.

With a more detailed consideration of the use of additional capacitances, one additional capacitance can be used to expand the measurement limits of electrostatistics of voltmeters (Fig. 23.3, A) or two additional containers can be used (Fig. 23.3, b).

For a circuit with one additional capacitance (Fig. 23.3, A) measured voltage U distributed between the voltmeter capacitance C y and additional capacity C is inversely proportional to the values S y and S

Considering that U c = U- Uy, can be written down

Rice. 23.3. Scheme for expanding electrostatic measurement limits

voltmeters:

A- circuit with one additional capacity; b- circuit with two additional containers; U- measured alternating voltage (rms value); C, C, C 2 - additional containers; Cv- capacity of the electrostatic voltmeter used V; U c- voltage drop across additional capacitance C; U v - electrostatic voltmeter reading

Solving equation (23.27) for U, we get:

From expression (23.28) it follows that the greater the measured voltage U Compared to the maximum permissible voltage for a given electrostatic mechanism, the smaller the capacitance should be WITH compared to capacity With u.

It should be noted that formula (23.28) is valid only with ideal insulation of the capacitors forming the capacitances WITH And C v . If the dielectric that insulates the capacitor plates from each other has losses, then additional errors arise. In addition, the voltmeter capacity C y depends on the measured voltage U, since from U The readings of the voltmeter and, accordingly, the relative positions of the moving and fixed plates that form the electrostatic measuring mechanism depend. The latter circumstance leads to the appearance of another additional error.

The best results are obtained if, instead of one additional capacitance, two additional capacitors C (and C 2) are used, forming a voltage divider (see Fig. 23.3, b).

For a circuit with two additional capacitors, the following relation is valid:

Where U a - voltage drop across capacitor C y

Considering that can be written down

Solving equation (23.30) for U, we get:

From expression (23.31) we can conclude that if the capacitance of the capacitor C 2 to which the voltmeter is connected significantly exceeds the capacitance of the voltmeter itself, then the voltage distribution is practically independent of the voltmeter reading. In addition, at C 2 " C y change in insulation resistance of capacitors C, and C 2 and frequency

Table 23.3

Limits and errors of measurement of alternating voltages

the measured voltage also has little effect on the instrument readings. That is, when using two additional containers, additional errors in measurement results are significantly reduced.

The limits for measuring alternating voltages with devices of different types and the smallest errors of these devices are given in Table. 23.3.

As examples, Appendix 5 (Table A.5.1) shows the technical characteristics of universal voltmeters that allow measuring, among other things, alternating voltages.

In conclusion, the following should be noted.

Errors in measuring currents (direct and alternating) with devices of the same type and under equal conditions are always greater than errors in measuring voltages (both direct and alternating). The errors in measuring alternating currents and voltages with devices of the same type and under equal conditions are always greater than the errors in measuring direct currents and voltages.

More detailed information on the issues raised can be obtained from.

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 supplies.

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 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 quantity 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:

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)

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, large 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 used widely enough, 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 1st circuit under study;

Large reduced error 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 the 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 on 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, you need 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

where is the signal repetition period;

- a function that describes the law of change in the instantaneous voltage value. The operator may not always 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 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:

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 :

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

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

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 voltage. 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.

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:

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.

Instruments for measuring voltage and current can be classified according to various criteria:

• - by type of reading device (analog and digital);
• - by measurement method (direct assessment (direct action) and comparison with the measure);
• - by the value of the measured voltage (peak values, average rectified values, rms values);
• - by type of entrance (open or closed).

Currently, a large number of electromechanical and electronic instruments for measuring voltages and currents are in use. Let's consider the principles of their construction.

Electromechanical voltmeters and ammeters

Electromechanical voltmeters and ammeters are direct-acting analog instruments in which the electrical measured quantity is directly converted into a reading from a reading device.

In the simplest case, electromechanical voltmeters and ammeters are a measuring mechanism with a reading device (see Chapter 1), equipped with input terminals for connection to the measurement object.

The generalized block diagram of an electromechanical voltmeter (ammeter) can be represented as a series-connected input measuring circuit and a measuring mechanism with a reading device. Note that the combination of a measuring mechanism and a reading device is usually called a meter.

The input measuring circuit (input device) contains, as a rule, one or more measuring transducers, with the help of which the measured quantity X converted to value Y, convenient for influencing the measuring mechanism.

Most often in electromechanical devices, scaling and normalizing measuring transducers are used, as well as value converters (see Chapter 1).

Almost most known types of measuring mechanisms (MM) can be used to measure voltages and currents.

To measure direct voltages in a wide range of values ​​(from fractions of millivolts to hundreds of volts), electromechanical voltmeters with a magnetoelectric measuring mechanism (MEMM) are used. These devices have a relatively high accuracy class (up to 0.05), but their input resistance does not exceed tens of thousands of ohms, which can lead to significant systematic errors. Systematic errors of voltmeters with MEIM are also of a temperature nature due to the dependence of the resistance of the device frame on the ambient temperature.

Less commonly, electromechanical voltmeters with electrostatic IM (ESIM), electromagnetic IM (EMIM) and electrodynamic IM (EDIM) are used to measure constant voltages.

Voltmeters with ESIM are usually used to measure high voltages (kilovoltmeters), and voltmeters with EDIM are used as reference instruments when testing measuring instruments of a lower accuracy class.

To measure direct currents in a wide range of values ​​(10 - 7 ... 50 A), electromechanical devices (ammeters) with MEIM are most widely used, as well as when measuring direct voltages. These devices are also characterized by a temperature systematic error (especially when using shunts), since in this case, due to different values ​​of the temperature coefficients of the frame and shunt material, a redistribution of the currents flowing through them occurs. To measure direct currents, ammeters with EMIM and EDIM are also used.

Measurement of alternating voltages is carried out with voltmeters with EMIM, EDIM, FDIM, ESIM, thermoelectric instruments, as well as rectifier voltmeters, i.e. voltmeters having a measuring mechanism of the magnetoelectric system and a rectifier (converter) connected at the input of the IM.

Alternating currents are measured with thermoelectric and rectifier ammeters, as well as ammeters with electromagnetic and electrodynamic IMs. Small alternating currents are usually measured with rectifier ammeters. The widest range of measured alternating currents is provided by rectifier ammeters; they are more often used to measure small currents. The widest frequency range of measured currents is provided by thermoelectric system ammeters.

Most electromechanical devices have low input resistance (kilo-ohms), so they are suitable for measuring voltage only in low-impedance circuits. In circuits with high-resistance loads (megaohms), these devices (with the exception of electrostatic ones) cannot be used, since when they are turned on, the load is shunted and thereby the electrical mode of the circuit changes. In addition, typical disadvantages for analog electromechanical devices are the small frequency range in which they give reliable readings, large input capacitances and inductances, and the dependence of the input resistance on frequency.

In practice, universal electromechanical instruments for measuring direct and alternating voltages and currents, as well as direct current resistance - avometers (multimeters) - have become widespread. They are a combination of additional resistors or shunts, converters of values ​​of measured alternating currents and voltages (semiconductor rectifiers) and an IM of a magnetoelectric system with a reading device.

A variant of the avometer circuit for measuring DC voltage is shown in Fig. 5.4.

Rice. 5.4.

The switch changes the measurement range, but the input resistance of the voltmeter, measured in [Ohm/V], usually remains constant when the range is changed due to the selection of resistors.

For example, if L, = 15 MOhm, I 2 = 4 MOhm, /?, = 800 kOhm, /? 4 = 150 kOhm, L 5 = 48 kOhm, and the ranges are respectively 1000,250,50, 10, 2.5 V, then if the device winding resistance is 2 kOhm, the input resistance of the device in any position of the range switch will be equal to 20 kOhm/V.

In practice, voltage measurements have to be performed quite often. Voltage is measured in radio engineering, electrical devices and circuits, etc. The type of alternating current can be pulsed or sinusoidal. Voltage sources are either current generators.

Types of voltage measurements

Pulse current voltage has amplitude and average voltage parameters. Sources of such voltage can be pulse generators. Voltage is measured in volts and is designated “V” or “V”. If the voltage is alternating, then the symbol “ ~ ", for constant voltage the symbol "-" is indicated. The alternating voltage in the home household network is marked ~220 V.

These are instruments designed to measure and control the characteristics of electrical signals. Oscilloscopes work on the principle of deflecting an electron beam, which produces an image of the values ​​of variable quantities on the display.

AC voltage measurement

According to regulatory documents, the voltage in a household network must be equal to 220 volts with a deviation accuracy of 10%, that is, the voltage can vary in the range of 198-242 volts. If the lighting in your home has become dimmer, lamps have begun to fail frequently, or household devices have become unstable, then to identify and eliminate these problems, you first need to measure the voltage in the network.

Before measurement, you should prepare the existing measuring device for use:

• Check the integrity of the insulation of control wires with probes and tips.
• Set the switch to AC voltage, with an upper limit of 250 volts or higher.
• Insert the test leads into the sockets of the measuring device, for example. To avoid mistakes, it is better to look at the designations of the sockets on the case.
• Turn on the device.

The measurement limit of 700 volts is selected on the multimeter. Some devices require that several different switches be set to the desired position in order to measure voltage: the type of current, the type of measurement, and also insert the wire tips into certain sockets. The end of the black tip in the multimeter is inserted into the COM socket (common socket), the red tip is inserted into the socket marked “V”. This socket is common for measuring any kind of voltage. The socket marked “ma” is used for measuring small currents. The socket marked “10 A” is used to measure a significant amount of current, which can reach 10 amperes.

If you measure the voltage with the wire inserted into the “10 A” socket, the device will fail or the fuse will blow. Therefore, you should be careful when performing measuring work. Most often, errors occur in cases where the resistance was first measured, and then, forgetting to switch to another mode, they begin to measure the voltage. In this case, a resistor responsible for measuring resistance burns out inside the device.

After preparing the device, you can begin measurements. If nothing appears on the indicator when you turn on the multimeter, this means that the battery located inside the device has expired and requires replacement. Most often, multimeters contain “Krona”, which produces a voltage of 9 volts. Its service life is about a year, depending on the manufacturer. If the multimeter has not been used for a long time, the crown may still be faulty. If the battery is good, the multimeter should show one.

The wire probes must be inserted into the socket or touched with bare wires.

The multimeter display will immediately display the network voltage in digital form. On a dial gauge, the needle will deviate by a certain angle. The pointer tester has several graduated scales. If you look at them carefully, everything becomes clear. Each scale is designed for a specific measurement: current, voltage or resistance.

The measurement limit on the device was set to 300 volts, so you need to count on the second scale, which has a limit of 3, and the readings of the device must be multiplied by 100. The scale has a division value equal to 0.1 volts, so we get the result shown in the figure, about 235 volts. This result is within acceptable limits. If the meter readings constantly change during measurement, there may be poor contact in the electrical wiring connections, which can lead to sparking and network faults.

DC voltage measurement

Sources of constant voltage are batteries, low-voltage or batteries whose voltage does not exceed 24 volts. Therefore, touching the battery poles is not dangerous and there is no need for special safety measures.

To assess the performance of a battery or other source, it is necessary to measure the voltage at its poles. For AA batteries, the power poles are located at the ends of the case. The positive pole is marked “+”.

Direct current is measured in the same way as alternating current. The only difference is in setting the device to the appropriate mode and observing the polarity of the terminals.

The battery voltage is usually marked on the case. But the measurement result does not yet indicate the health of the battery, since the electromotive force of the battery is measured. The duration of operation of the device in which the battery will be installed depends on its capacity.

To accurately assess the performance of the battery, it is necessary to measure the voltage with a connected load. For a AA battery, a regular 1.5-volt flashlight light bulb is suitable as a load. If the voltage decreases slightly when the light is on, that is, by no more than 15%, therefore, the battery is suitable for operation. If the voltage drops significantly more, then such a battery can only serve in a wall clock, which consumes very little energy.