Is it possible to measure frequency response with an alternating voltage voltmeter? Voltage measurement. Types and principle of measurements. Peculiarities. Amplitude voltmeters

The principle of operation of an electronic alternating voltage voltmeter is to convert alternating voltage into direct voltage, directly proportional to the corresponding value of alternating voltage, and measure DC voltage electromechanical measuring instrument or digital voltmeter.

The AC voltage value measured by an electronic voltmeter is determined by the type of AC-to-DC measuring converter used. Let's consider the design of electronic voltmeters of alternating voltages, requirements for individual elements, design features and their metrological characteristics.

Amplitude voltmeters

The deviation of the amplitude voltmeter indicator is directly proportional to the amplitude (peak) value of the alternating voltage, regardless of the shape of the voltage curve. None of the electromechanical measuring instrument systems have this property. Electronic peak-to-peak voltmeters use peak detectors with open and closed inputs.

The required sensitivity (the lower limit of measured voltages is a few millivolts) is achieved by using a UPT with a high gain after the detector.

Fig. Figure 2 shows a simplified block diagram of an amplitude voltmeter with a closed input, built according to a balancing conversion circuit.

Measured voltage U x supplied through an input device to the input of a peak detector with a closed input (VD1, C1, R1). To an identical detector (VD2, C2, R2) a compensating voltage is supplied with a frequency of about 100 kHz, generated in the circuit feedback. DC voltages equal to the amplitude values ​​of the measured signal and the compensating voltage are compared across resistors R1,R2. It should be noted that at low voltages the detectors will operate in quadratic mode, which will lead to an error in the amplitude value of the voltmeter.

The difference voltage is supplied to the UPT A1 with high gain. If the voltage at the output of the UPT has a positive polarity, which indicates that the signal voltage exceeds the compensating voltage or the absence of the latter, the previously locked generator-modulator is started, and the compensating voltage is supplied through the feedback divider to the detector VD2, R2, C2. The oscillator-modulator is a generator assembled using a capacitive three-point circuit, an amplifier and an emitter follower.

The excess of the compensating voltage over the measured one leads to blocking of the generator-modulator. The output voltage with an amplitude proportional to the amplitude of the measured voltage and a frequency of 100 kHz is supplied to the average rectified voltage detector U1 and is measured by a magnetoelectric voltmeter PV1.

An important requirement is the identity of the transfer characteristics of the signal detectors and the compensating voltage. Only with identical characteristics will the equality of the output voltages of the detectors indicate the equality of the input voltages.

In steady state on resistors R1 and R2 a certain voltage difference is formed and is equal to

(1)

Where TO and β are the transmission coefficients of the direct conversion and feedback circuit.

In this circuit, the direct conversion circuit includes a UPT, a generator-modulator, and the reverse circuit includes a divider in the feedback circuit and a compensating signal detector. Thus, to ensure high balancing accuracy, the gain of the amplifier and the generator-modulator must be quite high.

The components of the error are: the error of standard means during calibration, the random error of measuring direct voltage with a magnetoelectric device, the error caused by the instability of the feedback circuit transmission coefficient and the average-rectified transmission coefficient of the detector, non-identical characteristics of the detectors, and imbalance of the circuit.

Commercially produced amplitude millivoltmeters V3-6, V3-43 operate according to a similar scheme. The main error at frequencies up to 30 MHz is 4...6%, at frequencies up to 1 GHz – 25%. The scales of amplitude voltmeters are graduated in rms values ​​of sinusoidal voltage. The disadvantage is the large error when measuring voltages with a high level of harmonic components.

We have already considered that alternating voltage is characterized by instantaneous, average, average-rectified and root-mean-square values.

Most scales of voltmeters, except pulse ones, are calibrated in root-mean-square (rms) values, which are equal to 0.707 of the amplitude value. If the shape coefficients are known, then one of the parameters can be used to determine the others. When measuring sinusoidal voltages, the instantaneous value (amplitude) is determined as U=Umeas*1.41, where Umeas is the effective value or U=1.1*Usv (if the average rectified value is measured). When measuring non-sinusoidal signals, corrections must also be made to the readings.

Electromechanical, thermoelectric and electronic devices are used to measure alternating voltage. The choice of device is determined by the voltage limits, measurement conditions, and required accuracy.

Among electromechanical devices, devices of electromagnetic, electrodynamic and electrostatic systems are mainly used.

AC voltmeters are classified according to various criteria:

by purpose: pulse, alternating current, phase-sensitive, selective, universal;

by measurement method: direct assessment and comparison with the measure;

according to the measured voltage parameter: amplitude, root-mean-square and mean-rectified;

by type of indicator: pointer and digital.

Most electromagnetic system voltmeters are used at frequencies of 50 Hz. Accuracy class - 2.5 - 0.5. Electrodynamic voltmeters have the same frequency range, but more high class accuracy (0,1). The scale equation is quadratic in nature. Advantages: simplicity of design, possibility of direct use in alternating voltage circuits, reliability. Disadvantages - low sensitivity, high consumption from the measuring circuit, uneven scale.

Electrostatic voltmeters are used to measure high (up to 100 kV) voltages. Accuracy class 1.

Measuring high frequency voltage has its own characteristics. In order for the device not to influence the measuring circuit, it is necessary that its input resistance be large and the input capacitance as small as possible.

In the practice of radio-electronic measurements, electronic and rectifier voltmeters are most widely used. This is explained by the fact that electronic voltmeters have high input impedance at both high and low frequencies, high sensitivity when using an amplifier, and low consumption from the measuring circuit.

Measurement of alternating voltage by direct estimation method.

Electronic voltmeters.

Block diagrams of electronic voltmeters are built mainly according to two schemes: millivoltmeters and voltmeters for measurements high voltages. They are presented in Figure M2-8.

Figure M2-8. Electronic voltmeters for measuring alternating voltages.

Voltmeters for measuring high voltages consist of an input device, an AC-to-DC voltage converter (detector), a DC amplifier and a magnetoelectric system meter. Millivoltmeters are distinguished by the presence of an alternating voltage amplifier before the detector, which serves to increase sensitivity.

Average voltmeters are built according to structural diagram the first type from AC to DC voltage converters based on the average value. The simplest average voltmeters are rectifier voltmeters with converters made on diodes.

Selective voltmeters.

Selective, i.e. Selective microvoltmeters are widely used to study the spectrum of non-periodic signals. These are highly sensitive heterodyne receivers tuned to a specific frequency or a narrow frequency range. A simplified diagram of a selective voltmeter is shown in Figure M2-9.

Figure M2-9. Selective voltmeter circuit

The measured frequency signal Fc is fed through the input device to the mixer, where the signal from the local oscillator also arrives. In the mixer, the measured signal is converted to an intermediate frequency and amplified by the amplifier. At the output of the amplifier there is a voltmeter with a digital or dial indicator.

Pulse voltmeters. Pulse voltages are measured using pulse voltmeters, which are built according to the circuit of an analog electronic voltmeter with an amplitude detector. In these circuits, the pulse voltage is converted into DC voltage and its value is measured. In this circuit, it is possible to measure the amplitude of only positive pulses; for negative ones, the diode must be turned on in reverse. Special pulse voltmeters are calibrated in amplitude values. Very often, oscillographic measurement methods are used, which allow not only to measure the amplitude of pulses, but also to observe their shape.

Voltmeter is a measuring device that is designed to measure voltage permanent or AC in electrical circuits.

The voltmeter is connected in parallel to the terminals of the voltage source using remote probes. According to the method of displaying measurement results, voltmeters are divided into dial and digital ones.

The voltage value is measured in Voltach, indicated on instruments by the letter IN(in Russian) or Latin letter V(international designation).

On electrical diagrams The voltmeter is designated by the Latin letter V, surrounded by a circle, as shown in the photograph.

Voltage can be constant or alternating. If the voltage of the current source is alternating, then the sign " is placed in front of the value ~ "if constant, then the sign" – ".

For example, AC voltage household network 220 Volt is briefly denoted as follows: ~220 V or ~220 V. When marking batteries and accumulators, the sign " – " is often omitted, a number is simply printed. The voltage of the vehicle's or battery's power supply is indicated as follows: 12 V or 12 V, and batteries for a flashlight or camera: 1.5 V or 1.5 V. The housing must be marked near the positive terminal in the form of a " + ".

The polarity of alternating voltage changes over time. For example, the voltage in household electrical wiring changes polarity 50 times per second (the frequency of change is measured in Hertz, one Hertz is equal to one voltage polarity change per second).

The polarity of direct voltage does not change over time. Therefore, different measuring instruments are required to measure AC and DC voltage.

There are universal voltmeters that can be used to measure both alternating and direct voltage without switching operating modes, for example, the E533 type voltmeter.

How to measure voltage in household electrical wiring

Attention! When measuring voltages above 36 V, it is unacceptable for a person to touch the exposed wires, as they may receive an electric shock.

According to the requirements of GOST 13109-97, the effective voltage value in the electrical network must be 220 V ±10%, that is, it can vary from 198 V to 242 V. If light bulbs in the apartment begin to burn dimly or often burn out, or household appliances begin to work unstably, then in order to take action, you must first measure the voltage value in the electrical wiring.

When starting measurements, it is necessary to prepare the device: – check the reliability of the insulation of conductors with tips and probes; – set the switch of measurement limits to the position of measuring alternating voltage of at least 250 V;

– insert the connectors of the conductors into the sockets of the device, guided by the inscriptions next to them;

– turn on the measuring device (if necessary).

As you can see in the picture, the limit for changing the alternating voltage is 300 V in the tester, and 700 V in the multimeter. In many tester models, you need to set several switches to the required position at once. Type of current (~ or –), type of measurement (V, A or Ohms) and also insert the ends of the probes into the desired sockets.

In a multimeter, the black end of the probe is inserted into the COM socket (common for all measurements), and the red end into V, common for changing DC and AC voltage, current, resistance and frequency. The socket marked ma is used to measure small currents, 10 A when measuring current reaching 10 A.

Attention! Measuring voltage while the plug is inserted into the 10 A socket will damage the device. In the best case, the fuse inserted inside the device will blow out; in the worst case, you will have to buy a new multimeter. They especially often make mistakes when using instruments to measure resistance, and, forgetting to switch modes, measure voltage. I've met dozens of such faulty devices with burnt resistors inside.

After all the preparatory work has been completed, you can begin measuring. If you turn on the multimeter and no numbers appear on the indicator, it means that either the battery is not installed in the device or it has already exhausted its resource. Typically, multimeters use a 9 V Krona battery with a shelf life of one year. Therefore, even if the device has not been used for a long time, the battery may not work. When using the multimeter in stationary conditions, it is advisable to use a ~220 V/–9 V adapter instead of the crown.

Insert the ends of the probes into the socket or touch them to the electrical wires.

The multimeter will immediately show the voltage in the network, but you still need to be able to read the readings in a dial tester. At first glance, it seems difficult, since there are many scales. But if you look closely, it becomes clear on which scale to read the device. The TL-4 type device in question (which has served me flawlessly for more than 40 years!) has 5 scales.

The upper scale is used to take readings when the switch is in positions that are multiples of 1 (0.1, 1, 10, 100, 1000). The scale located just below is multiples of 3 (0.3, 3, 30, 300). When measuring AC voltages of 1 V and 3 V, 2 additional scales are marked. There is a separate scale for measuring resistance. All testers have a similar calibration, but the multiplicity can be any.

Since the measurement limit was set to ~300 V, it means that the reading must be made on the second scale with a limit of 3, multiplying the readings by 100. The value of a small division is 0.1, therefore, it turns out 2.3 + the arrow is in the middle between the lines, which means take the reading value 2.35×100=235 V.

It turned out that the measured voltage value is 235 V, which is within acceptable limits. If during the measurement process it is observed constant change the values ​​of the low-order digits, and the tester’s arrow constantly fluctuates, which means that there are bad contacts in the electrical wiring connections and it is necessary to inspect it.

How to measure battery voltage
battery or power supply

Since the voltage of DC sources usually does not exceed 24 V, touching the terminals and bare wires is not dangerous to humans and no special safety precautions are required.

In order to assess the suitability of a battery, accumulator or the health of the power supply, it is necessary to measure the voltage at their terminals. The terminals of round batteries are located at the ends of the cylindrical body, the positive terminal is indicated by a “+” sign.

Measuring DC voltage is practically not much different from measuring AC voltage. You just need to switch the device to the appropriate measurement mode and observe the polarity of the connection.

The amount of voltage that a battery creates is usually marked on its body. But even if the measurement result showed sufficient voltage, this does not mean that the battery is good, since the EMF (electromotive force) was measured, and not the capacity of the battery, on which the operating life of the product in which it will be installed depends.

To more accurately estimate the battery capacity, you need to measure the voltage by connecting a load to its poles. An incandescent flashlight light bulb rated for a voltage of 1.5 V is well suited as a load for a 1.5 V battery. For ease of operation, you need to solder conductors to its base.

If the voltage under load decreases by less than 15%, then the battery or accumulator is quite suitable for use. If not measuring instrument, then you can judge the suitability of the battery for further use by the brightness of the light bulb. But such a test cannot guarantee the battery life of the device. It only indicates that the battery is currently still usable.

To measure alternating voltage, analog electromechanical devices (electromagnetic, electrodynamic, rarely induction), 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//) the 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 arrows or digital devices 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. From these considerations, the quantitative stress values ​​given in textbook, are given, as a rule, in effective values ​​(see expression (23.25)).

When measuring variables great value has the shape of the desired voltages, 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 direct voltage 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 capacitors 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 the 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.

Limits for measuring alternating voltages with devices different types and the smallest errors of these devices are given in table. 23.3.

As examples, Appendix 5 (Table A.5.1) shows technical specifications universal voltmeters that allow you to measure, among other things, alternating voltages.

In conclusion, the following should be noted.

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

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.

Pulse current voltage has amplitude and average voltage parameters. The 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 devices designed to measure and control characteristics 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, the lamps have begun to fail more often, or household devices began to work unstably, then to identify and eliminate these problems, you first need to measure the voltage in the network.

Before measuring, you should prepare your 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 figure shows that the measurement limit of 300 volts is selected on the tester, and 700 volts 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 pointer device the arrow 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. Positive pole marked “+”.

D.C measured in the same way as a variable. 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 AA battery A regular 1.5 volt flashlight 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.