How to measure voltage with a multimeter. AC voltage measurement Voltage is measured using

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 chains direct current voltages up to 500V usually use additional resistances 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.

Voltmeter is a measuring device that is designed to measure voltage direct or alternating current 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, the alternating voltage of a household network of 220 Volts is briefly designated 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 on-board network of a car or battery is indicated as follows: 12 V or 12 V, and batteries for a flashlight or camera: 1.5 V or 1.5 V. On the body in mandatory marking is applied near the positive terminal in the form of a sign " + ".

Polarity AC 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, touching the bare wires is unacceptable, as this can lead to 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 the light bulbs in the apartment began to burn dimly or often burn out, the work became unstable Appliances, then 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 tester has selected the AC voltage measurement limit of 300 V, and the multimeter has 700 V. 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 required 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 applied. 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 there is a constant change in the value of the least significant digits, and the tester’s needle constantly fluctuates, it 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) is measured driving force), and not the capacity of the battery, which determines the operating life of the product in which it will be installed.

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 there is no measuring device, 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.

Basic unit of measurement electrical voltage is 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 electric circuit diagrams a voltmeter is indicated by a circle with a capital Latin letter " V" inside. Near symbol the voltmeter indicates it letter designation « 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.

Voltage is measured between two points schemes: in electronic circuits ah 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: DC voltage has five subranges, AC voltage has two.

Each subrange has its own maximum measurement limit, which is indicated 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 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 certainty that the voltage of a 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 it is possible to measure, for example, output voltage secondary windings of step-down transformers, or any other 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 measurement high voltage 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.

To measure the EMF, or the voltage on a specific current source, use a device called a Voltmeter. To connect the Voltmeter to the terminals of the measurement source, remote probes are used. By type, indicators are divided into digital and pointer.

In order to measure alternating or direct current, various instruments are used. The devices can be universal and measure both one and another type of current. These include a voltmeter brand “E533”

Both direct and alternating voltage are measured in Volts. In Latin it is designated “V”, in Russian “V”. If the voltage is constant, the symbol “-” is placed before the letter; if the voltage is variable “ ~ " Let's say the AC network is designated in two ways: ~220V or 220V. On accumulators and batteries the marking is applied without a sign.

The battery voltage is indicated 1.5V or 1.5V. The vehicle network is indicated as follows 12V, 12V. The positive terminal must be marked with a “+” sign. Different instruments are needed to measure each type of current. This is due to the fact that the polarity of direct current does not change over time, but that of alternating current does. For example, we have a household network that changes 50 times every 1 second. The frequency of changes is measured in Hertz, 1 Hz is equal to 1 change in voltage polarity per 1 second.

How to measure voltage in household electrical wiring

The requirements of GOST 13109-97 state that the voltage in the electrical network should not exceed 220V±10%. Minimum voltage in this case it will be equal to 198 V, the maximum is 242 V. If household appliances do not work stably, the light bulbs burn dimly or burn out, the first thing you should do is measure the voltage of the electrical wiring.

Before measurements, prepare the device:

The picture shows that the measurement limit in the tester is set to 300 V, in the multimeter - 700 V. Many tester models require switches to be set to several positions: type of measurement (Ohms, A, B); type of current (-, ~), and also install the ends of the probes into the required sockets. The multimeter requires installing a black probe in the COM port (whatever the measurement is), a red one in V (voltage, frequency, resistance measurement). The ma socket is designed to measure small currents, the 10 A socket is for currents not exceeding 10 A.

Carefully! If you insert a plug into the 10 A socket and measure the voltage, the device will fail. If there is a fuse, this will save the device. If not, you will have to purchase a new one. This happens quite often. I've come across quite a few devices with burnt out resistors. After everything has been done, you can start measuring.

If there are no numbers on the display when you turn on the device, the batteries are not inserted or are damaged. Most often, multimeters use Krona, which provides 9 V power. This battery will last for a year. Therefore, if the device was not used for a long time, the battery could be discharged. In stationary conditions, it is better to use a ~220V/–9V adapter instead of a crown. The ends of the probes are inserted into the socket.

The multimeter will start working, and here are the readings pointer device you need to be able to read. At first glance, this is a rather complicated operation. The “TL-4” device, which I have had for over 40 years, has 5 scales. The upper scale is for readings that are multiples of 1 (0.1, 1, and so on).

The scale is lower for numbers that are multiples of 3 (0.3, 3, and so on). If alternating current is measured, the value of which is 1 V, 3 V, there are 2 auxiliary scales. A special scale is applied under the resistance. All testers are made according to this principle, only the multiplicity of numbers may differ.

We will take readings from the second scale, multiplying them by 100. Because the probe is inserted at “~ 300V”. The small division price is 0.1. Therefore, 2.3 + take into account that the arrow is between the strokes, it turns out 2.35 * 100 = 235 V. This voltage is within the permissible range. If a constant deviation of the arrow is observed during measurements, it is necessary to check the contact of the connections. If it's bad, have it reviewed.

How to Measure the DC Voltage of a Battery
battery or power supply

Since the current source is no more than 24 V, which does not threaten human life, there is no need to adhere to safety measures. To determine the suitability of a battery, power supply or accumulator for further operation, you should measure the voltage at the terminals. On the battery, the terminals are located at the ends. Positive is marked as a “+” sign

Measuring direct current is no different from measuring alternating current. You just need to install the device in the required measuring range and measure, observing the polarity.

To make a more accurate assessment of the capacitance, it is necessary to measure the voltage under the load that is applied to the poles. For a battery with a voltage of 1.5 V, a load in the form of a 1.5 V incandescent lamp is suitable. To make it convenient to carry out test work, it can be soldered to the battery through conductors. If the voltage deviation from the norm is no more than 15%, the battery is suitable.

If the device is not available, you can determine the degree of discharge by the glow of the light bulb. However, this method will not provide a guarantee. It will only confirm that the battery can be used at this time. If the light bulb is dim, do not throw away the battery. It can be installed in a wall clock, in which it will serve for a long time. This is because the current consumption of the clock is too small.

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 converters, as well as converters of quantity values ​​are used (see Chapter 1).

Almost most known types of measuring mechanisms (MMs) 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 relatively high class accuracy (up to 0.05), however, 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 for testing measuring instruments more low class accuracy.

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 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.