Rules for measuring AC and DC current with a multimeter. A device for measuring current strength. How to measure current with a multimeter

Before we start talking about how to measure current with a multimeter, a few warnings need to be made. Firstly, if you have never used a multimeter or any other device, read the instructions carefully, because otherwise you may burn it on the first day. Secondly, before measuring any indicators, including the current strength in a high-voltage outlet or circuit, practice using more harmless power sources, for example, batteries. Thirdly, if you lack experience, carefully follow all instructions for the device.

Compliance with the rules for handling the device is imperative, since in the best case you can burn the device, and in the worst case you can even get an electric shock. This is due to the fact that all measurements are made under voltage. Also, during the measurement process, you should not neglect standard safety requirements.

How to measure current (amperage)

The current in a circuit is measured by connecting the device in series to it. In practice, this means that, to take measurements, you need to connect both probes from the multimeter to the broken wire. That is, the simplest circuit will look like this: power source – lamp – multimeter – power source. In this case, the device should be set to the A~ indicator (this is the AC symbol) and to the maximum value. The DC icon is very similar, so be careful not to get confused. Next you can take measurements.

Many people are interested in what the current strength is in a 220V outlet and how to check the current strength of a battery or battery. This type of question is incorrect for one simple reason - it is impossible to check the current strength of power supplies, since it is measured exclusively in the circuit. And to determine the current strength in a circuit, you need to create a circuit from a power source, some kind of device and a multimeter. However, we note that most modern household outlets are designed for a current of 16A.

How to measure the voltage in an outlet

Measuring the voltage at an outlet should only be done using multimeters rated for currents of 20A or more. If your device is designed for measurements in the range up to 6A, then when you try to take measurements it will simply burn out. Set the multimeter to measure AC voltage (V~ or AVC) and set the indicator to 750V. Next, connect the black probe to the COM port, and then connect the red probe. Now turn on the device and insert the probes into the socket, look at the screen and write down the readings.

To see how to do this correctly, watch the video:

How to check resistance with a multimeter

In order to measure resistance, set the multimeter control to the Ω (Ohm) sector and select the units of measurement K (KiloOhms) or M (MegaOhms). Next, we simply turn on the device, connect the probes to two contacts of the object being measured and look at the indicators. You should not try to measure the resistance in the socket; it is pointless and dangerous for the device. However, you can always measure the resistance of your own body; to do this, simply turn on the device, take the black probe in one hand and the red one in the other and look at the indicators.

Visualization of how to measure resistance

During the operation of an electrical network or any device, it is necessary to measure the current strength.

From this article you will learn what is meant by this term and what tools are used for this purpose.

At the same time, we’ll talk about safety measures when carrying out such work.

Current unit

In physics, current strength is usually called the amount of charge that crosses the cross section of a conductor per unit time. The unit of measurement is ampere (A). A current of 1 A is such that in 1 second a charge of 1 coulomb (C) passes through the cross-section of the conductor.

The current strength can be compared to the pressure of water. As you know, in the old days small rivers were blocked with dams to create a pressure capable of turning a mill wheel.

The stronger the pressure, the more productive the mill could be set in motion with its help.

In the same way, current strength characterizes the work that electricity can do. A simple example: a light bulb will burn brighter as the current in the circuit increases.

Why do you need to know how much current flows in a conductor? The strength of the current determines how it will act on a person in case of accidental contact with live parts. We display the effect produced by electricity in the table:

Here are a few more reasons:

1. The current strength characterizes the load on the conductor. The maximum throughput of the latter depends on the material and cross-sectional area. If the current is too high, the wire or cable will become very hot. This may cause the insulation to melt, resulting in a short circuit. This is why wiring is always protected from overloads by circuit breakers or fuses. Owners of apartments and houses with old wiring should pay special attention to the current flowing in the wires: due to the use of an increasing number of electrical appliances, it often becomes overloaded.
2. Based on the ratio of current values ​​in various circuits of an electrical appliance, we can conclude that it is working properly. For example, currents of equal strength must flow in the phases of an electric motor. If discrepancies are observed, the engine is faulty or is overloaded. The condition of a heating device or electric “warm floor” is determined in the same way: the current strength in all components of the device is measured.

The work of electricity, or more precisely its power (the amount of work per unit of time), depends not only on the current strength, but also on the voltage. As a matter of fact, the product of these quantities determines the power:

W = U * I,

• W – power, W;
• U – voltage, V;
• I – current strength, A.

Thus, knowing the voltage in the network and the power of the device, you can calculate how much current will flow through it provided it is in good condition: I = W/U. For example, if it is known that the power of the heater is 1.1 kW and it operates from a regular 220 V network, then the current strength in it will be: I = 1100 / 220 = 5 A.

Current measurement formula

It should be taken into account that, according to Kirchhoff’s laws, the current strength in the wire before branching is the sum of the currents in the branches. Since in an apartment or house all devices are connected in a parallel circuit, then if, for example, two devices with a current of 5 A are working simultaneously, then a current of 10 A will flow in the supply wire and in the common neutral wire.

The reverse operation, that is, calculating the consumer's power by multiplying the measured current by voltage, does not always give the correct result.

If the consumer device has windings, such as in electric motors, which have inductive reactance, part of the power will be spent on overcoming this resistance (reactive power).

To determine active power (useful work of electricity), you need to know the actual power factor for a given device, which is the ratio of active and reactive power.

Instruments for measuring current and voltage

Here are some measuring tools that will help an electrician in this matter:

Ammeter

1. There are several varieties of this device, which differ in their operating principles: Electromagnetic:
2. There is a coil inside, the current flowing through it creates an electromagnetic field. This field draws the iron core connected to the arrow into the coil. The greater the current, the more the core will be retracted and the more the needle will deviate. Thermal:
3. The device contains a tensioned metal thread connected to an arrow. The flowing current causes heating of the filament, the degree of which depends on the strength of the current. And the more the thread heats up, the more it will lengthen and sag, respectively, the more the arrow will deflect. Magnetoelectric:
4. The device has a permanent magnet, in the field of which there is an aluminum frame connected to the arrow with a wire wound around it. When electric current flows through a wire, the frame in a magnetic field tends to rotate through a certain angle, which depends on the strength of the current flowing. And the position of the arrow, which marks the current value on the scale, depends on the angle of rotation. Electrodynamic:
5. Inside the device there are two series-connected coils, one of which is movable. When current flows through the coils as a result of the interaction of the resulting electromagnetic fields, the moving coil tends to rotate relative to the stationary coil and at the same time pulls the arrow along with it. The angle of rotation will depend on the strength of the current flowing. current is passed through the windings of fixed coils connected by a magnetic system. As a result, a rotating or traveling electromagnetic field is formed, acting with some force (depending on the current strength) on a movable metal cylinder or disk. That one is connected to the arrow.
6. Electronic: Such devices are also called digital. There is an electrical circuit inside, information is displayed on a liquid crystal display.

Multimeter for measuring current

This is what is commonly called a universal electronic current parameter meter. It can switch both to ammeter mode and to voltmeter, ohmmeter and megohmmeter mode (large resistances, usually insulation, are measured).

Measuring current with a multimeter

The measurement results are displayed on a liquid crystal display. The device requires battery power to operate.

Tester

In terms of functionality, this is the same multimeter, but analog. The measurement results are indicated on the scale using an arrow; batteries are required only if you have an ohmmeter.

Clamp meters

Clamp meters are more practical. They just need to clamp the section of the wire being tested, after which the device will show the strength of the current flowing in it.

It should be taken into account that only the conductor being tested should be in the clamps. If you clamp several conductors, the device will show the geometric sum of the currents in them.

Clamp meters

Thus, when placing the entire 1-phase wire into a current clamp, the device will show “zero”, since multidirectional currents of the same magnitude flow in the phase and neutral conductors.

Measurement methods

The first three measuring instruments must be included in the load circuit in series with it, that is, in a wire break. For a 1-phase network, this can be either a phase or neutral wire. For a 3-phase - only phase, since at zero the geometric sum of currents flows in all phases (at the same load it is equal to zero).

Let us note two important circumstances:

1. Unlike a voltmeter (voltage meter), an ammeter cannot be used without a load, otherwise a short circuit will occur.
2. The probes of the device can touch wires or contacts only when there is no voltage, that is, the line being tested must be de-energized. Otherwise, an arc may occur between the closely spaced probe and the wire, generating enough heat to melt the metal.

All measuring instruments have a range switch that adjusts the sensitivity.

Grounding is necessary for the safe operation of electricity. – the most important component of the electrical network.

Transformer 220 to 12 Volts - you will find the purpose and recommendations for manufacturing.

Note that the current consumed by some devices, such as television and computer equipment, energy-saving and LED lamps, is not sinusoidal.

Therefore, some measuring instruments whose operating principle is oriented towards alternating voltage can determine the value of such current with an error.

Video on the topic

Determine the measuring span of your digital multimeter. A multimeter is a small hand-held instrument that can measure voltage, resistance, and current. Each model is designed to measure current within a specific range, and that range should be appropriate for the electrical system you are testing. For example, passing 200A through a multimeter rated at 10A maximum will cause the multimeter's fuse to fail. The maximum measured current is indicated on the multimeter itself or in its instructions.

Select the appropriate operating mode for the multimeter. Most multimeters can operate in several modes, measuring different quantities. To measure current, you must switch to A (current measurement) mode and either AC (alternating current) or DC (direct current), depending on the circuit being tested. The type of current is determined by the circuit's power source. For example, a household source provides AC, and a battery provides DC.

Set the measuring interval on the multimeter. To ensure you don't blow the multimeter's fuse, set the upper limit of this interval well above the expected current value. You can always lower the maximum if the multimeter does not show anything when connected to the circuit.

Insert the connectors into the appropriate sockets. Your multimeter comes with 2 cables, with a probe on one end and a connector on the other. Connect both cables to the sockets designed for measuring current; If these sockets are not clearly marked on the multimeter itself, you can install them by looking at the instructions.

To measure current, connect a multimeter to the circuit. This is extremely dangerous and can result in electric shock when measuring household AC current or current generated by other high voltage or current sources, and sometimes low power sources. Before touching any wires, especially bare wires, turn off all switches and check with an AC current probe to ensure the AC current in your circuit is zero. Do not work in a wet environment or even in high atmospheric humidity - moisture can conduct current. Put rubber gloves on your hands. Additional precautions may also be necessary. Consult a serious book on electrical work (not an online resource) before starting work. Keep in mind that the electrical insulation of the wire may have been damaged during assembly of the circuit or as a result of prolonged use. Insufficient insulation may result in electrical shock. Always have someone nearby with a cell phone to call 911 if necessary. Your partner should also be able to administer first aid and CPR. If you are electrocuted, your partner should pull you away using some non-conductive material (such as dry clothing, but other items may also be needed), otherwise they will also be electrocuted by touching you through the skin, and possibly and through clothing (or other insufficiently insulating material). In any case, consult an electrical safety book before you start measuring and find out what type of electrical signal you will be dealing with. Read in an electrical book (but not on an online resource) about the dangers that await you and how to avoid them. Cut the chain wire at a location that suits your needs. Secure both free ends of the wire and strip them. Securely connect one of them to one probe of the multimeter, the other to the second, so that they do not touch each other. Before taking measurements, make sure that the ends of the wire are pressed tightly against the probes of the device. Make sure the wire, especially the exposed ends, are not touching you. Turn on the circuit switches you previously turned off, and if there is no reading on the multimeter, adjust its scale.

Instruments for measuring alternating current can be different.

To measure industrial frequency current (50 – 100 Hz), direct assessment devices based on electromagnetic and electrodynamic systems, as well as thermoelectric systems, are mainly used.

In low-power high-frequency circuits, the current is measured by rectifier, thermoelectric, electronic digital and analog voltmeters using a resistor with a known resistance. The ammeter must have minimum values ​​of input resistance, inductance and capacitance.

Electromagnetic system devices. The principle of operation of these devices is based on the phenomenon of retraction of a steel plate connected to an arrow by the magnetic field of a coil. The deviation of the moving part of the measuring mechanism depends on the square of the measured current and can be used to measure both direct and alternating current with a frequency not exceeding 5 kHz. By selecting the shape of the core it is possible to obtain an almost uniform scale. Magnetoelectric system ammeters are produced as panel devices of accuracy classes 0.5, 1.0, 2.5 at frequencies up to 1500 Hz, and 0.5, 1.0 – up to 2400 Hz. To expand the limits of current measurement with an electromagnetic ammeter, not shunts are used, but sectional coils or transformers. Advantages: simplicity of design, low cost and reliability. Disadvantages: low accuracy and sensitivity. Electromagnetic ammeters are used for direct measurement of currents up to 200 A; the coil of the measuring mechanism is connected in series to the circuit of the measured current. The measurement limit is determined by the number of turns of the coil. The higher the limit, the fewer turns of thicker wire.

Electrodynamic devices. The principle of operation is based on the interaction of two magnetic fluxes created by currents flowing through two coils, one of which is movable. As a result of the interaction of the magnetic fields of the coils and counteracting springs, the moving coil rotates through a certain angle proportional to the currents in the coils. The effective (rms) value of the current is measured by these devices. The circuits for connecting the coil windings are different. When connected in series, small currents are measured (less than 0.5 A), the instrument scale is quadratic. When the windings are connected in parallel, large currents are measured, the scale is also quadratic. Electrodynamic ammeters are produced in various accuracy classes up to 0.1. They are mainly used at industrial frequencies. To expand the limits, switching the measuring mechanism coils from serial to parallel and current transformers are used.

Rectifying devices.

They are widely used to measure current in the audio frequency range. The operating principle is based on the rectifying properties of the diode. The direct component of the diode-rectified current is measured by a magnetoelectric system device. Typically, half-wave and full-wave rectifiers are used. Rectifier meters measure the average value of the alternating current, not the rms. The instrument scale is calibrated in root-mean-square values, so the readings are recalculated using the form factor. Rectifier instruments for measuring currents are widely used as components of combined instruments: testers, avometers used to measure currents, voltages, and resistances. When using appropriate diodes, rectifier devices can be used in the microwave range. Germanium and silicon diodes provide a frequency range of up to 100 MHz. The main advantages of rectifier devices are high sensitivity, low self-consumption and the ability to measure in a wide frequency range. Disadvantage: low accuracy. The main sources of errors are changes in diode parameters over time. The accuracy class of rectifier devices is 1.5 and 2.5, the measurement limits for current are from 2 mA to 600 A, for voltage from 0.3 to 600 V.

Thermoelectric devices.

They are used to measure high frequency currents. The device consists of a thermal converter, a thermoelement and a measuring device.

The measuring device I is made using a magnetoelectric system. The simplest thermal converter has a heater 2 and a thermocouple 1 made of two dissimilar conductors soldered together. If the measured current is passed through the thermoelement heater, then due to the heating of the junction in the circuit of the thermocouple and the device I, a constant voltage thermocurrent will flow. The device measures the effective value of alternating current. The scale of thermoelectric devices is close to quadratic. The sensitivity depends on the thermocouple material. The advantages of thermoelectric devices are high sensitivity, a large range of current measurements, a wide frequency range, and the ability to measure currents of arbitrary shape. Disadvantages are the unevenness of the scale, which in the initial part turns out to be compressed. In addition, the readings depend on temperature. The general frequency range of thermoelectric devices is from 45 Hz to 300 MHz, rated currents are from 1 mA to 50 A, accuracy classes are from 1.0 to 2.5.

Voltage measurement

DC voltage measurement

When using the direct assessment method, the voltmeter is connected in parallel to the section of the circuit where the voltage needs to be measured. The relative error of voltage measurement is
, i.e. The greater the internal resistance of the voltmeter, the smaller the measurement error.

DC voltage measurements can be performed with any DC voltage meters (magnetoelectric, electrodynamic, electromagnetic, electrostatic, analog and digital voltmeters.) The choice of a voltmeter is determined by the power of the measurement object and the required accuracy. The range of measured voltages ranges from fractions of microvolts to tens of kilovolts.

If the required accuracy can be provided by instruments of the electromechanical group, then this simple method of direct assessment should be preferred. When measuring voltages with higher accuracy, instruments based on the comparison method should be used. Any measurement method can use analog and digital readings.

Direct assessment devices.

Magnetoelectric devices are used to test the modes of radio circuits and are used to measure voltages in devices of other systems. In addition, they are used as indicators. Magnetoelectric system voltmeters have a uniform scale, high accuracy, high sensitivity, but low input resistance.

Electrostatic voltmeters have the advantage of low consumption, independence from ambient temperature, high input resistance, and the disadvantages are an uneven scale and the danger of breakdown between the plates.

Electronic voltmeters are most widely used to measure DC voltage. They can be analog and digital.

Analog electronic DC voltmeters.

Unlike voltmeters of the electromechanical group, electronic DC voltmeters have a high input resistance and low current consumption from the measuring circuit. Figure M2-6 shows a block diagram of an analog electronic voltmeter.

Figure M2-6. Block diagram of an analog electronic DC voltmeter.

The main elements are an input device, a DC amplifier and a magnetoelectric system measuring instrument. The input device contains input terminals, a voltage divider, and a pre-amplifier. A high-resistance resistor divider serves to expand the measurement limits. The DC amplifier serves to increase the sensitivity of the voltmeter and is a power amplifier of the measured voltage to the value necessary to create sufficient torque at the measuring device.

Constant voltage amplifiers are subject to requirements such as high linearity of characteristics and constant gain. The main technical characteristics of DC voltmeters are given in Table M2-3.

Table M2-3. Basic technical characteristics of DC voltmeters.

Measuring DC voltage with digital instruments.

Digital voltmeters are increasingly being used to measure voltages and currents. A simplified block diagram of a digital voltmeter is shown in Fig. M2-7.

Figure M2-7. Block diagram of a digital voltmeter

The input device contains a voltage divider. An analog-to-digital converter (ADC) converts an analog signal into digital form and represents it as a digital code. A digital readout device records the measured value.

Based on the type of ADC, digital voltmeters are divided into pulse-code and time-pulse. Because the ADC converts the DC signal into a digital code, digital voltmeters are considered DC voltage meters. To measure alternating voltage, a converter is installed at the output of the voltmeter.

Based on the type of quantity being measured, digital devices are divided into devices:

for measuring DC voltage;

for measuring alternating voltage;

multimeters (universal voltmeters for measuring voltage, resistance, current)

Digital voltmeters usually have a high input impedance of more than 100 MΩ, measuring ranges of 100 mV, 1 V, 10 V, 100 V, 1000 V. The sensitivity threshold on the 100 mV range can be 10 µV.

A multimeter is a device for measuring various electrical parameters. It allows you to measure direct and alternating voltage, current, resistance, as well as many specific parameters, such as the performance of diodes, transistors, and signal frequency. In order to know how to measure current with a multimeter, you need to understand the basic principles of operation of this device.

It is important to measure current strength when monitoring the correct operation of devices. Often you need to check the charging current level of a battery for a car, laptop, tablet, power-bank.

Various current measurements produced in different ways inside the measuring device. Therefore, there is always an element on the multimeter whose task is to select the parameter, measurement mode and signal level. Sometimes, in more advanced equipment, the signal level is determined automatically.

Typically, the parameter and measurement mode are selected by turning the knob on the multimeter body. The selected characteristics are grouped by their types. They are usually designated as follows:

To measure the necessary indicators, you first need to determine what type of current flows in the circuit being tested. This depends on the power source of the circuit. For example, accumulators and batteries are constant sources of power. To measure DC current, you need to set the rotary knob of the multimeter to the A -, DCA or I - icon, or press the button on the front panel corresponding to the desired mode. Both alternating and direct current are measured in amperes. Therefore, the value on the meter screen will be displayed in this value.

To understand how to measure amperes with a multimeter, you need to know that the current in a section of the circuit is always the same. When the ammeter is connected in series to the circuit (that is, the probes of the device are connected to different points of the circuit break), it will not create a noticeable change in the parameters of the circuit. In this case, it will be able to display the correct value of the flowing current. It is important to connect the meter in the correct polarity, that is, the red probe - to the branch that goes to the plus of the power source, and the black one - to the minus. Otherwise, the device will show negative values.

When preparing for a measurement, it is very important to know what signal level you need to check. If milliamps flow in the circuit, then the red probe must be connected to the meter socket on which V Ω mA is written, or there is a specific measurement limit (usually 300 - 400 mA). If you are checking a power circuit whose values ​​are measured in units of amperes, then the probe must be connected to the socket labeled A or NA (usually 5 to 10 amperes flow here). Neglecting this rule may damage the measuring device. There are more powerful ammeters, but they are used for special purposes.

Having correctly connected the device, you can start working.. The procedure for measuring amperage with a multimeter is as follows:

1. Install the probes into the appropriate sockets of the meter corresponding to the signal level.
2. Select DC mode using the regulator or by pressing the corresponding button on the front panel.
3. If necessary, select the level of the measured signal using a knob or button. The level should be chosen slightly higher than the expected value.
4. Connect the multimeter to the open circuit of the circuit branch, observing the polarity of the connection.
5. Turn on the power source.

In order to evaluate the performance of the simplest portable battery - a battery with a multimeter, just check its voltage and amperage, it is not necessary to use a load. To check, you need to install the red wire in the hole labeled A (NA), select the DC mode and the measurement limit on the front panel of the multimeter, and attach the probes in accordance with the polarity to the battery terminals - red to positive, black to negative. After a few seconds, the meter screen will display the direct current generated by the element.

If the values ​​are in the range of 4 - 6 amperes, then the battery is “fresh” and ready for use. With readings below 4 amperes, it can only be used in low-power devices. For values ​​below 2.5 A, it is better to refuse to use such an element.

The correct voltage values ​​must correspond to those indicated on the battery.

Among the battery parameters the current supplied is important. You can check it with a multimeter, but you need to connect a load in series with the meter. The load can be a regular incandescent lamp. Its resistance does not exceed several hundred ohms, and it can also be measured with a multimeter in resistance measurement mode. To do this, you need to attach the meter probes to the threads of the lamp base and the central terminal. The resistance value will be displayed on the screen.

If we consider the resistance of the multimeter to not make large changes in the current values, then its value should be equal to:

I = U / R, where I is the current in the circuit, amperes, U is the voltage supplied by the battery, and R is the load (lamp) resistance.

The readings of the measuring device must be compared with this calculated value. If the readings differ, the battery may be undercharged.

You can also check the battery leakage current. If you unhook the positive terminal and install a multimeter between it and the positive terminal of the battery, it will show a leak into the vehicle's on-board network. By pulling out the fuses in the car, you can even find out what the leakage is in different parts of the on-board network. With some experience, it is possible not only to learn how to measure amperes with a multimeter, but also to determine the causes of some car electrical faults.

Measuring current when charging a battery

Most car battery chargers have indicators that indicate charging parameters. But if they are faulty or missing, the charge current can be shown by a multimeter. When recharging the battery, you can connect a measuring device to the charging circuit. To display correct readings you need:

1. Install the red probe into the hole in the device marked A (NA), the black probe is usually connected to the input marked COM;
2. Select DC measurement mode and signal level;
3. Connect the positive terminal of the charger in series with the black probe of the multimeter, connect the red probe of the meter to the positive terminal of the battery, and connect the negative terminal of the battery to the negative terminal of the charger;
4. Next, you need to plug in the charger. The multimeter will display the current, which should not exceed 10% of the battery capacity.

Situations often arise when it is necessary to check the electrical network of a building. This is also the case with the usual electrical network in apartment buildings. Knowing how to measure the current strength with a multimeter in an alternating network, you can make minor repairs to the wiring at home.

The electrical outlet should also not be tested without a load.. The best load for an AC network would be an incandescent lamp. To take measurements you need to do the following:

Since the voltage in the network has a variable sinusoidal shape, the measuring device shows the effective value, which is 1.41 times less than the amplitude value.

Using the proposed method, you can check any variable circuit, including transformers, inductors, asynchronous and synchronous motors.

DC and AC voltage values You can also find out using a multimeter. To do this you need:

A multimeter is an indispensable device for efficient work with electrical circuits and signals. Using such a device, you can quickly identify a malfunction and determine the necessary signal parameters, so it is important to always have it on hand.