is a measuring device used to determine the value of resistance in electrical circuits. Resistance is measured in Omaha and is denoted by the Latin letter R. What Ohm is in a popular form is described in the website article “The Law of Current Strength”.

Block diagram and designation on Ohmmeter diagrams

The Ohmmeter measuring device is structurally a dial or digital indicator with a battery or power source connected in series, as shown in the photograph.

All combined instruments - pointer testers and digital multimeters - have the function of measuring resistance.

In practice, a device that measures only resistance is used for special cases, for example, to measure insulation resistance at elevated voltages, ground loop resistance, or as a reference device for testing other low-precision ohmmeters.

On electrical measuring circuits, an ohmmeter is designated by the Greek letter omega enclosed in a circle, as shown in the photograph.

Preparing an Ohmmeter for Measurements

Repair of electrical wiring, electrical and radio engineering products consists of checking the integrity of the wires and searching for contact failure in their connections.

In some cases, the resistance must be equal to infinity, for example, insulation resistance. And in others it is zero, for example, the resistance of wires and their connections. And in some cases it is equal to a certain value, for example, the resistance of the filament of a light bulb or heating element.

Attention! In order to avoid failure of the Ohmmeter, it is allowed to measure the resistance of circuits only when they are completely de-energized. You must unplug the plug from the socket or remove the batteries from the compartment. If the circuit contains electrolytic capacitors of larger capacity, then they must be discharged by short-circuiting the capacitor leads through a resistance rated at about 100 kOhm for a few seconds.

As with voltage measurements, before measuring resistance, it is necessary to prepare the device. To do this, you need to set the device switch to the position corresponding to the minimum measurement of the resistance value.

Before measurements, you should check the functionality of the device, since the batteries may be bad and the Ohmmeter may not work. To do this, you need to connect the ends of the probes together.

In this case, the tester’s needle should be set exactly to the zero mark; if it is not, then you can turn the “Set” knob. 0". If this does not work, you need to replace the batteries.

To test the continuity of electrical circuits, for example, when checking an incandescent light bulb, you can use a device whose batteries are dead and the needle does not set to 0, but reacts at least a little when the probes are connected. It will be possible to judge the integrity of the circuit by the fact that the arrow is deflected. Digital devices should also show zero readings, a deviation in tenths of ohms is possible due to the resistance of the probes and the transition resistance in the contacts connecting them to the terminals of the device.

When the ends of the probes are open, the tester arrow should be set to the point indicated on the scale ∞, and in digital instruments, the overload will flash or the number will be displayed 1 on the indicator on the left side.

The ohmmeter is ready for use. If you touch the ends of the probes to the conductor, then if it is intact, the device will show zero resistance, otherwise the readings will not change.

Expensive models of multimeters have a circuit continuity function with audio indication, indicated in the resistance measurement sector with a diode symbol. It is very convenient for testing low-impedance circuits, such as twisted pair cables for the Internet or household electrical wiring. If the wire is intact, then the continuity is accompanied by a sound signal, which eliminates the need to read readings from the multimeter indicator.

Examples from practice of measuring resistance of products

In theory, everything is usually clear, but in practice questions often arise that can best be answered by examples of checking the most common products with an ohmmeter.

Checking incandescent lamps

The incandescent light bulb in a lamp or car on-board devices has stopped shining, how can I find out the reason? The switch, electric socket or wiring may be faulty. Using the tester, any incandescent lamp from a home lamp or car headlight, filament of fluorescent lamps and energy-saving lamps can be easily checked. To check, just set the device switch to the minimum resistance measurement position and touch the ends of the probes to the terminals of the light bulb base.

The resistance of the light bulb filament was 51 Ohms, which indicates its serviceability. If the thread were broken, the device would show infinite resistance. The resistance of a 220 V halogen light bulb with a power of 50 watts when illuminated is about 968 Ohms, and a 12 volt car light bulb with a power of 100 watts is about 1.44 Ohms.

It is worth noting that the resistance of an incandescent lamp filament in a cold state (when the light bulb is not lit) is several times less than in a warm state. This is due to the physical property of tungsten. Its resistance increases nonlinearly with heating. Therefore, incandescent lamps usually burn out the moment they are turned on.

Checking sound-reproducing headphones

It happens with headphones in one of the emitters, or in both at once, the sound is distorted, periodically disappears or is absent. There are two possible options: either the headphones or the device from which the signal is received are faulty. Using an ohmmeter it is easy to check what the cause is and localize the location of the fault.

To check the headphones, you need to connect the ends of the probes to their connector. Typically, headphones are connected to the equipment using a 3.5 mm jack connector, shown in the photo.

One end of the probe touches the common terminal, and the other, in turn, touches the terminals of the right and left channels. The resistance should be the same and be about 40 ohms. Usually, the resistance is indicated in the passport for headphones.

If the resistance of the channels is very different, then there may be a short circuit or a broken wire in the wires. It is easy to verify this; just connect the ends of the probes to the terminals of the right and left channels. The resistance should be twice that of one earphone, that is, already 80 Ohms. In practice, the total resistance of series-connected emitters is measured.

If the resistance changes when the conductors move during measurements, it means that the wire is frayed in some place. Usually the wires fray where they exit the Jack or emitters.

To localize the location of the wire break, during measurements it is necessary to bend the wire locally, fixing the rest of it. Based on the instability of the ohmmeter readings, you will determine the location of the defect. If it’s a Jack, then you need to purchase a detachable connector, bite off the old one with a section of bad wire and solder the wire to the contacts of the new Jack.

If the break is located at the entrance to the headphones, then you need to disassemble them, remove the defective part of the wire, strip the ends and solder them to the same contacts to which the wires were soldered before. In the website article “How to solder with a soldering iron” you can learn about the art of soldering.

Measuring the resistor value (resistance)

Resistors (resistance) are widely used in electrical circuits. Therefore, when repairing electronic devices, it becomes necessary to check the serviceability of the resistor or determine its value.

On electrical diagrams, a resistor is designated as a rectangle, inside which its power is sometimes written in Roman numerals. I – one watt, II – two watts, IV – four watts, V – five watts.

You can check the resistor (resistance) and determine its value using a multimeter turned on in resistance measurement mode. In the resistance measurement mode sector, there are several switch positions. This is done in order to increase the accuracy of the measurement results.

For example, position 200 allows you to measure resistances up to 200 Ohms. 2k – up to 2000 Ohm (up to 2 kOhm). 2M – up to 2,000,000 Ohm. (up to 2 MOhm). The letter k after the numbers denotes the prefix kilo - the need to multiply the number by 1000, M stands for Mega, and the number needs to be multiplied by 1,000,000.

If the switch is set to position 2k, then when measuring a resistor with a nominal value of 300 kOhm, the device will show an overload. It is necessary to switch it to position 2M. In contrast to measuring voltage, it does not matter what position the switch is in; you can always switch it during the measurement process.

Online calculators for determining resistor values
by color marking

Sometimes when checking a resistor, the ohmmeter shows some resistance, but if the resistor, as a result of overloads, has changed its resistance and it no longer corresponds to the marking, then such a resistor should not be used. Modern resistors are marked using colored rings. The most convenient way to determine the value of a resistor marked with colored rings is to use an online calculator.

marked with 4 colored rings

Online calculator for determining the resistance of resistors
marked with 5 colored rings

Checking diodes with a multimeter or tester

Semiconductor diodes are widely used in electrical circuits to convert alternating current to direct current, and usually when repairing products, after an external inspection of the printed circuit board, the diodes are first checked. Diodes are made from germanium, silicon and other semiconductor materials.

In appearance, diodes come in different shapes, transparent and colored, in a metal, glass or plastic case. But they always have two conclusions and immediately catch the eye. The circuits mainly use rectifier diodes, zener diodes and LEDs.

The symbol for diodes in the diagram is an arrow pointing to a straight line segment. A diode is designated by the Latin letters VD, with the exception of LEDs, which are designated by the letters HL. Depending on the purpose of the diodes, additional elements are added to the designation scheme, which is reflected in the drawing above. Since there is more than one diode in a circuit, for convenience, a serial number is added after the letters VD or HL.

It is much easier to check a diode if you understand how it works. And the diode works like a nipple. When you inflate a ball, rubber boat or car tire, air enters it, but the nipple does not allow it back.

A diode works exactly the same. Only it passes in one direction not air, but electric current. Therefore, to check the diode, you need a direct current source, which can be a multimeter or a pointer tester, since they have a battery installed.

Above is a block diagram of the operation of a multimeter or tester in resistance measurement mode. As you can see, a DC voltage of a certain polarity is supplied to the terminals. It is customary to apply the plus to the red terminal, and the minus to the black. When you touch the diode terminals in such a way that the positive output of the device is on the anode terminal of the diode, and the negative output is on the cathode of the diode, then current will flow through the diode. If the probes are swapped, the diode will not pass current.

A diode can usually have three states - good, broken or broken. During a breakdown, the diode turns into a piece of wire; it will pass current no matter the order in which the probes touch. If there is a break, on the contrary, the current will never flow. Rarely, but there is another condition when the transition resistance changes. Such a malfunction can be determined by the readings on the display.

Using the above instructions, you can check rectifier diodes, zener diodes, Schottky diodes and LEDs, both with leads and in SMD version. Let's look at how to test diodes in practice.

First of all, it is necessary, observing the color coding, to insert the probes into the multimeter. Usually a black wire is inserted into COM, and a red wire into V/R/f (this is the positive terminal of the battery). Next, you need to set the operating mode switch to the dialing position (if there is such a measurement function), as in the photo, or to the 2kOm position. Turn on the device, close the ends of the probes and make sure it is working.

We’ll start the practice by checking the ancient germanium diode D7, this specimen is already 53 years old. Germanium-based diodes are now practically not produced due to the high cost of germanium itself and the low maximum operating temperature, only 80-100°C. But these diodes have the smallest voltage drop and noise level. They are highly valued by tube amplifier builders. In direct connection, the voltage drop across a germanium diode is only 0.129 V. The dial tester will show approximately 130 Ohms. When the polarity is changed, the multimeter shows 1, the dial tester will show infinity, which means a very high resistance. This diode is OK.

The procedure for checking silicon diodes is no different from checking those made of germanium. The cathode terminal is usually marked on the diode body; it can be a circle, line or dot. In direct connection, the drop across the diode junction is about 0.5 V. For powerful diodes, the drop voltage is less, and is about 0.4 V. Zener diodes and Schottky diodes are checked in the same way. The voltage drop of Schottky diodes is about 0.2 V.

For high-power LEDs, more than 2 V drops at the direct junction and the device can show 1. But here the LED itself is an indicator of serviceability. If, when turned on directly, you can see even the faintest glow of the LED, then it is working.

It should be noted that some types of high-power LEDs consist of a chain of several LEDs connected in series and this is not noticeable from the outside. Such LEDs sometimes have a voltage drop of up to 30 V, and they can only be tested from a power supply with an output voltage of more than 30 V and a current-limiting resistor connected in series with the LED.

Checking electrolytic capacitors

There are two main types of capacitors, simple and electrolytic. Simple capacitors can be included in the circuit in any way you like, but electrolytic capacitors can only be connected with polarity, otherwise the capacitor will fail.

On electrical diagrams, a capacitor is indicated by two parallel lines. When designating an electrolytic capacitor, its connection polarity must be indicated with a “+” sign.

Electrolytic capacitors have low reliability and are the most common cause of failure of electronic components of products. A swollen capacitor in the power supply of a computer or other device is not a rare sight.

Using a tester or multimeter in resistance measurement mode, you can successfully check the serviceability of electrolytic capacitors, or, as they say, ring. The capacitor must be removed from the printed circuit board and be sure to be discharged so as not to damage the device. To do this, you need to short-circuit its terminals with a metal object, such as tweezers. To test the capacitor, the switch on the device must be set to resistance measurement mode in the range of hundreds of kilo-ohms or mega-ohms.

Next, you need to touch the terminals of the capacitor with the probes. At the moment of contact, the instrument needle should sharply deviate along the scale and slowly return to the position of infinite resistance. The speed at which the needle deflects depends on the capacitance value of the capacitor. The larger the capacitor capacity, the slower the shooter will return to its place. A digital device (multimeter), when touching the probes to the terminals of the capacitor, will first show a small resistance, and then increasingly increasing up to hundreds of megohms.

If the behavior of the devices differs from that described above, for example, the resistance of the capacitor is zero Ohm or infinity, then in the first case there is a breakdown between the windings of the capacitor, and in the second, a break. Such a capacitor is faulty and cannot be used.

The list of possible applications of a multimeter in the practice of a radio amateur is huge. We will be interested in one question here: is it possible and how to check the resistance with a multimeter? Of course, you can check, because an ohmmeter is inserted into the design of this device. It is with its help that you can measure the resistance of cable lines, all radio components, transformers, inductors, fuses and capacitors.

If we look at the schematic diagram of an ohmmeter, then this is a circle, inside of which there is this letter of the Latin alphabet - “Ω” (omega), as well as two terminals, which represent the two probes of the device. By the way, the letter omega means resistance in physics.

Since there is quite a wide variety of multimeter models on the market, the location of the designations on the housing may be different. But since our task is to measure resistance with a tester, we will be interested in the panel where this very letter “Ω” is located. There is also a manual switch and several measurement limits. On some models there may be five, on others seven. The designation is made by numbers and letters.

For example, the limit may be “200”, which means the resistance is measured up to 200 Ohms. There may be either such a designation as “2000”, or such as “2k”. This is the same thing - the limit defines up to 2000 Ohms or 2 kOhms, which is the same indicator. The same is true with the following designations: 2M or 2000k - up to 2,000,000 Ohms. So that you understand what we are talking about, below is a photograph of the multimeter panel, where everything is clearly visible:

Let's give an example. You have a coil or any radio component on your hands, the approximate resistance of which is 1000 Ohms or 1 kOhm, then you need to set the resistance limit higher than the approximate one. If you look at the photo, you will understand that the measured resistance will be a limit of 2 kOhms. On some models there is no such indicator, so it is set to 20 kiloohms.

Now the measurement process itself. But first we need to remind you (who doesn’t know) that the red probe is inserted into the hole (socket) “V/Ω”, and the black one into “com”. In this case, a check is made, that is, both probes are connected. The display should show zeros. Of course, the switch itself must first be set to the range designated omega.

Measuring indicators of the multimeter

So, the approximate resistance is 1 kOhm. An inspection is underway. Now pay attention to the display, if one appears on it, then the part being tested has a higher resistance. This means that you need to reinstall the multimeter to a higher position. In our case, according to the photo it is 20 kOhm. We install it and take additional measurements.

Attention! Do not touch bare areas of probes and leads of radio components. The thing is that the human body also has its own resistance, which means that the multimeter will show on the display the total indicator: the resistance of the body and the radio components. If there is a need to hold the probe or part, then this can be done with only one hand.

Features of measuring with a multimeter

• It often becomes necessary to measure the resistance of a part that is soldered into the plateau. If you check the assembly, the indicator will be incorrect. Why? Because the element being tested will be connected by a circuit to other radio components, and, therefore, the multimeter will show the general indicator. Therefore, before testing, it is necessary to unsolder one pin of the element from the board, that is, disconnect it from the circuit.
• When testing multi-pin elements, they must be completely dismantled. And after that, check their resistance to ensure a correct determination of the serviceability of the device.
• The serviceability and integrity of the probes also affects the accuracy of the multimeter reading. It has already been discussed above how to check the device for its serviceability. But let’s add that if the probes are placed next to each other or moved over each other, and if in this case the display readings jump (first one thing, then the other), then this means that there is a defect in the probes. This is a guarantee of an incorrect measurement. Therefore, it is worth replacing the probes with new ones.
• The battery, built into the device and serving as a power source, plays an important role in the quality of testing. Practice shows that as soon as the battery begins to discharge, the tester immediately begins to lie. Therefore, you should pay attention to the icon that indicates the battery and shows its charging. If it is reduced, then the battery must be replaced with a new one or the device must be recharged.

Let's return to the position of how to measure resistance. What I would like to add. All radio components have a resistance that is known, and it is marked or indicated in tables. This is no secret for radio amateurs. All elements have certain limits and tolerances. For example, resistors have a tolerance of plus or minus 10%. For example, when testing a resistor with a nominal resistance of 1 Megohm, you can get different results: from 990 kOhm to 1.1 Megohm. And this will be considered the correct indicator.

There are often questions regarding the accuracy of the test performed. Again, let's give an example based on a 1000 Ohm resistor. If you check it at the limit of 2000, then the readings will be “1” on the display. If you turn the switch to the limit up to 20k, then the readings may be, for example, 1.12 or something else, that is, more accurate. Therefore, when checking a radio component for resistance, it is necessary to carry out testing at different limits and select the most accurate indicator.

Please note that measuring current and voltage with a multimeter should start with high limits. With resistance it’s the other way around; you have to start from low positions. Why is this so? Because at low limits, if you measure an element with high resistance, the display will always show one. This means that by moving up the line of limits, you can reach the required indicator, which will show a reliable result.

Insulation resistance test

How to measure the insulation resistance of cable lines? The question is actually very serious. And let's start answering it with warnings. The insulation resistance of cables and wires can only be measured in the warm season or in heated rooms. Because ice can form inside the cable braid - frozen droplets of water. And everyone knows that ice is a dielectric, a material that does not conduct. This means that these ice inclusions will not be detected by resistance meters. After thawing, moisture will appear inside the wiring, which negatively affects the cable as a whole.

So, let's test. An insulation resistance meter must be installed by installing the two ends of the measuring instrument (megaohmmeter) on the end of the phase wire located in the distribution board, and on the end of the neutral wire located in the same place. In this case, their ends must be disconnected from the terminals. The measured resistance must be within certain limits, which are determined by the PUE. By the way, these rules contain tables with indicators of limits. Based on them, you will have to compare the obtained indicators, which will depend on the brand of cable and its cross-section.

Testing insulation resistance is a basic process that electricians typically use when checking the integrity of electrical wiring inside buildings (residential and non-residential).

Conclusion on the topic

Let's summarize the question of how to check resistance with a tester (multimeter)? In fact, this process is simple. The main thing is to correctly understand how to measure a given value, how to correctly set the device, and what limits should be used. Since the device itself is manual, you will need to remember all the manipulations with switches and probes. If you understand and remember this, then you will not have problems with testing.

Instructions

Insert the black test lead into the COM jack of the multimeter, then insert the test lead into the VΩmA jack. Turn on the measuring range switch to turn on the device. For small measurements, turn the switch to the Ω sector and set it to the position opposite 200 (Measurement range 0.1 - 200 Ohms). Connect the probes to each other (check the measuring circuit for ), a digital value in the range of 0.3 – 0.7 should appear on the display. This is the resistance of the test leads. Every time you turn on the multimeter, check the resistance value of the test leads. If it increases to 0.8 Ohm, replace the test leads. When the wires are open, the display should show the number 1 in the leftmost register (very high resistance, ).

To measure, touch the contacts of the circuit being tested at the same time. If the circuit or current consumer is working properly, the multimeter readings will change: it will show a certain resistance. In the case of checking for a broken power cord, fuse or “continuity” of wires, the resistance should be very low (within 0.7 - 1.5 Ohms). And when checking current consumers (light bulbs, heating elements, network windings of transformers), it can rise to 150 - 200 Ohms. Moreover, such a dependence can be traced - the more powerful the current consumer, the lower its resistance.

If the multimeter readings have not changed, switch the resistance measurement range by placing the switch opposite the number 2000 (0 - 2000 Ohms). If the display readings do not change here, switch to the next range and measure again. Please note: when the switch knob is positioned opposite the number 2000k, the sensitivity of the multimeter is very high and if you simultaneously grasp the probe contacts with the fingers of your left and right hands, the device will show the body resistance, which will distort the multimeter readings.

Video on the topic

note

All circuits and current consumers being tested must be de-energized!

Helpful advice

Before each measurement, check the measuring circuit for short circuit. Don’t forget to check the battery status: when the device is turned on and the battery is inserted, a battery symbol will appear on the display.

Sources:

• how to check resistance

Electrical resistance is a physical quantity that characterizes the properties of a conductive material. Resistance is defined as the ratio of the voltage at the ends of a conductor to the current flowing through it.

You will need

• Ohmmeter (multimeter, tester)

Instructions

In order to measure the value, you will need an ohmmeter. Today, the most commonly used devices in practice are called testers or multimeters. These universal devices are capable of measuring not only current, but also current, capacitance characteristics, and resistance.

The tester is equipped with two leads (probes). To measure resistance, it is necessary to connect the first probe to one terminal of the product being tested (conductor), and the other probe to the second terminal.

A regular tester contains a number of ranges for measuring electrical resistance; special modes are also possible for “testing” conductors and checking transistor junctions. The presence of different modes is usually determined by the specific model of the device.

“Dialing” can be indispensable when searching for a short circuit. If there is a short circuit, an audible signal is issued, but only if the measured resistance is less than a certain permissible limit.

When measuring resistance, precautions should be taken. Thus, measurements cannot be taken in a circuit that contains power supplies. This may cause permanent damage to the instrument being measured.

Some circuit elements have a resistance that can depend on the magnitude and direction of the current, as well as the voltage applied to them. These are the so-called elements with nonlinear resistance. The resistance of the elements also depends on temperature. An increase in temperature can lead to both an increase and a decrease in resistance. Specific features depend on the material from which the element is made.

Video on the topic

Sources:

• How to measure

There are three types of instruments that allow you to measure resistance: digital, pointer and bridge. The methods for using these meters vary. An experienced DIYer should be able to measure resistance using any of these.

You will need

• digital multimeter, pointer tester, ohmmeter or bridge resistance meter.

Instructions

Regardless of which device you are going to use, the resistor whose resistance will be measured should be removed from the circuit. First, it should be disconnected from the power source and the capacitors in it should be discharged.

To measure resistance digitally, select the resistance measurement switch and the coarsest mode. Connect the wires to the device sockets corresponding to the resistance measurement mode, and then connect a resistor to the probes. If the resistance is not a resistor, but an element that depends on the direction of the current, note that the digital multimeter has a positive voltage on the red probe. By sequentially switching the switch towards more precise limits, ensure that the overload disappears. Read the indicator readings, and by the position of the switch, find out in what units they are expressed.

Pointer resistance measurement tester is done in the same way, but taking into account a number of its features, namely: - in a dial tester in resistance measurement mode, the positive pole is in most cases located on the black probe;
- the zero of the resistance scale is at its end;
- after each switching of the limit, the probes of the device should be closed, the arrow should be set to zero using a special regulator, and only after that the measurement should be carried out;
- for some pointer testers, the limit is selected not by turning the knob, but by moving the plug;
- also, some pointer instruments require, in addition to selecting a limit, to turn on the resistance measurement mode with a separate switch.

This is how a bridge meter is used. By connecting a resistor to it, move the limit switch to one of the extreme positions. Rotate the regulator from one end of the scale to the other. If the bridge balance indicator (light, sound or pointer) never works, choose another limit. On it the regulator is again scrolled from one end to the other. This operation is repeated until the bridge can be balanced. Now the scale on the regulator determines the resistance, and the position of the limit switch determines in what units it is expressed.

A resistor is one of the main elements of any electrical circuit. Its main purpose is to create a certain resistance. Resistance can be measured with special instruments or determined by special markings on the resistor body.

You will need

• - tester;
• - calculator;
• - tables of markings.

Instructions

Take a tester that can operate in ohmmeter mode. Connect it to the resistor contacts and take a measurement. Since the resistance of resistors can be very different, set the device. If the tester can only measure current and resistance, take a current source and assemble an electrical circuit using a resistor. When connecting a circuit, be sure to control the current that passes through it so as not to cause a short circuit. After changing the current in amperes, switch the tester to measure voltage. Connect it in parallel with the resistor, and take readings in volts. Then find the resistance of the resistor by dividing the voltage U by the current I (R=U/I). If the source is DC, when connecting devices

If the resistor is marked, find its resistance without resorting to additional operations. Resistors are marked either with numbers, or a combination of numbers with letters, or a set of colored stripes.

If three digits are indicated, then use the first two digits to determine the tens and units of the number, and the third digit is the power of 10 to which it must be raised to obtain the correct value. For example, if the numbers 482 are printed on a resistor, then its resistance is 48∙10²=4800 Ohms.

When the SMD marking is applied to a resistor, the first two digits are taken as a coefficient, and the letter corresponds to the power of 10 by which it must be multiplied. Take all coefficient values ​​and letter designations in the EIA SMD resistor marking table. The resistor may also have a fourth letter, indicating its accuracy class. For example, if a resistor is marked 21VF, then its resistance will be 162∙10=1620 Ohm ±1%.

If there are colored stripes on the resistor, use the color-coded resistor resistance chart. The first three marks correspond to the numbers from which the coefficient is made, and the fourth is the power of 10 by which the resulting coefficient must be multiplied.

Resistance as a physical quantity

The electrical resistance of a conductor is a physical quantity denoted by the letter R. The unit of resistance is taken to be 1 ohm - the resistance of a conductor in which the current strength is 1 ampere with voltage at the ends. Briefly this is written by the formula:

Resistance units can also be multiples. So, 1 (mOhm) is 0.001 Ohm, (kOhm) is 1000 Ohm, 1 (MOhm) is 1,000,000 Ohm.

What is the cause of electrical resistance in conductors?

If electrons moving in an orderly manner in a conductor did not experience any obstacles on their way, they could move by inertia for an arbitrarily long time. But in reality this does not happen, since electrons interact with ions located in the crystal lattice of the metal. This slows down their movement, and in 1 second a smaller number of charged particles pass through the cross-section of the conductor. Therefore, the charge transferred by electrons in 1 second decreases, i.e. the current decreases. Thus, every conductor seems to counteract the current moving in it, resisting it.

The cause of resistance is the collision of moving electrons with ions of the crystal lattice.

How is Ohm's law expressed for a section of a circuit?

In any electrical circuit, a physicist deals with three physical quantities - current, voltage and resistance. These quantities do not exist separately by themselves, but are interconnected by a certain ratio. Experiments show that the current strength in a section of a circuit is directly proportional to the voltage at the ends of this section and inversely proportional to the resistance of the conductor. This is Ohm's law, discovered by the German scientist Georg Ohm in 1827:

where I is the current strength in the section of the circuit, U is the voltage at the ends of the section, R is the resistance of the section.

Ohm's law is one of the fundamental laws of physics. Knowing the resistance and current, you can calculate the voltage in a section of the circuit (U=IR), and knowing the current and voltage, you can calculate the resistance of the section (R=U/I).

Resistance depends on the length of the conductor, cross-sectional area and the nature of the material. The lowest resistance is characteristic of silver and copper, and ebonite and porcelain almost do not conduct electric current.

It is important to understand that the conductor resistance, expressed from Ohm’s law by the formula R=U/I, is a constant value. It does not depend on either current or voltage. If the voltage in a given area increases several times, the current will increase by the same amount, but their ratio will remain unchanged.

When repairing any equipment, both domestic and professional, a frequently performed operation is measuring resistance. The easiest and fastest way to check the resistance is with a multimeter. As a rule, the measurement technique is similar for various electronic devices, but there are also features associated with their physical properties.

General information about resistance

In science, the concept of resistance denotes a physical quantity characterizing the ability of a conductor to impede the passage of an electrical signal flowing in it.

Resistance in an alternating current circuit is called impedance, and in an electromagnetic field it is called wave resistance. There is also an element of the electrical network - a resistor, which is often called resistance. Unit of measurement of physical quantity is Om. On diagrams and in literature, resistance is designated using the Latin letter R.

The most popular method is to check the resistance of a resistor or junctions of semiconductor devices with a multimeter, while special devices are used to measure the wave parameter of a cable, for example, an oscilloscope or an LC meter.

The impedance value of the resistor is indicated on its body by applying numbers or stripes. Actual resistor resistance, even in good working order, may differ from the nominal value by the value of the permissible deviation. The whole test comes down to measuring the resistance value with a tester and comparing the result with the declared one.

Semiconductors. The operation of semiconductor elements is based on the properties of a p-n junction to easily pass current in one direction and resist its passage in the other.

When checking electrical objects, measuring the insulation resistance of wires is of particular importance. Typically, readings are taken relative to the phase conductor and the surface of its insulation. The measuring device used for this is called a megohmmeter.

Types of devices for taking measurements

Almost all multifunctional measuring instruments have the ability to measure the impedance value. Based on their operating principle and functionality, manufactured devices can be digital or analog. At the same time, their important characteristics are the error and measurement range.

Before you start working with the tester, you need to make sure that its batteries are in good condition. If on a digital type of device indicator with blinking battery, this means the battery needs to be replaced. For a pointer instrument, a signal to replace the power elements will be the inability to set the pointer to the zero position.

To obtain the correct result, it is necessary not only to use the configured device, but also to monitor the ambient temperature. As is known from the laws of physics, when heated, the resistance value of conductors increases, and that of semiconductors decreases. The optimal temperature is considered to be 20 degrees Celsius.

Digital multimeter

The main feature of a digital multimeter is the presence of a screen; the measured value is clearly displayed on it. The operating principle of the device is based on comparing the measured signal with a reference signal; for this, an analog-to-digital converter is used.

To carry out a measurement, the tester is connected with a set of wires to the element being measured. At one end of each wire There is a plug intended for installation in the meter socket, and on the other there is a contact probe. The procedure for measuring the resistance of a resistor with an electronic multimeter can be represented as the following:

1. Pressing the ON/OFF button turns on the device.
2. The probes are connected to the two ends of the resistor, the reverse ends of the wires to the Ω and COM connectors.
3. The switch sets the approximate resistance.
4. If a unit is displayed on the indicator, the switch should be moved up one position, i.e., increase the measurement limit.
5. If, when taking readings, numbers other than one are displayed on the screen, this will be the resistance value.

In the same way, you can measure the resistance of the p-n junction of a semiconductor. A digital device is convenient to measure constant resistance, but it is useless when you need to find out its variable value. For such measurements, it is preferable to use a pointer instrument.

Pointer device

The very first measuring instruments were equipped with a pointer device. This device was an electromechanical head. Structurally, it is made in the form of a frame located in a magnetic field. To this head through various resistances an electrical signal is given. Depending on the current strength, the arrow in the frame deviates, settling in a certain position. The range of the needle deflection is calibrated, according to these values ​​and the required value is calculated.

The technical capabilities of an analog tester are largely determined by the sensitivity of the magnetoelectric measuring device. Its main advantage is its inertia and immunity to interference during the measurement of DC voltage and resistance value.

Pointer instruments are ideal for displaying signal dynamics. The tester instantly shows its change. At the same time, such a device has a large error when measuring in high-resistance circuits, and there is some difficulty in interpreting the measurement results.

The device is turned on according to the instructions indicated on the back of the battery cover. The switch button selects the operating mode for constant, variable value or resistance (respectively “- » , «~ » , «Ω »). A double click is used for a measurement pair. The calculation range switch is set to a fixed value corresponding to the expected measurement value.

Before measuring the resistance value, the tester is adjusted by rotating the zero knob until the arrow points to the value “∞ » . When selecting the measuring range "Ω » resistance values ​​are not marked with maximum numbers in this range, but have the following form: x1, x10, x100. This means that the resulting value will be measured in Ohms, kOhms, and MOhms. Active resistance is measured from a direct current source (battery) installed in the device.

Having turned on and prepared the tester, you need to attach the probes to the object being tested. According to the arrow readings, the result will appear on the measuring scale, which is then multiplied by the range multiplier.

Using a Megger

A megohmmeter is a specialized measuring device. Before starting measurements, you must strictly adhere to the requirements of the PUE (electrical installation rules). The basic rules include:

A megohmmeter is a complex device consisting of a current generator and a measuring head. Also included are: current-limiting resistors, terminal blocks, a dielectric housing and a mode switch.

The device has three terminals for external connection of wires. The ground is connected to one, the line to the other, and the screen to the third. Which wire is connected where is indicated in the instructions for the device.

The ground and line terminals are used for any insulation readings relative to the ground loop, and the shield contact is needed to reduce the influence of leakage currents. Such currents appear when measuring between two wire strands located parallel to each other. The screen contact is connected with a special wire that comes with the device.

After connecting all the probes on old-style devices, you will need to twist the knob, which will ensure the operation of the internal generator and the supply of voltage to the object being tested. In modern devices, a button is used instead of a handle, and power is taken from installed batteries or galvanic batteries. The generator voltage can range from 100 volts to 2.5 kV. As soon as the voltage is applied, for a pointer instrument, readings are taken from the arrow on the scale corresponding to the selected range, and for a digital type of instrument, readings are taken in the form of numbers on the indicator.

Example of practical measurement

An example of measuring impedance with a digital tester is a soldering iron. First, connect the device plugs according to the instructions, and then set the measurement limit to 10 kOhm. You need to touch the contact plug of the device with the probes and look at the indicator. It will display a number - for example, 320. This means that the soldering iron has a resistance of 320 Ohms. Having obtained the resistance value, you can calculate the power of the soldering iron. It is calculated using the formula: P = U*U/R, where:

For the example given, it will be 150 W, when connected to the network - 220 volts. This way it is possible to take readings from incandescent lamps and any heating elements.

Using a multimeter, you can ring an electrolytic capacitor, i.e., determine its serviceability by changes in resistance. Initially, the capacitor is desoldered from the board and discharged by shorting its contacts to each other. The tester switch is set to the highest range position, after which you need to touch the legs of the radio element with probes.

If measurements are carried out with a pointer instrument, its needle should deflect to the zero position and then slowly move to the infinity position. The speed of its return depends on the capacity of the element. The larger it is, the slower the arrow returns.

Before measuring the resistance with a multimeter, you must remove at least one of the radio element leads from the board. This is due to the fact that, being in the board, the terminals of the element can be shunted by other radio components, i.e. another radio element can be connected in parallel to these terminals. This means that the result will be determined incorrectly.

A home technician periodically needs to measure circuit parameters. Check what voltage is currently in the network, whether the cable is frayed, etc. For these purposes there are small devices - multimeters. With small size and cost, they allow you to measure various electrical parameters. Let's talk about how to use a multimeter further.

External structure and functions

Recently, specialists and radio amateurs mainly use electronic models of multimeters. This does not mean that switches are not used at all. They are indispensable when electronic devices simply do not work due to strong interference. But in most cases we are dealing with digital models.

There are different modifications of these measuring instruments with different measurement accuracy and different functionality. There are automatic multimeters in which the switch has only a few positions - they select the nature of the measurement (voltage, resistance, current) and the device selects the measurement limits itself. There are models that can be connected to a computer. They transfer measurement data directly to a computer, where they can be saved.

But most home craftsmen use inexpensive models of the middle class of accuracy (with a bit resolution of 3.5, which ensures an accuracy of 1%). These are common multimeters dt 830, 831, 832, 833. 834, etc. The last number shows the “freshness” of the modification. Later models have wider functionality, but for home use these new features are not critical. Working with all these models is not much different, so we will talk in general about the techniques and procedures.

Structure of an electronic multimeter

Before using a multimeter, let's study its structure. Electronic models have a small liquid crystal screen on which measurement results are displayed. Below the screen there is a range switch. It rotates around its axis. The part on which the red dot or arrow is marked indicates the current type and range of measurements. There are marks around the switch that indicate the type of measurements and their range.

Below on the case there are sockets for connecting probes. Depending on the model, there are two or three sockets; there are always two probes. One is positive (red), the second negative is black. The black probe is always connected to a connector labeled “COM” or COMMON or that is labeled “ground.” Red - into one of the free slots. If there are always two connectors, no problems arise; if there are three sockets, you need to read the instructions for which measurements to insert the “positive” probe into which socket. In most cases, the red probe is connected to the middle socket. This is how most measurements are carried out. The upper connector is necessary if you are measuring a current of up to 10 A (if more, then also in the middle socket).

There are tester models in which the sockets are located not on the right, but at the bottom (for example, the Resanta DT 181 multimeter or the Hama 00081700 EM393 in the photo). In this case, there is no difference in connection: black to the socket with the inscription “COM”, and red, depending on the situation - when measuring currents from 200 mA to 10 A - to the far right socket, in all other situations - to the middle one.

There are models with four connectors. In this case, there are two sockets for measuring current - one for microcurrents (less than 200 mA), the second for current strength from 200 mA to 10 A. Having understood what is in the device and why, you can begin to figure out how to use a multimeter.

Switch position

The measurement mode depends on the position of the switch. There is a dot at one of its ends, it is usually tinted white or red. This end indicates the current operating mode. In some models, the switch is made in the form of a truncated cone or has one pointed edge. This sharp edge is also a pointer. To make your work easier, you can apply bright paint to this pointing edge. This could be nail polish or some kind of abrasion-resistant paint.

By turning this switch you change the operating mode of the device. If it stands vertically up, the device is turned off. In addition, there are the following provisions:

• V with a wavy line or ACV (to the right of the “off” position) - AC voltage measurement mode;
• A with a straight line—DC measurement;
• A with a wavy line - determination of alternating current (this mode is not available on all multimeters; it is not present in the photos above);
• V with a straight line or the inscription DCV (to the left of the off position) - for measuring DC voltage;
• Ω - resistance measurement.

There are also provisions for determining the gain of transistors and determining the polarity of diodes. There may be others, but their purpose must be found in the instructions for a specific device.

Measurements

Using an electronic tester is convenient because you don’t have to look for the right scale, count divisions, and determine the readings. They will be displayed on the screen accurate to two decimal places. If the measured value has polarity, then a minus sign will also be displayed. If there is no minus sign, the measurement value is positive.

How to measure resistance with a multimeter

To measure resistance, move the switch to the area indicated by the letter Ω. Select any of the ranges. We apply one probe to one input, the second to the other. The numbers that appear on the display are the resistance of the element you are measuring.

Sometimes what appears on the screen is not numbers. If 0 “jumps out,” then you need to change the measurement range to a smaller one. If the words “ol” or “over” are highlighted, the value is “1”, the range is too small and needs to be increased. That's all the tricks for measuring resistance with a multimeter.

How to measure current

To select a measurement mode, you must first determine whether the current is direct or alternating. There may be problems with measuring AC parameters - this mode is not available on all models. But the procedure is the same regardless of the type of current - only the position of the switch changes.

D.C

So, having decided on the type of current, we set the switch. Next, you need to decide which socket to connect the red probe to. If you don’t even know approximately what values ​​to expect, so as not to accidentally burn the device, it is better to first install the probe in the upper (leftmost in other models) socket, which is labeled “10 A”. If the readings are small - less than 200 mA, move the probe to the middle position.

The situation is exactly the same with the choice of measurement range: first set the maximum range, if it turns out to be too large, switch to the next smaller one. Do this until you see the readings.

To measure current, the device must be connected to an open circuit. The connection diagram is shown in the figure. In this case, it is important to install the red probe on the “+” of the power source and touch the black probe to the next element of the circuit. When measuring, do not forget that there is power, work carefully. Do not touch the bare ends of the probe or circuit components with your hands.

Alternating current

You can try the AC current measurement mode on any load connected to a household electrical outlet and thus determine the current consumed. Since in this mode the device must be connected to an open circuit, difficulties may arise with this. You can make a special cord for measurements, as in the photo below. There is a plug at one end of the cord, a socket at the other, cut one of the wires, attach two WAGO connectors to the ends. They are good because they also allow you to clamp the probes. After the measuring circuit is assembled, we proceed to measurements.

Move the switch to the “alternating current” position, select the measurement limit. Please note that exceeding the limits may damage the device. At best, the fuse will burn out, at worst, the “filling” will be damaged. Therefore, we act according to the scheme proposed above: first we set the maximum limit, then gradually reduce it. (don’t forget about rearranging the probes in the sockets).

Now everything is ready. First, connect the load to the outlet. Maybe a table lamp. We insert the plug into the network. Numbers appear on the screen. This will be the current consumed by the lamp. In the same way, you can measure the current consumption for any device.

Voltage measurement

The voltage can also be variable or constant; accordingly, select the required position. The approach to choosing a range is the same: if you don’t know what to expect, set it to the maximum, gradually switching to a smaller scale. Do not forget to check whether the probes are connected correctly and into the correct sockets.

In this case, the measuring device is connected in parallel. For example, you can measure the voltage of a battery or a regular battery. We set the switch to the DC voltage measurement mode position, since we know the expected value, we select the appropriate scale. Next, use the probes to touch the battery on both sides. The numbers on the screen will be the voltage that this battery produces.

How to use a multimeter to measure AC voltage? Yes, exactly the same. Just choose the right measurement limit.

Testing wires using a multimeter

This operation allows you to check the integrity of the wires. On the scale we find a continuity sign - a schematic representation of the sound (look at the photo, but there is a double mode, or maybe there is only a continuity sign). This image was chosen because if the wire is intact, the device makes a sound.

We put the switch in the desired position, the probes are connected as usual - into the lower and middle sockets. We touch one edge of the conductor with one probe, and the other with the other. If we hear a sound, the wire is intact. In general, as you can see, using a multimeter is not difficult. Everything is easy to remember.