Contacts
home/ Finish

The multimeter shows 0. How to check resistance with a multimeter - practical application and useful tips

Hi all! Today we will talk again about such a device as a multimeter. This device, also called a tester, is designed to measure the basic characteristics of an electrical circuit, electrical appliances, in cars - in general, wherever there is electricity. We have already talked a little about multimeters, today we will touch in more detail on what and how they can measure. Once upon a time, the multimeter was the domain of only electricians. However, now many people use it.

There are many different models of multimeters. There is a class of instruments for measuring only certain characteristics. Multimeters are conventionally reduced to two types:

  1. analog multimeters - data is displayed by an arrow. These are multimeters that are still used by people of the old school; they often cannot or do not want to work with modern instruments;
  2. digital multimeters – data is displayed in numbers. This type of tester has replaced the pointer tester; for example, I prefer to use such a device.

Since digital devices are now the most common, we will consider the description of this device using its example. Below are the main symbols that are found on almost any multimeter model.

If you examine the front panel of the multimeter, you can see eight blocks with different symbols:

What does the multimeter show when selecting different operating modes?

They are located around a round switch, with which you can set the desired mode. On the switch, the contact point is indicated by a dot or a raised triangle. Designations are divided into sectors. Almost all modern multimeters have a similar layout and a round switch.

sector OFF. If you set the switch to this position, the device is turned off. There are also models that automatically turn off after a while. This is very convenient, because for example, I forget to turn it off while working, and it’s not convenient when you measure, then solder, turn it off all the time. The battery lasts a long time.

2 and 8– two sectors with the designation V, this symbol indicates voltage in volts. If just a symbol V– then the DC voltage is measured if V~, AC voltage is measured. The numbers next to them show the range of the measured voltage. Moreover, constant is measured from 200m (millivolts) to 1000 volts, and variable is measured from 100 to 750 volts.

3 and 4– two sectors for measuring direct current. Only one range is highlighted in red for measuring current up to 10 amperes. The remaining ranges are: from 0 to 200, 2000 microamps, from 0 to 20, 200 milliamps. In ordinary life, ten amperes are enough; when measuring current, the multimeter is connected to the circuit by connecting the probes to the desired socket, specially designed for measuring current. One day I tried to measure the current in an outlet for the first time with my first simple tester model. I had to replace the probes with new ones - the standard ones were burnt out.

5 (fifth) sector. The icon looks like WiFi. 🙂 Setting the switch in this position allows you to conduct an audible test of a circuit, such as a heating element.

6 (sixth) sector – setting the switch to this position checks the serviceability of the diodes. Checking diodes is a very popular topic among motorists. You can check the serviceability of, for example, the diode bridge of a car generator:

7 - symbol Ω . Here resistance is measured from 0 to 200, 2000 Ohm, from 0 to 20, 200 or 2000 kOhm. This is also a very popular mode. In any electrical circuit there are the most resistance elements. It happens that by measuring resistance you quickly find a fault:

What is HFE mode on a multimeter?

Let's move on to more advanced functions. The multimeter has the following type of measurements: HFE. This is a test of transistors, or the current transfer coefficient of a transistor. There is a special connector for this measurement. Transistors are an important element; perhaps only the light bulb does not have them, but even there they will probably appear soon. The transistor is one of the most vulnerable elements. They burn out most often due to power surges, etc. I recently replaced two transistors in my car battery charger. To check, I used a tester and unsoldered the transistors.

The connector pins are marked with letters such as "E, B and C". This means the following: "E" is emitter, "B" is base, and "C" is collector. Typically all models have the ability to measure both types of transistors. With inexpensive models of multimeters, it can be very inconvenient to check soldered transistors because of their short, cut legs. And the new ones are the best :):). Let's watch a video on how to check the serviceability of a transistor using a tester:

The transistor, depending on its type (PNP or NPN), is inserted into the corresponding connectors and, according to the readings on the display, it is determined whether it is working or not. If there is a fault, the display shows 0 . If you know the current transfer coefficient of the transistor being tested, you can check it in the HFE by checking the tester readings and the transistor data sheet

How is resistance indicated on multimeters?

One of the main measurements taken by a multimeter is resistance. It is indicated by a horseshoe symbol: Ω, Greek Omega. If there is only such an icon on the multimeter body, the device measures the resistance automatically. But more often there is a range of numbers nearby: 200, 2000, 20k, 200k, 2000k. Letter " k" after the number denotes the prefix "kilo", which in the measurement system SI corresponds to the number 1000.

Why is there a hold button in a multimeter and what is it for?

Button Data hold, which the multimeter has, is considered useless by some, while others, on the contrary, use it often. It means data retention. If you press the hold button, the data displayed on the display will be fixed and will be displayed continuously. When pressed again, the multimeter will return to operating mode.

This function can be useful when, for example, you have a situation where you alternately use two devices. You have carried out some kind of standard measurement, displayed it on the screen, and continue to measure with another device, constantly checking with the standard. This button is not available on all models; it is intended for convenience.

Designations of direct current (DC) and alternating current (AC)

Measuring direct and alternating current with a multimeter is also its main function, as is measuring resistance. You can often find the following symbols on the device: V And V~ — DC and AC voltage respectively. On some devices, constant voltage is designated DCV, and alternating voltage ACV.

Again, it is more convenient to measure current in automatic mode, when the device itself determines how many volts, but this function is available in more expensive models. In simple models, direct and alternating voltage during measurements must be measured with a switch depending on the range being measured. Read about this in detail below.

Decoding the symbols 20k and 20m on a multimeter

Next to the numbers indicating the measurement range, you can see letters such as µ, m, k, M. These are so-called prefixes, which indicate the multiplicity and fractionality of units of measurement.

  • 1µ (micro) – (1*10-6 = 0.000001 from unit);
  • 1m (millies) – (1*10-3 = 0.001 from unit);
  • 1k (kilo) – (1*103 = 1000 units);
  • 1M (mega) – (1*106 = 1,000,000 units);

For example, to check the same heating elements, it is better to take a tester with a megometer function. I had a case where a malfunction of the heating element in a dishwasher was detected only by this function. For radio amateurs, of course, more complex devices are suitable - with the function of measuring frequencies, capacitor capacity, and so on. Nowadays there is a very large selection of these devices; the Chinese don’t do anything.

To measure resistance we need.

In order to measure resistance, we need to turn the knob to “measure resistance”. This is our entire top row in green. The letter “K” tells us that we are going to measure kilo-ohms, and the letter “M” means that we are going to measure mega-ohms. The measurement limit is shown before the letter. If a 1 lights up on the multimeter display when measuring resistance, then we switch the knob to a higher limit.


How to measure resistance with a multimeter

Let's take this constant


We see the inscription “82R” on it. It means that its resistance should be 82 Ohms. You can read more about resistor markings. To do this, apply one probe to one end of the resistor, and the other probe to the other end.


As you can see, the multimeter almost accurately showed the resistance value of this resistor.

How to test a variable resistor

Let's measure the resistance of the variable resistor. As you know, with a variable resistor we can change the resistance manually. The same applies to tuning resistors - this is one of the types of variable resistors.


This is his view from below. Here we see the inscription 47 KM. This means its resistance should be 47 KiloOhms between the two extreme contacts.

Using a stick, we can twist it clockwise or counterclockwise, thereby changing the resistance between the middle contact and the two outer contacts


And here is its circuit designation:


We place the probes at the extreme contacts. We measure the total resistance of the variable resistor.


Hmmm... A little different resistance. Our variable resistor is too old, which may be why its resistance does not match what is written. In order to check whether it is working, turn the variable resistor knob all the way counterclockwise and measure the resistance between the left and middle contacts. It should be close to zero.


Turn the handle clockwise, but not all the way. We measure the resistance between the middle and left contacts again.


We measure the resistance between the middle and right contacts.


The total should be the result of the resistance of the two extreme contacts. 12.2+27.6=39.8 Almost everything is correct. Therefore, our variable resistor is working properly. Some variable resistors have a range not from zero, but from some other value, for example from 10 to 100 KOhm. Be careful when checking.

Rules for measuring resistance

  1. Press the probes with some force onto the resistor terminals. In this way, you will eliminate the appearance of contact resistance, which, when pressed lightly, will add up to the measured resistance.
  2. Do not measure resistance under voltage! This could damage the multimeter or cause you an electric shock!
  3. When measuring the resistance of a resistor on a printed circuit board, double-check that the board is de-energized. Then unsolder one end of the resistor and then measure its resistance.
  4. Do not touch the resistor leads when measuring its resistance! The average human body has a resistance of about 1 KiloOhm and depends on many factors. Therefore, by touching the resistor terminals when measuring resistance, you introduce an error into the measurements.
  5. If you want to measure the resistor's resistance as accurately as possible, clean its terminals either with a knife or with the mildest sandpaper. In this case, you will remove the oxide layer, which in some cases introduces a noticeable error in the resistance measurement.

In amateur radio practice and the work of any masters tuning household electrical appliances, a multimeter is of great importance.

This device can measure all electrical parameters: voltage, resistance, current and others.

To know how to test resistance with a multimeter, you need to study the preparation of this device and testing methods.

How the device works

According to Ohm's law, resistance is calculated as the voltage across a section of a circuit divided by the amount of current in that circuit. This operating principle is used in the simplest magnetoelectric ohmmeters, which are capable of measuring values ​​from hundreds of ohms to several megaohms. Several methods of measuring resistance are used:

  • Magnetoelectric. Direct measurement of current at a known voltage is used.
  • Ratiometric. It is based on a comparison of the strengths of two currents, one of which flows through the measured resistor. If the forces are different, the ratiometer shows their difference, which is proportional to the resistance value.
  • Analog electronic. Converts the resistance value into a proportional voltage using an op-amp.
  • Digital electronic. The measured resistor is installed in one of the arms of the bridge and the value is automatically selected using a digital method.
  • Four-wire connection for low resistance measurements. Used to exclude the influence of wire resistivity on the experiment.

Setting up the multimeter

In order for the meter to show the correct values, it needs to be prepared for operation. The multimeter can measure a large number of electrical quantities:

  • Voltage constant and variable;
  • Current strength;
  • Resistance;
  • Frequency.

They can also test diodes, transistors and capacitors. The multimeter can be configured to test different levels of values, from milliohms to gigaohms, you just need to select the correct measurement limit.

To know how to correctly measure resistance with a multimeter, you need to determine the type of test, since the traces of printed circuit boards are tested at one limit, and the insulation resistance at a completely different one.

Digital device

Setting up a measuring device with a digital scale differs from setting up an analog dial gauge. Digital multimeters can be configured with a knob that switches modes, or with mode selection buttons. Sometimes the meter itself determines the signal level and this parameter does not need to be adjusted. But in most cases it is necessary to perform the following procedure to prepare for measurements:

Analog meter

Such a device usually has several scales at once, the readings on which are displayed by an arrow. In order to determine which scale to take readings on, you need to set the measurement value to be determined using the knob on the front panel and select its nature: direct, alternating current or voltage, or resistance (in ohms or kilo-ohms) using switching buttons. Each of the scales is labeled, so you just need to find the inscription corresponding to the selected measurement and count the readings using it.

For measurements, you also need to correctly connect the probes to the correct sockets labeled Ω and COM. If necessary, adjust the zero using a special knob. To do this, with the probe contacts closed, see if the device needle is at zero. If there is a deviation, then adjust it by rotating the knob labeled “Adjustment.” zero."

Resistance measurement methods

Depending on the resistance being measured, the verification algorithm may vary. You can check either a standard resistor for use in radio equipment, or ring the contacts of the circuit to search for a break, or test the insulation resistance.

Resistor check

Beginning radio amateurs often immediately try to assemble equipment using various elements. For this purpose, resistors, capacitors, LEDs and other products are ordered. When installing resistors in a circuit, you must know exactly the value it has. Therefore, all elements must be tested for their ratings with a multimeter. Principles that you need to remember in order to know how to measure the resistance of a resistor with a multimeter:

  • The measurement must be made on a non-conductive surface;
  • It is forbidden to touch the ends of the probes and the resistor leads with your hands;
  • Before checking, you need to properly configure the measuring device.

Inside the diagram

There are situations when it is necessary to check the resistance of a single resistor in a finished circuit, for example, when identifying malfunctions or inaccuracies in the operation of the product. At the same time, if you try to check its value directly by touching the terminals, the value will be given incorrectly, since the multimeter measures the resistance of the entire circuit located between the probe terminals parallel to the resistor being measured.

Therefore, to understand how to test a resistor with a multimeter without desoldering, you need to know that only one element is connected to the measuring device. To do this, unsolder one of the resistor terminals and measure its value using the standard method.

Continuity of circuits

To test the functionality of an electrical appliance, it is necessary to test all its connections using the resistance test method. Cable lines are also monitored for integrity using this method. For example, direct connections between connectors or long interface lines. These connections can also be checked with a multimeter in ohmmeter mode. To do this you need:

  1. Use the knob on the front panel of the meter to set the resistance test mode and the minimum measurement limit (2-20 Ohms);
  2. Identify the pins of a circuit or cable between which there is a direct connection. This can be seen in the circuit diagram;
  3. Apply the probes of the measuring device to the selected contacts. If the screen displays a reading in the order of units of Ohms (up to hundreds of Ohms for a cable), then the circuit is operational.

Insulation check

When laying home or office electrical wiring, to control its integrity and safety, you need to check the insulation resistance between live parts. This test is also used in the development of any electrical appliances, as proper insulation increases the reliability of the product. Insulation is usually checked with a special device - a megger or teraohmmeter. But it is possible to test the insulation with a simple tester. In this case, the result may be obtained with a significant error.

To measure resistance with a multimeter, you need to remember that insulation of conductive products is a specific thing. Therefore, it is necessary to follow safety rules when working with it.

The first is that all work is carried out with the power off: turn off the mains switch, remove power from the circuit. Secondly, remember that the insulation is tested only at positive air temperatures outside or in a heated room, since the appearance of frozen condensation inside the cable braid greatly distorts its resistance. For the same reason, it is impossible to test the insulation immediately after moving the cable from conditions with negative temperatures to a warm room.

To check with a tester, you need to select the maximum resistance measurement limit and touch the opposite cable terminals with the probes.

For single-phase home wiring, these are the neutral and phase wires. In electrical appliances, this is the supply wire and the housing terminal. The resulting value must correspond to that specified in the operating instructions for the electrical device, cable or electrical installations. Typically this value should not be less than 20 megaohms.

Measuring the value of resistors with a multimeter is quite simple. Circuit continuity testing and insulation testing require some skill. In any case, safety precautions are required when carrying out this work.

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 resistor resistance may differ significantly from the nominal value, since there is a 10% tolerance on this parameter.

How to measure resistance with a multimeter, because a radio amateur often requires more accurate data, and if the element is not new, it is necessary to check its performance.

Connecting probes

Supplied with two probes - red and black. Apart from coloring, they are no different.

The working part of the probe is a pointed metal rod, partially hidden in an insulating shell. The pointed profile allows you to pierce wire insulation and work with closely spaced conductors, for example, microcircuit terminals.

A wire with a connector at the opposite end is connected to the working part for connection to the multimeter ports. There are four such ports, they are marked with the symbols:

  1. COM: from English. common - general;
  2. V/Ω (voltage and resistance);
  3. mA (current);
  4. 20A max (current over 200 mA).

One probe is always connected to the COM port, the second - in accordance with the measured parameter. To measure resistance, it is connected to the “V/Ω” port. In this mode, the multimeter supplies constant voltage to the probes and it is important to know how the potentials are distributed:

  • port "COM": "minus";
  • "V/Ω" port: positive.

Polarity is taken into account when measuring the resistance of p-n junctions of semiconductor devices - diodes, transistors, etc.

When buying a multimeter, you should pay attention to the quality of the probes - the accuracy of the measurements depends on this. In cheap models, the connectors in the ports do not hold well, causing contact to be broken. To check, switch the device to resistance measurement mode and apply the probes one to the other. If the readings on the display are constantly changing, the contact is unreliable and it is better to abandon this model.

Low-quality probes are reworked: the wires are replaced with thicker ones (they have lower resistance), they are soldered to the tips so that they do not fall off (in this case the tips must be tinned) and the connectors are adjusted.

The color of the probe does not matter, but it is customary to include a black probe in the “COM” port with a negative potential. It is recommended to adhere to this rule, since all instructions for measuring resistance in transistors and other semiconductor elements are oriented towards it.

Measuring range selection

The multimeter is adjusted by turning the multi-position switch. Its positions are divided into several sectors with the following designations:

  • DCV (-) or V-: DC voltage measurement;
  • ACV (~) or V~: alternating voltage;
  • DCA (-) or A-: DC current;
  • ACA (~) or A~: ;
  • Ω: resistance.

Additional sectors available on some models:

  • CX: ;
  • hFE:: gain determination;
  • icon, image of a light bulb or sound waves: dialing mode.

Resistance measurement

Each sector, with the exception of “hFE” and “continuity”, contains several positions that determine the sensitivity of the device, that is, the range of measured values.

In the “Ω” sector there are usually positions:

  1. 2000;
  2. 200K;
  3. 2000K.

The letter "K" is short for the prefix "kilo". That is, the maximum resistance displayed by the device is 2 million ohms or 2 megohms.

To measure resistance up to 200 Ohms, the switch is set to position “200”. If the measured value lies within the range of 200 - 2000 Ohms, the switch is set to the “2000” position, etc. That is, the rule applies: the nearest largest value is selected relative to the expected value of the measured resistance.

If it’s difficult to guess with an interval - when the range:

  • underestimated: the display shows “1” - the infinity symbol;
  • overestimated: displays a number with two leading zeros, for example "005".

When measuring unknown quantities, the switch is set to the maximum value and then lowered step by step until the extreme readings on the multimeter are replaced by some finite ones.

If this rule is not mandatory when measuring resistance - if the range is too low, nothing threatens the device, then voltages of an unknown order should be observed strictly: if the range is too high, the tester may burn out. True, modern models are equipped with interlocks that open the internal circuits of the multimeter in case of an error, but it is better to be safe.

Some multimeters can measure resistance up to 200 megohms. In the “Ω” sector they have 7 positions:

  1. 200K;
  2. 200M.

But this is still not enough to measure the insulation resistance of wires. Such problems are solved using a megger - a tester that supplies voltage up to 2500 V and is capable of determining resistance up to 300 Ohm.

There are ones that determine the measurement range independently. Their cost is higher than regular ones.

Resistance measurement circuit

When measuring resistance, proceed in the following order:

  1. The black probe is plugged into the “COM” connector.
  2. Red - to the “V/Ω” connector.
  3. Set the switch to one of the positions of the “Ω” sector or to the largest one - if the order of the measured resistance is unknown.
  4. Check the functionality of the device by touching the probes to each other. If the device is working properly, a certain meager value will appear on the display. If it constantly changes or is high, this indicates poor contact - the measurement accuracy will be low.
  5. De-energize the circuit or element under study.
  6. Touch the ends with probes, the resistance of which needs to be measured. When determining the resistance of a semiconductor device, it is important to observe polarity. So, to measure the resistance of a diode, the black probe is shorted with the cathode, the red one with the anode.
  7. If “1” is displayed on the screen, which means an extremely high value, the device switch is moved to a lower position and the measurements are repeated.

The principle of measuring resistance in an electrical circuit

When measuring the resistance of a resistor, the following abnormal situations are possible:

  • The multimeter displays “1” in any range: the resistor has burned out;
  • The resistance value is significantly lower than expected: an interturn short circuit has occurred in the resistor.

In resistance measurement mode, the multimeter supplies the voltage generated by a 9 V battery to the probes. The result is determined based on an analysis of the current in the circuit in accordance with Ohm's law (R = U/I).

Measuring on the contacts of the circuit under test

It is convenient to check the integrity of the electrical circuit in the continuity mode. At the same time, the multimeter also measures the resistance and if it is below 50 Ohms, it gives a signal (buzzer). This feature makes it easier to do a number of things:

  • audit of the integrity of wires and electrical connections;
  • detection of short circuits;
  • search for a core of a multi-core cable;
  • checking diodes and other semiconductor elements for breakdown.

When using the dialing mode, the user does not have to read the readings on the display.

Measuring small resistances

When measuring resistances of several ohms, the multimeter's error becomes excessive. The situation is aggravated by the fact that the device itself and its probes have a resistance of about 0.3 - 0.7 Ohms. Therefore, resistors with a small value are checked using the indirect method:

  1. A circuit is formed from resistors connected in series: the test resistor and the reference resistor. A high-precision resistor is used as a standard - the tolerance does not exceed 0.05%. The color marking of such elements contains a gray stripe (not to be confused with a silver stripe). The denomination is also small. For example, to measure a resistance of about 1.5 ohms, a 2.7 ohm reference resistor is suitable.
  2. The circuit is powered from a 12 V DC source. This option is recommended as the most affordable: this voltage is generated by a car battery or computer power supply. If there are sources with a higher voltage, but with a permissible voltage for these resistors, you should use them. The higher the voltage, the more accurate the measurements.
  3. Use a multimeter to measure the voltage drop across the resistor under test (potential difference). The device determines voltage with much greater accuracy than resistance - up to 0.1 mV. This feature encourages the use of an indirect measurement method.

Multimeter equivalent circuit for measuring voltage and current

Calculate the resistance of the resistor under study from the proportion: (12 – U) / U = Raet / R. That is, R = Raet * U / (12 – U), Where

Rat- resistance of the reference resistor, Ohm;
R- resistance of the resistor under study, Ohm;
12 - current source voltage, V;
U- voltage drop across the resistor under study.

To measure the parameter U, the probes of the multimeter touch the contacts of the resistor under test: red - from the “plus” side, black - from the “minus” or ground (ground) side, if the circuit is powered from a car battery.

Resistance measurement of nonlinear elements

The resistance of semiconductor elements depends on the voltage at the terminals.

The voltage on the probes of different models of multimeters in the “Ω” mode is different, and therefore they will show different resistance. Because of this, diodes are checked like this:

  1. In the “Ω” mode, touch the medium-capacity capacitor with probes until it is fully charged (“1” lights up on the display).
  2. Switch the multimeter to the DC voltage measurement mode (DCV (-) or V- sector) and determine the voltage at the capacitor terminals. It will be equal to the voltage supplied to the probes in the “Ω” mode.
  3. Using the formula I = U / R, the current flowing through the diode is calculated.

Check whether the point with coordinates U and I lies on the graph of the current-voltage characteristic of the diode. If it is on the side of exceeding permissible deviations, but the diode opens and closes, it can be used in circuits with low accuracy requirements.

Measuring resistance with a multimeter is the simplest operation, but it also has its own subtleties. By following the tips outlined above, the user will be able to take measurements safely and with accurate results.