How to check a resistor for operation with a multimeter

A resistor or constant resistance is both the simplest and most common element in electrical circuits; it is installed in all devices. But, despite its simplicity, in case of violation of operating modes or thermal conditions, it can burn out. This raises the question of how to check the resistor for performance with a multimeter. Home health check technology will be outlined in this article.

Troubleshooting Algorithm

Visual inspection

Any repair begins with an external examination of the board. It is necessary to look through all the units without instruments and pay special attention to yellowed, blackened parts and units with traces of soot or carbon deposits. A magnifying glass or microscope can help you with an external examination if you are working with a tight assembly of SMD components. Broken parts can indicate not only a local problem, but also a problem with the trim elements of that part. For example, an exploded transistor could pull a couple of elements in the strapping along with it.

The area on the board, which is not always yellowed from temperature, indicates the consequences of burnout of the part. Sometimes this happens as a result of a long operation of the device, when checking all the parts may turn out to be intact.

In addition to inspecting external defects and traces of burning, it is worth sniffing to check if there is an unpleasant smell like from burnt rubber. If you find a blackened item, you need to check it. It can have one of three faults:

1. Break.
2. Short circuit.
3. Inconsistency with par.

Sometimes a breakdown is so obvious that it can be determined without a multimeter, as in the example in the photo:

Checking the resistor for an open

You can check the serviceability with a regular dial or a tester in the diode test mode with sound indication (see. photo below). It should be noted that only resistors with resistance in units of Ohm - tens of kOhms can be checked by dialing. And 100 kOhm is not enough for every dial-up.

To check, you just need to connect both probes to the terminals of the resistor, it does not matter if it is an SMD component or an output component. A quick check can be carried out without unsoldering, after which suspicious elements can be removed and checked again for a break.

Attention! When checking a part without unsoldering it from the printed circuit board, be careful - you can be confused by parallel elements. This is true both when checking without instruments, and when checking with a multimeter. Do not be lazy and better solder the suspicious detail. This way you can check only those resistors where you are sure that nothing is installed in parallel in the circuit.

Short circuit test

In addition to breaking, the resistor could short-circuit. If you use a dial tone, it should be low-impedance, for example, on an incandescent lamp. Because high-resistance LED dials "ring" the circuit with resistance and tens of kOhm without significant changes in the brightness of the glow. Audible indicators do a better job at this test than LEDs. By the frequency of the beeping, one can judge the integrity of the circuit, in the first place in terms of reliability are complex measuring instruments, such as a multimeter and an ohmmeter.

Checking for a short circuit is carried out in one way, we will consider the instructions step by step:

1. Measure a section of the circuit with an ohmmeter, continuity or other device.
2. If its resistance tends to zero and the continuity indicates a short circuit, the suspicious element is soldered off.
3. Check the section of the circuit already without an element, if the short circuit is gone - you have found faults, if not - the neighboring ones are soldered until it goes away.
4. The rest of the elements are mounted back, the one after which the short circuit is gone is replaced.
5. Check the results of work for the presence of short circuit.

Here is a good example of a burnt-out resistor leaving traces on adjacent resistors, it is likely that they are damaged too:

The resistor turned black from high temperature, not only traces of burning are visible on the neighboring elements, but also traces of overheated paint, its color changed, part of the conductive resistive layer could be damaged.

The video below clearly shows how to check a resistor with a multimeter:

Determine the value of the resistor

For Soviet resistances, the denomination was indicated in an alphanumeric way. In modern output resistors, the denomination is encrypted with color stripes. To replace the resistance after checking for serviceability, you need to decipher the marking of the burnt one.

There are a lot of free applications for android to determine the marking by colored stripes. Previously, tables and special tools were used.

You can make the following cheat sheet to check:

Cut out the colored circles, pierce them in the center and connect, the largest back, the smallest in the front. By aligning the circles, you determine the resistance of the element.

By the way, on modern ceramic resistors, explicit markings are also used with an indication of the resistance and power of the element.

If we are talking about SMD elements, everything is quite simple here. Let's say the marking "123":

12 * 10³ = 12000 ohms = 12 k ohms

There are also other markings of 1, 2, 3 and 4 characters.

If the part burns out so that the marking is not visible at all, you should try to rub it with your finger or an eraser, if this does not help - we have three options:

1. Search on the electrical schematic diagram.
2. In some circuits, there are several of the same circuits, in which case you can check the rating of the part on the adjacent cascade. Example: pull-up resistors on buttons for microcontrollers, limiting resistances of indicators.
3. Measure the resistance of the surviving section.

There is nothing to add about the first two methods, let's find out how to check the resistance of a burnt out resistor.

To begin with, you need to clean the coating of the part. After that, turn on the resistance measurement mode on the multimeter, it is usually signed "Ohm" or "Ω".

If you are lucky and the area directly near the terminal is burnt out, simply measure the resistance at the ends of the resistive layer.

In the example, as in the photo, you can measure the resistance of the resistive layer or determine by the color of the marking strips, here they are not covered with soot - a fortunate coincidence.

Well, if you are unlucky and part of the resistive layer has burned out, it remains to measure a small section and multiply the result by the number of such sections along the entire length of the resistance. Those. in the picture you can see that the probes are connected to a piece equal to 1/5 of the total length:

Then the impedance is:

R_measured* 5 = R_nominal

Such a check allows you to get a result close to the real denomination of the burnt element. This method is detailed in the video:

How to check variable resistor and potentiometer

To understand what testing a potentiometer is, let's look at its structure. A variable resistor differs from a potentiometer in that the first is adjusted with a screwdriver, and the second with a handle.

The potentiometer is a three-legged part. It consists of a slider and a resistive layer. The slider slides over the resistive layer. The outer legs are the ends of the resistive layer, and the middle one is connected to the slider.

To find out the impedance of the potentiometer, you need to measure the resistance between the outer legs. And if you check the resistance between one of the extreme legs and the central one, you will find out the current resistance on the engine relative to one of the edges.

But the most common malfunction of such a resistor is not burning off the ends, but wear of the resistive layer. Because of this, the resistance changes incorrectly, contact may be lost in certain areas, then the resistance jumps to infinity (open circuit). When the slider takes the position in which the contact of the slider with the coating reappears, the resistance becomes "correct" again. You may have noticed this problem when adjusting the volume on an old speaker or amplifier. The problem is that when the knob is rotated, clicks or loud knocks are heard periodically in the speakers.

In general, checking the smoothness of the potentiometer is more clearly carried out with an analog multimeter with an arrow, because on a digital screen, you simply may not notice the defect.

Potentiometers can be double, sometimes they are called "stereo potentiometers", then they have 6 pins, the test logic is the same.

The video below clearly shows how to check a potentiometer with a multimeter:

The methods for testing resistors are simple, but a multimeter or ohmmeter with multiple ranges is needed to get a normal test result. With its help, you can also measure voltage, current, capacitance, frequency and other quantities, depending on the model of your device. It is the main tool of the electronics repair technician. Resistances sometimes fail with external integrity, sometimes deviate from the nominal resistance value. Checking is needed to determine the compliance of parts with ratings, as well as to make sure that the element is working or not. In practice, the verification methods may differ from those described, although the principle is the same, it all depends on the situation.

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