The resistor burned out how to find out what it was. How to find out the resistance of a burnt resistor

Most often, there are resistor malfunctions associated with the burnout of the conductive layer or a violation of the contact between it and the collar. For all cases of defects, there is a simple test. Let's figure out how to check the resistor with a multimeter.

Types of multimeters

The device is pointer or digital. The first does not require a power source. It works as a microammeter with switching shunts and voltage dividers in the specified measurement modes.

The digital multimeter shows on the screen the results of comparing the difference between the reference and measured parameters. It needs one that affects the accuracy of measurements as it discharges. It is used to test radio components.

Types of faults

A resistor is an electronic component with a specific or variable value of electrical resistance. Before checking the resistor with a multimeter, it is inspected, visually checking the serviceability. First of all, the integrity of the hull is determined by the absence of cracks and chips on the surface. Leads must be securely fastened.

A faulty resistor often has a completely burnt surface or partially in the form of rings. If the coating has darkened a little, this does not yet characterize the presence of a malfunction, but only speaks of its heating, when the power released on the element at some point exceeded the permissible value.

The part may look like new even if the contact breaks inside. Many people have problems here. How to check the resistor with a multimeter in this case? It is necessary to have a circuit diagram, according to which voltage measurements are made at certain points. To facilitate troubleshooting in the electrical circuits of household appliances, control points are highlighted with an indication of the value of this parameter on them.

Resistors are checked last, when there is no doubt about the following:

• semiconductor parts and capacitors are intact;
• there are no burnt tracks on printed circuit boards;
• there are no breaks in the connecting wires;
• connector connections are secure.

All of the above defects appear with a much higher probability than the failure of the resistor.

Characteristics of resistors

The resistance values ​​are standardized in series and cannot take any values. Permissible deviations from the nominal value are set for them, depending on the manufacturing accuracy, ambient temperature and other factors. The cheaper the resistor, the greater the tolerance. If, during the measurement, the resistance value goes beyond its limits, the element is considered faulty.

Another important parameter is the power of the resistor. One of the reasons for the premature failure of a part is its incorrect choice for this parameter. Power is measured in watts. It is chosen the one for which it is designed. On the symbol diagram, the power of the resistor is determined by the signs:

• 0.125 W - double slash;
• 0.5 W - straight longitudinal line;
• Roman numeral - power value, W.

The replacement resistor is selected according to the same parameters as the faulty one.

Checking resistors for compliance with ratings

To check, you need to find the resistance values. They can be seen by the serial number of the element on the diagram or in the specification.

Resistance measurement is the most common way to test a resistor. In this case, compliance with the face value and tolerance is determined.

The resistance value must be within the range set by the switch on the multimeter. The probes are connected to the COM and VΩmA jacks. Before checking the resistor with a tester, the serviceability of its wires is first determined. They are closed together, and the device should show a resistance value equal to zero or a little more. When measuring low resistances, this value is subtracted from the instrument readings.

If the energy of the batteries is not enough, the resistance is usually different from zero. In this case, the batteries should be replaced as the measurement accuracy will be poor.

Beginners, not knowing how to check the resistor for performance with a multimeter, often touch the probes of the device with their hands. When values ​​are measured in kiloohms, this is unacceptable, since distorted results are obtained. Here you should know that the body also has a certain resistance.

When the device fixes the resistance value equal to infinity, this is an indicator of the presence of a break (on the screen "1" is lit). It is rare that a resistor has a breakdown when its resistance is zero.

After measurement, the obtained value is compared with the nominal value. This takes into account the tolerance. If the data matches, the resistor is good.

When there are doubts about the correctness of the readings of the device, you should measure the resistance value of a good resistor with the same rating and compare the readings.

How to measure resistance when the value is unknown?

Setting the maximum threshold when measuring resistance is optional. In ohmmeter mode, you can set any range. The multimeter will not fail because of this. If the instrument reads "1", which means infinity, the threshold should be increased until the result appears on the screen.

Calling function

And how to check the resistor with a multimeter for serviceability? A common way is dialing. The switch position for this mode is indicated by a signal diode icon. The sign of the signal can be separately, the upper limit of its operation does not exceed 50-70 ohms. Therefore, resistors whose values ​​​​exceed the threshold do not make sense to ring. The signal will be weak and may not be heard.

At circuit resistance values ​​below the limit value, the device emits a squeak through the built-in speaker. The continuity is done by creating a voltage between the points of the circuit, selected using probes. For this mode to work, you need suitable power sources.

Checking the health of the resistor on the board

Resistance is measured when the element is not connected to the rest in the circuit. To do this, you need to release one of the legs. How to check a resistor with a multimeter without soldering it out of the circuit? This is done only in special cases. Here it is necessary to analyze the connection diagram for the presence of shunt circuits. Semiconductor parts especially affect the readings of the device.

Conclusion

When deciding how to check a resistor with a multimeter, you need to figure out how electrical resistance is measured and what limits are set. The device is intended for manual use and you should remember all the methods of using the probes and the switch.

Repair of electronics, as well as its reverse engineering, are interesting, but still quite difficult activities. One of the difficulties of such a pastime is an attempt to recognize the denominations of burnt components. When there is no device diagram at hand, this recognition becomes almost the mystery of the century. Resistors, due to their greater distribution on printed circuit boards and their greater tendency to burn out, are desirable objects in terms of finding out their ratings with almost completely charred cases.

Despite the seeming impossibility of determining the resistance of a burned-out resistor, its value can still be found out. There are three methods for determining resistance.

First method. First, remove the outer covering, which is most likely already charred. Clean the burnt section of the resistor where any conduction has already disappeared. Measure the resistance from one end of the resistor to the damaged area. Then measure the resistance from the damaged area to the other end of the resistor. Add up these two measured resistances. This will be the approximate value of the burnt resistor. For a slightly more accurate value of the final resistance, you can add a small value of the resistance of the burnt area to this sum. Let's say the value of the burnt resistor was 1 kΩ, but you got 970 ohms. So just add 30 ohms and you have 1 ohm.

Second method. This method can also be used to determine the value of a resistor, and it can also be used on connected resistors in a circuit in case you don't know about resistor color coding, i.e. what the stripes on a resistor mean. It should be noted that the resistor should show at least some signs of life, that is, it should not be completely burned out. So, first connect a resistor to a multimeter and measure the voltage drop across the resistor of interest. Now measure the current flowing through the resistor. Multiply both values ​​and you get the power of the resistor, divide the voltage by the current, you get the resistance (Ohm's law).

Third method. This method can be used better if you know the expected output voltage of the circuit and you have a set of resistors with the same wattage as the burnt resistor. Start with a high resistance value and temporarily connect such a resistor in place of the burnt resistor. Measure the expected output voltage of the circuit. If you get the same voltage as the expected voltage, then you have found the desired resistance. If not, then continue to decrease the value of the resistor until you are satisfied with the operation of the circuit.

The resistor or constant resistance is at the same time the simplest and most common element in electrical circuits, it is installed in all devices. But, despite its simplicity, if the operating modes or thermal conditions are violated, it can burn out. This raises the question of how to check the resistor for performance with a multimeter. The technology for checking health at home will be described in this article.

Troubleshooting algorithm

visual inspection

Any repair begins with an external inspection of the board. It is necessary to inspect all nodes without instruments and pay special attention to yellowed, blackened parts and nodes with traces of soot or soot. For visual inspection, a magnifying glass or microscope can help if you are working with densely mounted SMD components. Broken parts can indicate not only a local problem, but also a problem in the strapping elements of this part. For example, an exploding transistor could drag a couple of elements in the harness along with it.

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

In addition to examining 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 element, you need to check it. It can have one of three faults:

1. Break.
2. Denomination mismatch.

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

Checking the resistor for a break

You can check the serviceability with a regular dialer or a tester in the diode test mode with sound indication (see photo below). It is worth noting that only resistors with a resistance of a few ohms - tens of kOhms can be checked by dialing. And not every dial can handle 100 kOhm.

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 one. A quick check can be carried out without desoldering, after which all the same, desolder suspicious elements and check again for a break.

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

Short circuit test

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

Checking for a short circuit is carried out in one way, 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, a suspicious element is soldered.
3. Check the section of the circuit already without an element, if the short circuit is gone - you have found a malfunction, if not - solder the neighboring ones until it goes away.
4. The remaining elements are mounted back, the one after which the short circuit is gone is replaced.
5. Check the results of the work for the presence of a short circuit.

Here is a clear example of the fact that a burned-out resistor left traces on neighboring resistors, it is likely that they are also damaged:

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

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

Determine the value of the resistor

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

To determine the markings on the colored stripes, there are a lot of free applications for android. Previously, tables and special devices were used.

You can make a checklist like this:

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

By the way, on modern ceramic resistors, explicit marking is also used indicating the resistance and power of the element.

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

12 * 10 3 \u003d 12000 ohms \u003d 12 kOhm

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

If the part is burned so that the marking is not visible at all, it is worth trying to rub it with your finger or an eraser, if this does not help, we have three options:

1. Search on the electrical circuit diagram.
2. In some circuits there are several identical circuits, in which case you can check the value 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 resistor.

Let's start with the fact that 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 output burns out, just 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 lucky coincidence.

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

Then the total resistance is:

R measured *5=R nominal

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

How to test variable resistor and potentiometer

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

The potentiometer is a piece with three legs. It consists of a slider and a resistive layer. The slider slides over the resistive layer. The extreme 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 extreme 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 the burning of the ends, but the 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 occupies 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 old speakers or an amplifier. The problem is that when the knob is rotated, clicks or loud knocks are periodically heard in the speakers.

In general, it is more convenient to check the smoothness of the potentiometer with an analog multimeter with an arrow, because on a digital screen, you just might not notice the defect.

Potentiometers can be dual, 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 the potentiometer with a multimeter:

Methods for testing resistors are simple, but to get a normal test result, you need a multimeter or ohmmeter with several measurement limits. With it, you can also measure voltage, current, capacitance, frequency and other quantities, depending on the model of your device. This is the main tool of the electronics repairman. Resistances sometimes fail with external continuity, sometimes they go beyond the nominal resistance value. A check is needed to determine whether the parts correspond to the ratings, and also 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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