A 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 operating conditions or thermal conditions are violated, it can burn out. This raises the question of how to test a resistor for functionality with a multimeter. The technology for checking serviceability at home will be outlined in this article.
Any repair begins with an external inspection of the board. It is necessary to examine all components without instruments and pay special attention to yellowed, blackened parts and components with traces of soot or soot. For external inspection, a magnifying glass or microscope can help you if you are working with dense mounting of SMD components. Torn parts may indicate not only a local problem, but also a problem in the strapping elements of that part. For example, an exploding transistor could drag down a couple of elements in the harness.
An area on a board that turns yellow due to temperature does not always indicate the consequences of part burnout. Sometimes this happens as a result of long-term operation of the device; when checked, all 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 odor like burnt rubber. If you find a blackened element, you need to check it. It may have one of three malfunctions:
Sometimes a breakdown is so obvious that it can be determined without a multimeter, as in the example in the photo:
You can check the serviceability using a regular dial tone or a tester in diode testing mode with sound indication (see photo below). It is worth noting that by testing you can only check resistors with a resistance of units of Ohms - tens of kOhms. And not every continuity can handle 100 kOhm.
To check, you just need to connect both probes to the terminals of the resistor, it doesn’t matter whether it’s an SMD component or an output one. A quick check can be carried out without desoldering, after which you can still desolder the suspicious elements and check again for a break.
Attention! When checking parts without desoldering them from the printed circuit board, be careful - you may be misled by parallel elements. This is true both when checking without instruments and when checking with a multimeter. Don't be lazy and better unsolder the suspicious part. This way you can only check those resistors where you are sure that nothing is installed parallel to them in the circuit.
In addition to the break, the resistor could have short-circuited. If you use a dial, it should be low-impedance, for example, on an incandescent lamp. Because high-resistance LED dials “ring” the circuit with a resistance of tens of kOhms without significant changes in the brightness of the glow. Sound indicators cope with this test better than LEDs. By the frequency of beeping one can judge the integrity of the circuit; complex measuring instruments such as a multimeter and ohmmeter are in first place in terms of reliability.
Checking for short circuit is carried out in one way, let's look at the instructions step by step:
Here is a clear example that a burnt resistor has left marks on neighboring resistors; there is a possibility that they are also damaged:
The resistor has turned black from the high temperature, not only traces of burning are visible on the neighboring elements, but also traces of overheated paint, its color has changed, and part of the conductive resistive layer could be damaged.
The video below clearly shows how to check a resistor with a multimeter:
For Soviet resistance denominations, the denomination was indicated alphanumerically. In modern output resistors, the value is encrypted with color stripes. To replace the resistance after checking for serviceability, you need to decipher the markings of the burnt one.
There are a lot of free Android applications for identifying markings by color stripes. Previously, tables and special devices were used.
You can make a cheat sheet like this to check:
Cut out colored circles, pierce them in the center and connect them, the largest one to the back, the smallest one to the front. By combining the circles, you determine the resistance of the element.
By the way, modern ceramic resistors also use explicit markings indicating the resistance and power of the element.
If we are talking about SMD elements, everything is quite simple here. Let's say the marking is “123”:
12 * 10 3 = 12000 Ohm = 12 kOhm
There are also other markings of 1, 2, 3 and 4 characters.
If a part is burned so that the markings are not visible at all, you should try rubbing it with your finger or an eraser; if this does not help, we have three options:
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 by cleaning the part's coating. After this, turn on the resistance measurement mode on the multimeter, it is usually labeled “Ohm” or “Ω”.
If you are lucky and the area directly next to the output burns out, simply measure the resistance at the ends of the resistive layer.
In the example shown in the photo, you can measure the resistance of the resistive layer or determine it by the color of the marking stripes; here they are not covered with soot - a fortunate coincidence.
Well, if you are unlucky and part of the resistive layer has burned out, all that remains is 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
This check allows you to get a result close to the real value of the burnt element. This method is described in detail in the video:
To understand what testing a potentiometer involves, 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 part with three legs. 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 total resistance of the potentiometer, you need to measure the resistance between the outer legs. And if you check the resistance between one of the outer 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 takes the position in which the contact of the slider with the coating reappears, the resistance again becomes “correct”. You may have noticed this problem when adjusting the volume on older speakers or an amplifier. The problem is that when you turn the knob, clicks or loud knocks are periodically heard in the speakers.
In general, it is more obvious to check the smoothness of the potentiometer with an analog multimeter with an arrow, because on a digital screen you may simply 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 the potentiometer with a multimeter:
Methods for testing resistors are simple, but to obtain a normal test result you need a multimeter or ohmmeter with several measurement limits. With its help, you can also measure voltage, current, capacitance, frequency and other quantities depending on the model of your device. This is the electronics repairman's essential tool. Resistances sometimes fail with external integrity, sometimes they deviate from the nominal resistance value. Checking is needed to determine whether the parts comply with the nominal values, and also to make sure whether the element is working or not. In practice, verification methods may differ from those described, although the principle is the same, it all depends on the situation.
Useful
Most often, resistor malfunctions are associated with burnout of the conductive layer or poor contact between it and the clamp. For all cases of defects there is a simple test. Let's figure out how to test a resistor with a multimeter.
The device can be pointer or digital. The first one does not require a power source. It works as a microammeter with switching of shunts and voltage dividers into specified measurement modes.
The digital multimeter shows on the screen the results of a comparison of the difference between the reference and measured parameters. It requires something that affects the accuracy of measurements as it discharges. It is used to test radio components.
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 its serviceability. First of all, the integrity of the body is determined by the absence of cracks and chips on the surface. The terminals 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 indicate the presence of a malfunction, but only indicates 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 inside breaks. 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 identified with the value of this parameter indicated on them.
Checking resistors is done as a last resort, when there is no doubt about the following:
All of the above defects appear with a much higher probability than resistor failure.
Resistance values are standardized in series and cannot take any value. For them, permissible deviations from the nominal value are specified, depending on the manufacturing accuracy, ambient temperature and other factors. The cheaper the resistor, the greater the tolerance. If, during 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 premature failure of a part is its incorrect selection according to this parameter. Power is measured in watts. It is chosen the one for which it is designed. In the symbol diagram, the power of the resistor is determined by the signs:
The replacement resistor is selected according to the same parameters as the faulty one.
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.
Measuring resistance is the most common way to test a resistor. In this case, compliance with the rating and tolerance is determined.
The resistance value must be within the range that is set by the switch on the multimeter. The probes are connected to the COM and VΩmA sockets. Before checking a resistor with a tester, the serviceability of its wires is first determined. They are connected to each other, and the device should show a resistance value equal to zero or slightly more. When measuring small resistances, this value is subtracted from the instrument readings.
If the energy of the batteries is insufficient, a resistance other than zero is usually obtained. In this case, the batteries should be replaced as the measurement accuracy will be low.
Beginners, not knowing how to test a resistor for functionality with a multimeter, often touch the probes of the device with their hands. When quantities are measured in kilo-ohms, this is unacceptable, since distorted results are obtained. Here you should know that the body also has a certain resistance.
When the device registers a resistance value equal to infinity, this indicates the presence of a break ("1" lights up on the screen). It is rare to see a breakdown of a resistor when its resistance is zero.
After measurement, the resulting value is compared with the nominal value. In this case, tolerance is taken into account. If the data matches, the resistor is OK.
When doubts arise about the correctness of the instrument readings, you should measure the resistance value of a working resistor with the same rating and compare the readings.
Setting the maximum threshold when measuring resistance is not necessary. In ohmmeter mode, you can set any range. The multimeter will not fail because of this. If the device shows "1", which means infinity, the threshold should be increased until the result appears on the screen.
How can you check the resistor with a multimeter for serviceability? A common method is dialing. The switch position for this mode is indicated by a diode icon with a signal. The sign of the signal can be different, the upper limit of its response does not exceed 50-70 Ohms. Therefore, it makes no sense to ring resistors whose values exceed the threshold. The signal will be weak and may not be heard.
When the circuit resistance values are below the limit value, the device emits a squeak through the built-in speaker. Continuity testing 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.
Resistance is measured when the element is not connected to the others in the circuit. To do this, you need to release one of the legs. How to check a resistor with a multimeter without desoldering it from 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.
When deciding how to test a resistor with a multimeter, you need to understand how electrical resistance is measured and what limits are set. The device is intended for manual use and you should remember all the techniques for using the probes and switch.
Repairing electronics, as well as its reverse engineering, are interesting, but still quite difficult activities. One of the difficulties of this pastime is trying to recognize the values of the burnt components. When you don’t have a device diagram at hand, this recognition becomes almost a mystery of the century. Resistors, due to their greater distribution on printed circuit boards and greater tendency to burn out, are desirable objects in terms of determining their values with almost completely charred cases.
Despite the seeming impossibility of determining the resistance of a burnt resistor, its value can still be found out. There are three methods for determining resistance.
First method. First remove the outer covering, which is likely already charred. Clean the burnt section of the resistor where any conductivity 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 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 burned area to this amount. Let's say the value of the burned resistor was 1K ohms, but you get 970 ohms. So just add 30 ohms and you have 1 kohm.
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 are not aware of resistor color coding, i.e. what the stripes on a resistor mean. It should be noted that the resistor must show at least some signs of life, that is, it should not be completely burnt out. So, first connect the resistor to the 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 and 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 power 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 resistance you are looking for. If not, then continue decreasing the resistor value until you are satisfied with the circuit's performance.
