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How to Test a Resistance Detection

There are many types of resistors, including fixed resistors, cement resistors, fuse resistors, potentiometers, varistors and so on. In this article, we will talk about the detection methods and experience of common resistors.

Detection of Fixed Resistors

The actual resistance value can be measured by connecting the two pens (positive and negative) with the pins at the two ends of the resistance respectively. In order to improve the measurement accuracy, the measurement range should be selected according to the nominal value of the measured resistance. Because of the nonlinearity of the ohmic block scale, the middle part of the scale is more precise. Therefore, the pointer indication value should fall as far as possible to the middle part of the scale, i.e. within the 20%-80% radian range from the beginning of the full scale, in order to make the measurement more accurate. It varies according to the error level of resistance. The errors between the reading and the nominal resistance are allowed to be (+5%), (+10%) or (+20%) respectively. If not, beyond the error range, it means that the resistance value has changed.

Note: When testing, especially when measuring resistance above tens of k_, do not touch the conductive part of the pen and resistance by hand; when the detected resistance is welded down from the circuit, at least one head should be welded in order to avoid other components in the circuit affecting the test and causing measurement errors; although the resistance value of the color ring resistance can be improved by The color ring sign is determined, but it’s better to test its actual resistance with a multimeter when using it.

Detection of Cement Resistance

The method and precautions of testing cement resistance are exactly the same as that of testing ordinary fixed resistance.

Detection of Fuse Resistor

In the circuit, when the fuse resistor breaks the circuit, it can be judged by experience: if the surface of the fuse resistor is blackened or burnt, it can be concluded that the load is too heavy and the current through it exceeds the rated value many times; if there is no trace on the surface of the fuse resistor, the current flowing through the circuit is just equal to or slightly larger. At its rated fuse value. For the judgement of the fuse resistor without any trace on the surface, it can be measured by the multimeter R * 1 block. In order to ensure the measurement accuracy, one end of the fuse resistor should be welded off the circuit.

If the measured resistance value is infinite, it indicates that the fuse resistor has failed to open the circuit. If the measured resistance value is far from the nominal value, it indicates that the resistance variable value should not be used again. In the maintenance practice, it is found that a few fuse resistors are short-circuit breakdown in the circuit, so attention should be paid to the detection.

Detection of potentiometer

When checking potentiometer, we should first rotate the handle to see if it rotates smoothly, whether the switch is flexible, whether the “click” sound is clear and crisp when the switch is on or off, and listen to the sound of contact points and resistance friction inside the potentiometer, such as “sandy” sound, indicating that the quality is not good. When testing with a multimeter, the appropriate resistance of the multimeter is selected according to the resistance value of the potentiometer to be measured, and then the multimeter can be tested in the following way.

The Ohm block of A multimeter is used to measure the “1” and “2” ends. Its reading should be the nominal resistance value of the potentiometer. If the pointer of the multimeter is fixed or the resistance value differs greatly, it indicates that the potentiometer has been damaged.

B detects whether the movable arm of the potentiometer is in good contact with the resistor. Measure the ends of “1”, “2” (or “2”, “3”) with the ohmmeter. Rotate the axis of the potentiometer counterclockwise to the position close to “off”. The smaller the resistance, the better. Then slowly rotate the shank clockwise, the resistance value should gradually increase, and the pointer in the head should move smoothly. When the shank rotates to the extreme position “3”, the resistance value should be close to the nominal value of the potentiometer. If the pointer of the multimeter runs out during the rotation of the shaft handle of the potentiometer, it indicates that the movable contacts have bad contact.

Detection of Positive Temperature Coefficient Thermistor (PTC)

When testing, the multimeter R * 1 gear is used, which can be operated in two steps.

A normal temperature test (indoor temperature is close to 25 C); the actual resistance value of the two pins contacting the PTC thermistor of the two meter pens is measured and compared with the nominal resistance value, the difference between the two is normal within (+2_). If the difference between the actual resistance and the nominal resistance is too large, it indicates that its performance is poor or damaged.

B heating test; On the basis of normal room temperature test, the second step test-heating test can be carried out. A heat source (e.g. soldering iron) is heated near the PTC thermistor. At the same time, the multimeter is used to monitor whether its resistance value increases with the increase of temperature. If so, it indicates that the thermistor is normal, and if the resistance value does not change, it means that the thermistor is normal. Its performance deteriorates and can not continue to use. Be careful not to make the heat source close to the PTC thermistor or directly contact the thermistor to prevent it from scalding.

Detection of Negative Temperature Coefficient Thermistor (NTC)

(1) The method of measuring nominal resistance value Rt by multimeter is the same as that of measuring ordinary fixed resistance. That is to say, the actual value of Rt can be measured directly by choosing appropriate resistance according to nominal resistance value of NTC thermistor. However, because NTC thermistor is very sensitive to temperature, the following points should be paid attention to when testing:

ART is measured by the manufacturer when the ambient temperature is 25 C. Therefore, when measuring Rt with multimeter, it should also be carried out when the ambient temperature is close to 25 C to ensure the reliability of the test.

The measurement power of B must not exceed the prescribed value in order to avoid the measurement error caused by the thermal effect of current.

C. Pay attention to correct operation. When testing, do not pinch the thermistor body with your hand to prevent the human body temperature from affecting the test.

(2) To estimate the temperature coefficient alpha t, the resistance value Rt1 is measured at room temperature t1, and then the resistance value RT2 is measured near the thermistor Rt by using electric soldering iron as heat source. Meanwhile, the average temperature T2 on the surface of the thermistor RT at this time is measured by a thermometer and calculated.

Detection of varistor

Measuring the positive and reverse insulation resistance between the two pins of varistor by R * 1K block of multimeter is infinite, otherwise, it means that the leakage current is large. If the measured resistance is very small, it means that the varistor has been damaged and can not be used.

Detection of Photoresistors

A. Cover the transparent window of the photosensitive resistor with a black paper sheet. At this time, the pointer of the multimeter is basically fixed, and the resistance value is nearly infinite. The larger the value, the better the photoresistor performance. If this value is very small or close to zero, it means that the photoresistor has been burned out and damaged, and can not be used again.

B. When a light source is aligned with the transparent window of the photosensitive resistor, the pointer of the multimeter should oscillate considerably and the resistance value will obviously decrease. The more this value, the better the performance of the novel bright photoresistor. If this value is large or even infinite, it indicates that the open circuit inside the photoresistor is damaged and can not be used again.

C. The translucent window of the photoresistor is directed at the incident light, and the upper part of the light-shielding window of the photoresistor is shaken with small black paper, so that it receives light intermittently. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the multimeter pointer always stops at a certain position and does not oscillate with the paper, it indicates that the photosensitive material of the photoresistor has been damaged.


5 thoughts on “How to Test a Resistance Detection

  1. Hi sir
    It’s really a good post, I have learn something from it. Keep up!

    Thanks very much.

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