1. Visual inspection
For fixed resistors, first check that the mark is clear, the protective paint is intact, no scorching, no scars, no cracks, no corrosion, and the resistor body is in close contact with the pins, etc. For potentiometers, it should also be checked that the hinge is flexible, properly tightened, and feels comfortable. If there is a switch, check whether the switch action is normal.
2. Multimeter detection
(1) Detection of fixed resistance
Use the resistance of the multimeter to measure the resistance, and choose the different multiplier of the multimeter for the resistance of different resistance values. For the analog multimeter, because the indication of the resistance is non-linear, the larger the resistance, the denser the indicator, so the selection of the appropriate range, should make the deflection angle of the watch hand larger, indicating at 1/3~2/3 full scale, the reading is more accurate. If the measured resistance exceeds the error range of the resistor, the resistance value is infinite, the resistance value is 0, or the resistance value is unstable, the resistor is broken.
During the measurement, pay attention to the hand holding the resistor should not contact the two pins of the resistor, which will make the resistance presented by the hand and the measured resistance in parallel, which will affect the accuracy of the measurement. In addition, the resistance value of the resistor in the circuit can be detected by the resistance of the multimeter resistance when it is not live. The in-line test should first de-energize, then disconnect the resistance from the circuit, and then take a measurement.
(2) Detection of fuse resistance and sensitive resistance
The fuse resistor is generally only a few to tens of ohms, and if the measured resistance is infinite, it has been blown off. It can also detect the quality of the fuse resistance online, measure the voltage to the ground at both ends respectively, if one end is the power supply voltage, and the voltage at one end is 0 volts, the fuse resistance has been blown off.
There are many types of sensitive resistors, taking thermistors as an example, they are divided into positive temperature coefficient and negative temperature coefficient thermistors. For positive temperature system (PTC) thermistor, the general resistance value is not large at room temperature, in the measurement with a hot electric soldering iron close to the resistance, then the resistance value should be significantly increased, indicating that the resistance is normal, if there is no change, the component is damaged, negative temperature thermistor is the opposite.
The resistance value of the photoresistor is large when there is no light (covering the light with hands or objects), and the resistance value is significantly reduced when there is a light hand hand indicating the resistance. If there is no change, the component is damaged.
(3) Detection of variable resistors and potentiometers
First measure whether the resistance value between the two fixed ends is normal, if it is infinitely large or zero ohm, or the difference with the nominal is large, exceeding the allowable range of error, it means that it has been damaged;
3. Measure the resistance with a bridge
If accurate measurement of the resistance of a resistor is required, it can be tested with a bridge (digital). The resistor is inserted into the measuring end of the bridge element, and the resistance value of the resistor can be read out from the display by selecting the appropriate range. For example, when a self-made resistor is used with a resistance wire or a fixed resistor to obtain a more accurate resistance value, the resistance of the homemade resistor must be measured with an electric bridge.