Resistor Detection

1. Visual inspection
For fixed resistors, first check that the markings are clear, the protective paint is intact, without burning, scars, cracks, corrosion, and close contact between the resistor body and pins. For potentiometers, it is also necessary to check that the rotating shaft is flexible, appropriately tightened, and feels comfortable. If there is a switch, check whether the switch action is normal.
2. Multimeter detection
① Detection of fixed resistance
Measure the resistance using the resistance range of a multimeter, and select different multiplier ranges of the multimeter for measuring different resistance values. For a pointer type multimeter, due to the nonlinearity of the resistance range, the larger the resistance value, the denser the reading. Therefore, when selecting a suitable range, the deviation angle of the meter needle should be larger, indicating 1/3 to 2/3 of the full range, and the reading should be more accurate. If the measured resistance exceeds the error range of the resistor, the resistance is infinite, the resistance is 0, or the resistance is unstable, it indicates that the resistor is faulty.
Be careful not to touch the two pins of the resistor with the hand holding the resistor during measurement, as this will cause the resistance presented by the hand to be in parallel with the measured resistance, affecting measurement accuracy. In addition, it is not possible to use a multimeter to measure the resistance value of the resistor in the circuit when it is live. Online testing should first cut off the power, then disconnect the resistance from the circuit, and then measure it.
② Detection of fuse resistance and sensitive resistance
The resistance value of a fuse is usually only a few to tens of ohms. If the measured resistance value is infinite, it is already blown. The resistance of the fuse can also be tested online by measuring the voltage to ground at both ends. If one end is the power voltage and the other end is 0V, the fuse resistance has blown.
There are many types of sensitive resistors, taking thermistors as an example, which can be divided into positive temperature coefficient and negative temperature coefficient thermistors. For positive temperature series (PTC) thermistors, the resistance value is generally not high at room temperature. When measuring, use a hot electric soldering iron to approach the resistance. At this time, the resistance value should increase significantly, indicating that the resistance is normal. If there is no change, it indicates that the component is damaged, while for negative temperature series thermistors, the opposite is true.
The resistance value measured by the multimeter is high when there is no light (covering the light with hands or objects), while the resistance value indicated by the needle of the meter decreases significantly when there is light. If there is no change, the component is damaged.
③ Detection of variable resistors and potentiometers
Firstly, measure whether the resistance value between two fixed terminals is normal. If it is infinite or zero ohms, or if it differs significantly from the nominal value and exceeds the allowable error range, it indicates that it has been damaged; The resistance value of the resistor body is normal. Then, connect one lead of the multimeter to the sliding end of the potentiometer and the other lead to any fixed end of the potentiometer (adjustable resistor). Slowly rotate the shaft handle to observe whether the needle changes smoothly. When rotating from one end to the other end, the resistance value changes from zero ohms to the standard value (or vice versa), and there is no jumping or shaking phenomenon, indicating that the potentiometer is normal. If there is any jumping or shaking phenomenon during the rotation process, The sliding point shows poor contact with the resistor body.
3. Using a bridge to measure resistance
If precise measurement of the resistance value of the resistor is required, it can be tested through a bridge (digital). Insert the resistor into the measuring end of the bridge element and select the appropriate range to read the resistance value of the resistor from the display. For example, when using a resistance wire to make a homemade resistor or processing a fixed resistor to obtain a more accurate resistance value, it is necessary to use a bridge to measure the resistance value of the homemade resistor.