How to measure the quality of components with a multimeter
Use a multimeter to measure whether the components are good or bad and whether the circuit is working normally, which can be divided into online measurement and offline measurement.
1. Offline measurement
Without any connection with the working circuit, the measurement of the components alone is an offline measurement. Here are a few types in brief:
1. Inductance, such as the 220V end of a 1W-500W power transformer, the DC resistance is generally between a few KΩ-tens of Ω, and the greater the wattage, the lower the resistance. The coil resistance of the electromagnetic relay is generally within this range; in addition, the DC resistance of the transformer used in the switching power supply is relatively low, generally between a few tenths to tens of Ω. The higher the power and the higher the frequency, the lower the DC resistance. The DC resistance of small finished inductors is also within this range.
The inductance measurement has one thing in common, that is, the test results are the same no matter whether it is positive measurement, reverse measurement, or any gear of the pointer meter or digital meter.
2. Measurement of semiconductor devices: when measuring diodes, the forward resistance is generally between a few Ω and several hundred Ω, and the test results will be different with different meters and different gears. The reverse resistance will be very large, usually between several megabytes and ∞, but the germanium tube will be smaller, generally above hundreds of KΩ. If you use a pointer meter Rx10K to measure a diode with a stabilized voltage value lower than 9v, the forward and reverse resistances will be very low, which is a normal phenomenon. When measuring NPN or PNP transistors, b, c, and e can be understood as two diodes connected together, and can be measured by the above method. For a meter with a special gear for the triode, you can use this gear for direct measurement.
3. Measurement of capacitance
For a digital meter, it can be directly measured with a capacitance file. If it is a pointer meter, use the Rx1 or Rⅹ10 file to measure the capacitance above 100μF, and use the Rx1K or Rx10K file to measure the capacitance below 100μF. It is better to return to the original point after the pointer swings (the larger the capacity, the larger the swing), otherwise there is leakage or exchange the test leads to test again, because the leakage of the electrolytic capacitor will increase when the reverse voltage is applied. For the capacitor that has just been removed from the circuit, it must be measured after discharge to prevent damage to the multimeter.
4. Rough test integrated circuit
There should be a good IC for reference. For the forward and reverse resistances of any two pins, comparisons are made respectively. If the resistance value between a certain pin is quite different from that of a good integrated circuit, it can be preliminarily judged that the integrated circuit is damaged.
2. On-line measurement
Online measurement belongs to live measurement. To do this work, you should first understand what the normal voltage or current of the measured point is. When measuring voltage, it is best to use a digital meter with high internal resistance to reduce the impact on the working state of the circuit under test, otherwise the reliability of the test data will be greatly reduced. On-line voltage measurement is an important means to judge whether a circuit or a component is good or bad.
When measuring the current, first disconnect the point to be measured when the power is off, connect the multimeter in series, and set the gear to a position slightly larger than the current of the point to be measured, and then turn on the power to test.
Finally, a reminder, if the circuit has a voltage exceeding 36V during online measurement, pay attention to personal safety. In addition, it is also necessary to pay attention to the accurate start of the test pen, and do not cause a short circuit to the circuit under test.
