Use a multimeter to measure the quality of components and whether the circuit is working properly
1. Inductors, such as the 220V end of a 1W to 500W power transformer, the DC resistance is generally between a few KΩ to dozens of Ω. The larger 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 and tens of Ω. The greater the power and the higher the frequency, the lower the DC resistance. The DC resistance of small finished inductors is also within this range.
One thing that all inductance measurements have in common is that the test results are the same regardless of whether they are measured forward, backward, or using an analog meter or a digital meter.
2. Measurement of semiconductor devices: When measuring diodes, the forward resistance will generally be between a few Ω and a few hundred Ω. The test results will be different for different meters and different gears. The reverse resistance will be very large, usually between several megabytes and ∞, but the germanium tube will be smaller, usually above hundreds of KΩ. If you use the Rx10K range of the pointer meter to measure a diode with a voltage regulation value lower than 9v, the forward and reverse resistances will be very low, which is normal. When measuring NPN or PNP transistors, b, c, and e can be understood as two diodes connected together, and can be measured using the above method. For meters with a special gear for transistors, this gear can be used for direct measurement.
3. Measurement of capacitance
For digital meters, you can directly use the capacitance scale to measure. If it is a pointer meter, you can use Rx1 or Rⅹ10 gear to measure capacitance above 100μF, and use Rx1K or Rx10K gear to measure capacitance below 100μF. It is better for the pointer to return to the original point after swinging (the larger the capacity, the larger the swing). Otherwise, there is leakage or exchange the test leads and try again, because the leakage of the electrolytic capacitor will increase when reverse voltage is applied. For capacitors that have just been removed from the circuit, be sure to measure them after discharge to prevent damage to the multimeter.
4. Rough test of integrated circuits
There should be a good integrated circuit for reference. For the forward and reverse resistance of any two feet, compare and contrast them respectively. If the resistance between a certain pin is significantly different from that of a good integrated circuit, you can initially determine that the integrated circuit is damaged.
Online measurement
Online measurement is a live measurement. To do this work, you should first know what the normal voltage or current is at the point being measured. 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 credibility of the test data will be greatly compromised. Measuring voltage online is an important means to judge the quality of circuits or components.
When measuring current, you should first disconnect the measured point when the power is off, connect a multimeter in series, and set the gear to a position slightly larger than the current of the measured point, and then turn on the power to test.
One last reminder: if the circuit has a voltage exceeding 36V during online measurement, please pay attention to personal safety. In addition, pay attention to the accuracy of the test pen to avoid short circuiting the circuit under test.
