How to use a multimeter to check transistors?

1, Preparation work
Before starting the inspection, it is necessary to ensure that the multimeter is in good condition, the battery is fully charged, and the appropriate test gear is selected. For transistor inspection, it is usually necessary to use the resistance range (Ω) and voltage range (V), and sometimes the current range (A) may also be required. However, direct measurement of transistor current is rare and is usually indirectly judged by measuring other components in the circuit.
2, Pin identification
The pins of a transistor are usually divided into a base (B), an emitter (E), and a collector (C). Before conducting testing, it is necessary to correctly identify these pins. For most transistors, the pin arrangement has a fixed identification or color code, but the most reliable method is to consult the data manual or use a pin identifier. If these tools are not available, one can also try to infer pin functionality by measuring resistance values, but this requires some experience and skills.
3, Resistance test
Resistance testing is a common method for checking whether a transistor is damaged. Firstly, adjust the multimeter to the resistance range and select the appropriate range. Then, proceed with the following steps for testing:
Base emitter resistance: Connect one probe of a multimeter to the base and the other probe to the emitter, and record the resistance value. For NPN transistors, the normal resistance should be several hundred to several thousand ohms; For PNP type, it may be tens to hundreds of ohms. If the resistance value is close to zero or infinite, it may indicate that the transistor is damaged.
Base collector resistance: Similarly, connect one probe of a multimeter to the base and the other probe to the collector, and record the resistance value. This value is usually larger than the base emitter resistance, but it can also vary depending on the transistor type and specific model.
Emitter collector resistance: Connect a multimeter directly between the emitter and collector to measure their resistance values. For unbiased transistors, this value may be large because the transistor is in the off state at this time. However, please note that this test cannot directly determine the quality of the transistor, as even if the transistor is normal, the resistance value will be affected by other parts of the circuit.
4, Voltage testing
Voltage testing usually needs to be conducted while the circuit is in operation. Set the multimeter to the voltage range and select the appropriate range. Then, connect the probes to the base, emitter, and collector of the transistor respectively, and measure the voltage values at each point. By comparing the measurement results with the voltage values during normal operation, the working state of the transistor can be preliminarily determined. If the voltage value is abnormal, it may indicate that the transistor is not functioning properly or has been damaged.
5, Current testing (indirect)
Due to the difficulty and insecurity of directly measuring transistor current, the current is usually indirectly calculated by measuring the voltage drop across other components in the circuit, such as resistors. This method requires a certain level of circuit knowledge and computational ability. For example, a known resistance resistor can be connected in series with the emitter or collector of a transistor, and the voltage drop across the resistor can be measured to calculate the current passing through the transistor using Ohm's law. If the current value is abnormal, it may indicate that there is a problem with the transistor.
6, Precautions
Before conducting any testing, please ensure that the circuit is powered off to avoid the risk of electric shock.
Be careful when handling multimeter probes during testing to avoid damaging transistor pins or other components on the circuit board.
If you have any questions or uncertainties about the test results, please refer to the relevant data manual or consult a professional.
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