How to use a multimeter to measure the three pins of a transistor
1. Identify the base and determine the transistor type (NPN or PNP)
For PNP transistors, the C and E poles are respectively the positive poles of the two PN junctions inside, and the B pole is their common negative pole. However, for NPN transistors, the opposite is true: the C and E poles are respectively the negative poles of the two PN junctions, and the B pole is their common positive pole. Based on the small forward resistance and large reverse resistance of the PN junction, it is easy to determine the type of base and transistor. The specific method is as follows:
Set the multimeter to the R × 100 or R × 1K position. The red pen contacts a certain pin, and the black pen is connected to the other two pins separately. This way, three sets of readings (twice per set) can be obtained. When one of the sets has a low resistance value of several hundred ohms in the second measurement, if the common pin is the red pen, it contacts the base, and the transistor type is PNP; If the common pin is a black probe, it is also in contact with the base, and the transistor type is NPN.
2. Distinguish between emitter and collector electrodes
Due to the different doping concentrations in the two P regions or two N regions during the production of a transistor, if the emitter and collector are used correctly, the transistor has strong amplification ability. Conversely, if the emitter and collector are used interchangeably, the amplification ability is very weak, which can distinguish the emitter and collector of the transistor.
After identifying the transistor type and base b, the following methods can be used to distinguish the collector and emitter.
Set the multimeter to the R × 1K gear. Pinch the base and the other pin together by hand (be careful not to let the electrodes directly touch each other). To make the measurement phenomenon obvious, moisten your fingers and connect the red probe to the pin pinched together with the base and the black probe to the other pin. Pay attention to the amplitude of the multimeter pointer swinging to the right. Then swap the two pins and repeat the above measurement steps. Compare the amplitude of the pointer swinging to the right in two measurements and find the one with the larger swing amplitude. For PNP type transistors, connect the black probe to the pin that is pinched together with the base, repeat the above experiment, and find the one with the largest swing amplitude of the probe. For NPN type transistors, connect the black probe to the collector and the red probe to the emitter. For PNP type, the red probe is connected to the collector and the black probe is connected to the emitter.
The principle of this electrode discrimination method is to use the battery inside the multimeter to apply voltage to the collector and emitter of the transistor, giving it amplification capability. When the base and collector are pinched by hand, it is equivalent to applying a forward bias current to the transistor through the resistance of the hand, making it conductive. At this time, the amplitude of the pointer swinging to the right reflects its amplification ability, so the emitter and collector can be correctly distinguished.
