Special Operating Skills of Digital Multimeters
Identifying Transistors with the Diode Test Range and 200 MΩ Range
Set the multimeter to the diode test range. The diode range of a digital multimeter provides an output voltage of approximately 2.7 V. The unidirectional conductivity of the PN junction is used to identify the base (b) and distinguish NPN and PNP transistors. (1) Assume one pin of the transistor is the base. Connect the red probe to this assumed base, and touch the black probe to the other two pins separately to measure resistance. If both readings are low and roughly equal, swap the probes. If both readings become high and approximately equal, the pin connected to the red probe is the base, and the transistor is NPN type. (2) If the red probe is connected to the assumed base and both readings are high and equal, swap the probes. If both readings turn low and equal, the pin connected to the black probe is the base, and the transistor is PNP type. (3) If one reading is low and the other is high in the above test, the assumed base is incorrect. Try another pin as the base and repeat the test. If no valid result can be obtained after testing all three pins, the transistor is damaged.
Set the multimeter to the 200 MΩ resistance range. For an NPN transistor, assume one pin is the collector (c). Connect the red probe to the assumed collector and the black probe to the emitter (e). Alternatively, pinch the base and collector with your fingers (ensure they do not touch each other). This applies a bias resistor between b and c to supply forward base current and turn on the transistor. Record the reading. Swap the red and black probes and measure again, then record the second reading. Compare the two resistance values. The smaller reading indicates the correct assumption; the pin connected to the red probe in this test is the collector. For a PNP transistor, the pin connected to the black probe is the collector.
Note: The polarity of the red and black probes on a digital multimeter is opposite to that of an analog multimeter.
