How to Test a Transistor with an Analog Multimeter
Testing with power on allows measurement of the base voltage. For silicon transistors, this is typically 0.7V; for germanium transistors, 0.2–0.3V.
Normal voltage indicates proper operation; otherwise, the transistor is in cutoff.
With power off, check whether the forward and reverse resistances of the transistor's PN junctions are normal.If a transistor cannot be tested properly in-circuit due to parallel low-value resistors or inductors, it should be removed and tested separately.
Transistor pins must be correctly identified; otherwise, the circuit will not function normally and the transistor may be damaged.With the transistor type and electrodes known, the method to test a transistor using an analog multimeter is as follows:
① Testing an NPN transistor:Set the multimeter to the Ω × 100 or Ω × 1k range.Connect the black probe to the base, then touch the red probe to the other two terminals in turn.If both resistance readings are low, then connect the red probe to the base and the black probe to the other two terminals.If both readings are very high, the transistor is good.
② Testing a PNP transistor:Set the multimeter to the Ω × 100 or Ω × 1k range.Connect the red probe to the base, then touch the black probe to the other two terminals in turn.If both resistance readings are low, then connect the black probe to the base and the red probe to the other two terminals.If both readings are very high, the transistor is good.
When markings on the transistor are unclear, the multimeter can be used to preliminarily determine whether the transistor is good, identify its type (NPN or PNP), and distinguish the e (emitter), b (base), and c (collector) electrodes.
① Identifying the base (b) and transistor type using an analog multimeter:Set the multimeter to Ω × 100 or Ω × 1k.Assume one pin is the base and connect the black probe to it. Touch the red probe to the other two pins.
If both readings are low (several hundred to several thousand ohms), the assumed base is correct and the transistor is NPN.
If both readings are high (several thousand to tens of thousands of ohms), the assumed base is correct and the transistor is PNP.
If one reading is high and the other low, the assumed base is wrong - repeat with a different pin.
② Identifying the collector (c) and emitter (e):Keep the multimeter on Ω × 100 or Ω × 1k.Using an NPN transistor as an example:Connect the black probe to the assumed collector (c) and the red probe to the assumed emitter (e).Pinch the b and c terminals with your fingers (without direct contact), using your body as a bias resistor between b and c. Note the resistance reading, then reverse the probes and test again.
If the first reading is smaller than the second, the original assumption is correct: lower resistance between c and e means higher current through the multimeter and proper bias.
Most modern analog multimeters have an hFE socket for measuring transistor current gain, which can be used to estimate the amplification factor.
