Methods for Measuring Transistors Using Analog Multimeters
The pins of a transistor must be correctly identified. Otherwise, the transistor will fail to operate normally and may even burn out when connected into a circuit. Given the known transistor type and electrodes, the method for judging whether a transistor is good or bad with an analog multimeter is described as follows:
① Testing an NPN transistor: Set the multimeter ohm range to "R×100" or "R×1k". Connect the black test lead to the base, and touch the red test lead to the other two electrodes successively. If both measured resistance values are relatively small, then connect the red test lead to the base and the black test lead to the remaining two electrodes in sequence. If both resistance readings are very large, the NPN transistor is in good condition.
② Testing a PNP transistor: Set the multimeter ohm range to "R×100" or "R×1k". Connect the red test lead to the base, and touch the black test lead to the other two electrodes successively. If both measured resistance values are relatively small, then connect the black test lead to the base and the red test lead to the remaining two electrodes in sequence. If both resistance readings are very large, the PNP transistor is in good condition.
When the markings on the transistor are unclear, an analog multimeter can be used to preliminarily verify its quality, determine its type (NPN or PNP), and identify the three electrodes e, b and c. The test procedures are given below:
① Identifying the base and transistor type with an analog multimeter: Set the multimeter ohm range to "R×100" or "R×1k". Assume one pin is the base. Connect the black test lead to this hypothetical base, and apply the red test lead to the other two pins one‑by‑one. If both resistance readings are low (roughly several hundred to several thousand ohms), the assumed base is correct and the device under test is an NPN transistor. By the same token, if both readings are high (roughly several kilo‑ohms to tens of kilo‑ohms), the assumed base is correct and the transistor is PNP. If one reading is large and the other is small, the assumed base is wrong. Another pin should be hypothesized as the base and the above test shall be repeated.
② Distinguishing collector c and emitter e: Keep the analog multimeter at the "R×100" or "R×1k" ohm range. Take the NPN transistor as an example. Connect the black test lead to the presumed collector c and the red test lead to the presumed emitter e. Pinch pins b and c with your fingers (do not allow direct contact between b and c). The human body acts as a biasing resistor between b and c. Read the resistance value on the meter scale. Then reverse the two test leads and perform the measurement again. If the resistance obtained in the first measurement is smaller than the second reading, the original assumption of c and e is valid. A lower resistance between c and e indicates a larger current through the multimeter, which means proper biasing. Modern analog multimeters are equipped with sockets for measuring transistor current gain \(H_{FE}\), so the current amplification factor of a transistor can also be estimated.
