How to Test the Quality of a Transistor Using an Analog Multimeter
In-circuit testing can be divided into testing under power-on or power-off conditions. Under power-on conditions, the base voltage can be measured. Generally, it is 0.7V for silicon transistors and 0.2-0.3V for germanium transistors, indicating normal operation; otherwise, the transistor is in a cut-off state. Under power-off conditions, the forward and reverse resistances of the PN junctions of the transistor can be measured to check if they are normal. If a transistor cannot be properly tested due to parallel small resistors or inductors in the circuit, it can be removed for measurement.
The pins of a transistor must be correctly identified; otherwise, connecting it to a circuit may not only prevent normal operation but also burn out the transistor. Given the type and electrodes of a transistor, the methods for determining the quality of a transistor using an analog multimeter are as follows:
Testing an NPN Transistor:
Set the multimeter's ohmmeter to "R × 100" or "R × 1k". Connect the black test lead to the base and the red test lead to each of the other two electrodes successively. If the resistance values measured twice are both small, then connect the red test lead to the base and the black test lead to each of the other two electrodes successively. If the resistance values measured twice are both large, the transistor is good.
Testing a PNP Transistor:
Set the multimeter's ohmmeter to "R × 100" or "R × 1k". Connect the red test lead to the base and the black test lead to each of the other two electrodes successively. If the resistance values measured twice are both small, then connect the black test lead to the base and the red test lead to each of the other two electrodes successively. If the resistance values measured twice are both large, the transistor is good.
When the markings on a transistor are unclear, a multimeter can be used to preliminarily determine the quality and type (NPN or PNP) of the transistor and identify the three electrodes (e, b, c). The testing method is as follows:
Determining the Base (b) and Transistor Type with an Analog Multimeter:
Set the multimeter's ohmmeter to "R × 100" or "R × 1k". First, assume a certain electrode of the transistor is the "base" and connect the black test lead to the assumed base. Connect the red test lead to each of the other two electrodes successively. If the resistance values measured twice are both small (or approximately several hundred ohms to several thousand ohms), the assumed base is correct, and the transistor is an NPN type. Similarly, if the resistance values measured twice are both large (approximately several thousand ohms to several tens of thousand ohms), the assumed base is correct, and the transistor is a PNP type. If the two measured resistance values are one large and one small, the originally assumed base is incorrect. In this case, another electrode must be assumed to be the "base", and the above test must be repeated.
