There are three types of transistor testers each performing a unique operation.
In addition, curve tracers are reliable indicators of transistor performance.
A circuit tester is used to check whether a transistor which has previously been performing properly in a circuit is still operational. The transistor's ability to "amplify" is taken as a rough index of its performance. This type of tester indicates to a technician whether the transistor is dead or still operative. The advantage of this tester is that the transistor does not have to be removed from the circuit.
These devices usually perform three types of checks:
Some service testers include a go/no-go feature, indicating a pass when a certain h<sub>fe</sub> is exceeded. These are useful, but fail some functional but low h<sub>fe</sub> transistors.
Some also provide a means of identifying transistor elements, if these are unknown. The tester has all these features and can check solid-state devices in and out of circuit.
Transistor h<sub>fe</sub> varies fairly widely with Ic, so measurements with the service type tester give readings that can differ quite a bit from the h<sub>fe</sub> in the transistor's real life application. Hence these testers are useful, but can't be regarded as giving accurate real-life h<sub>fe</sub> values.
This type of tester is used for measuring transistor parameters dynamically under various operating conditions. The readings they give are absolute. Among the important characteristics measured are:
Transistor testers have the necessary controls and switches for making the proper voltage, current and signal settings. A meter with a calibrated "good" and "bad" scale is on the front. In addition, these transistor testers are designed to check the solid-state diodes. There are also testers for checking high transistor and rectifiers.