Transistors are semiconductors that are used as amplifiers or electronic switches. Transistors are made in either one of two standard types, NPN or PNP, which refer to the configuration of the layers of the semiconductor materials that are used to manufacture the transistor. Transistors have three connections -- the base, the emitter and the collector. For an NPN transistor, the transistor is turned on when the base is at high relative to the emitter. The PNP transistor is turned on when the base is low relative to the emitter. Transistors can be tested with a multimeter, which can verify a transistor is working correctly and also help to identify its connections.
Label the pins of the transistor to be tested as 1, 2 and 3, working from left to right. This is used as a guide for test purposes only. These are not the actual connections.
Using the diode test setting on a digital multimeter, connect the positive red and negative black probes to the pins of the transistor in the following order, and record the output from the multimeter:
1 Positive - 2 Negative
2 Positive - 1 Negative
1 Positive - 3 Negative
3 Positive - 1 Negative
2 Positive - 3 Negative
3 Positive - 2 Negative
Each of the three connections is being tested, with each test performed with the leads connected both ways round. The multimeter will either show "OL," indicating an open circuit, or will show a voltage reading, indicating the forward voltage of the transistor junction.
Analyse the results. When the tests have been completed, output similar to the following will be seen:
1 Positive - 2 Negative - OL
2 Positive - 1 Negative - OL
1 Positive - 3 Negative - 0.675 volts
3 Positive - 1 Negative - OL
2 Positive - 3 Negative - 0.635 volts
3 Positive - 2 Negative - OL
The only positive readings are for wires 1 and 3, and 2 and 3. The emitter-base junction always has the higher reading -- 0.665 volts in the example -- and the base-collector junction has the lower reading, here 0.635 volts.
Identify the pins. Find the pin that is common to both readings. In Step 3 that is pin 3. This is the transistor base pin, which means that pin 1 is the emitter and pin 2 is the collector. As the voltage readings were obtained when pin 3, the base, was connected to the negative probe, the transistor type is PNP. If the base was connected to the positive probe when voltage readings were obtained, the transistor type is NPN.
If results similar to those above are not obtained, such as two voltage readings and four open circuit readings, it indicates the transistor is faulty.
If the transistor is part of an electrical circuit, it should be desoldered and removed before being tested to get valid results.