Transistor test using an analogue multimeter
The diode test using an analogue multimeter can be extended to give a simple and straightforward confidence check for bipolar transistors. Again the test using a multimeter only provides a confidence check that the device has not blown, but it is still very useful.
The test relies on the fact that a transistor can be considered to comprise of two back to back diodes, and by performing the diode test between the base and collector and the base and emitter of the transistor using an analogue multimeter, the basic integrity of the transistor can be ascertained.
It should be noted that a transistor cannot be functionally replicated using two separate diodes because the operation of the transistor depends upon the base which is the junction of the two diodes, being one physical layer, and also very thin.
Step by step instructions:
The instructions are given primarily for an NPN transistor as these are the most common types in use. The variations are shown for PNP varieties.
Set the meter to its ohms range - any range should do, but the middle ohms range if several are available is probably best.
Connect the base terminal of the transistor to the terminal marked positive (usually coloured red) on the multimeter
Connect the terminal marked negative or common (usually coloured black) to the collector and measure the resistance. It should read open circuit (there should be a deflection for a PNP transistor).
With the terminal marked negative still connected to the base, repeat the measurement with the positive terminal connected to the emitter. The reading should again read open circuit (the multimeter should deflect for a PNP transistor).
Now reverse the connection to the base of the transistor, this time connecting the negative or common (black) terminal of the analogue test meter to the base of the transistor.
Connect the terminal marked positive, first to the collector and measure the resistance. Then take it to the emitter. In both cases the meter should deflect (indicate open circuit for a PNP transistor).
It is next necessary to connect the meter negative or common to the collector and meter positive to the emitter. Check that the meter reads open circuit. (The meter should read open circuit for both NPN and PNP types.
Now reverse the connections so that the meter negative or common is connected to the emitter and meter positive to the collector. Check again that the meter reads open circuit.
If the transistor passes all the tests then it is basically functional and all the junctions are intact.
Notes:
The final checks from collector to emitter ensure that the base has not been "blown through". It is sometimes possible that there is still a diode present between collector and base and the emitter and the base, but the collector and emitter are shorted together.
As with the germanium diode, the reverse readings for germanium transistors will not be as good as for silicon transistors. A small level of current is allowable as this results from the presence of minority carriers in the germanium.
