Test transistor using digital multimeter
taken from: http://www.kpsec.freeuk.com/components/tran.htm
Testing a transistor
Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it:
|Testing an NPN transistor|
1. Testing with a multimeter
Test each pair of leads both ways (six tests in total):
- The base-emitter (BE) junction should behave like a diode and conduct one way only.
- The base-collector (BC) junction should behave like a diode and conduct one way only.
- The collector-emitter (CE) should not conduct either way.
The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.
|A simple switching circuit
to test an NPN transistor
2. Testing in a simple switching circuit
Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard for example. Take care to include the 10kresistor in the base connection or you will destroy the transistor as you test it!
If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.
To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.
Some multimeters have a ‘transistor test’ function which provides a known base current and measures the collector current so as to display the transistor’s DC current gain hFE.
There are three main series of transistor codes used in the UK:
- Codes beginning with B (or A), for example BC108, BC478
The first letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable.
- Codes beginning with TIP, for example TIP31A
TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings.
- Codes beginning with 2N, for example 2N3053
The initial ‘2N’ identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.