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  Testing
Transistors
with an Ohm Meter
   
A few simple checks with an ohm-meter can tell you a lot about a transistor, especially if you only want to know if it is good or bad. Once in a while, more sophisticated tests may require the use of a transistor tester or curve tracer.
 
The Diode Test
A good bipolar transistor exhibits diode-like behavior between the base and emitter connections, and also between the base and collector connections. If your meter has a diode measurement capability, use it to verify the forward voltage for each of these diodes. (Look for 0.2V to 0.3V for Germanium devices and 0.6V to 0.7V for silicon devices).
 
If your meter does not have diode capability, measure the resistance between base and emitter, then switch leads and measure the resistance again. One polarity should indicate a low resistance and the other should indicate a very high resistance. Measure the base-collector diode in the same way, reversing the leads for the second measurement. Again one polarity should indicate high, and the other low. Both diodes must be present. If one or the other cannot be measured, the device is open.
 
The Finger Bias Test
If you don't have any other test equipment, you can perform a cheap and dirty test of transistor action using just an ohm-meter and your fingers. Set the ohm-meter to a very high resistance scale (say 20M ohms full scale) and connect the more positive lead (usually red) to the collector and the more negative lead (usually black) to the emitter. (For PNP devices, use the opposite polarity.) Don't make any connection to the base. You should read a very high resistance value, usually infinity. If you don't see a high resistance value, the transistor may have a leakage problem.
 
Keep the ohm-meter connected between collector and emitter. Wet your finger and thumb, then use them to touch the collector and the base. The leakage from collector to base through your wet skin provides a little bit of base current. With this bias current flowing, the ohm meter should read much less than infinity. On my B&K Circuitmate DM27, the reading is about 200K for a medium beta small signal silicon transistor. When this test works, you can have some confidence in the transistor. When it fails, you shouldn't throw the transistor away until after it has failed other more definitive measurements.
 
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Last modified on  2/29/00 10:06:30 PM