Tips on Testing a Transistor

How to test a bipolar transistor. This is a handy testing method to know if you are an electronics hobbyist or just have a need to test a transistor but have no need or wish to purchase a transistor testing unit.

A transistor is a solid state semiconductor device that most closely resembles a diode. In fact two thirds of the device IS a diode. Testing it is very easy. I find the best way to test a transistor is by using an analog ohm meter.

Just attach one probe of the meter to the left most leg of the transistor. This is now leg number one. The middle leg is leg two and the third leg is of course leg three. Now attach the other probe of your meter to leg two. Record the reading. A large swing in your meter's needle indicates a low resistance. A small swing of the needle indicates high resistance. Using a note pad mark this as an "H" for high or "L" for low. Leave probe one on leg one where it is and attach probe two to the third leg of the transistor. Again, a large swing is for low resistance and a small swing indicates a high resistance. Now...Move probe ONE to leg TWO..and start over. You can see what is happening here.

You attach probe one to leg one, then leg two and finally to leg three progressively and use the remaining probe to test the two remaining legs of your transistor. You are moving progressively through the legs of the transistor checking the resistance in both directions. Do this for all three legs and record each reading. When you have finished you should have recorded six readings. These readings are leg one and two, leg one and three. Leg two and one, leg two and three. And lastly leg three and one and leg three and two.

What all this testing is doing is checking the resistance in all possible configurations of the three legged device. At the end of the test what you are looking for is two and only two, low resistance readings. This indicates the transistor is "probably" good. I say probably good because this test only looks at the resistance paths through the device. It doesn't place any sort of load on the device and will not let you know certain particulars about the device that a true transistor testing unit would reveal.

If you find you have more than two low resistance readings this is almost a certain indicator that the transistor has over heated and burned a low resistance path through one of it's junctions. So don't take a chance you should just throw the device away. It can't be repaired and placing it back into the circuit board is just asking for trouble by placing a low resistance path to other delicate and voltage sensitive components.

A diode is much simpler to test. Just place an ohm meter probe on either leg and look at the reading. It will be either high or low. Now reverse the leads and test the other way. It will be high or low again. A good diode will read high in one direction and low in the other direction. Two lows or two highs indicate a bad diode.

There are quite a few different types of transistors and the test just described is primarily for bipolar transistors and silicon and germanium diodes. There are also several different types of diodes. So this test may not be suitable for say a zener or a tunnel diode.