If not, then you can run this test with a real resistor instead of your fingers. Most of the time, one orientation has a obvious higher gain. That apparent lower resistance is due to the transistor amplifying the base current. You should see a lower resistance than if you used your fingers to bridge C and E. With the base floating, there should be no current so the meter should read infinite resistance.

I usually did this by connecting the meter across C-E. A transistor will still work with C and E flipped, but the gain will be higher when connected as intended.

On a power package, C is usually connected to the case or tab or whatever.Īnother way to test for C versus E is to measure the gain. The next problem is to figure out which are the C and E leads. The 'N' and 'P' in the names tells you the voltages required to make the diodes conduct.įiguring out whether you have a NPN or PNP and which lead is the base is therefore easy. In a NPN, it takes positive voltage on the base relative to E or C to make the diodes conduct, and the other way around with a PNP. From the view of probing with a two-wire ohmmeter, a bipolar transistor looks like two diodes back to back. In this case, you definitely want to forward bias the junctions.Ī bipolar transistor has only three leads, so only 6 possible two-wire measurements when taking polarity into account.

Especially with the smaller packages, you're going to have to experiment.įirst, make sure the ohmmeter is not set to some extra low voltage mode intended to not forward bias diodes. However, all too often there is no manufacturer's part number, or its just a short code, or its a in-house number. In that case, you can find the datasheet and get the parameters outright. I used to do this routinely in high school when salvaging parts from unknown discarded boards.Īs Spehro said in a comment, sometimes you can find a part number.

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