Allen's Wet Finger Transistor Test


Salvaging electronic components from obsolete or broken circuit boards can save the electronic hobbyist a lot of money. It's also quite satisfying to rip apart a broken clock radio and turn it into something cool! Resistors, capacitors, and diodes are usually clearly labeled, but transistors usually aren't. It can be difficult to find the leads and polarity by the part number because each manufacturer has its own system. This page explains my method for figuring out whether an unknown transistor is NPN or PNP, and which leads are the emitter, base, and collector.

First, remove the transistor from the board. Components can't be tested on circuit boards because they're connected to other components. You'll need a meter with a diode test function because meters don't put out a high enough voltage on their resistance test to turn on semiconductor junctions. Meters with diode test functions are not expensive. I've seen them for as little as $10.

Select the diode test function on your meter. Pick one of the three transistor leads at random and hook up the positive meter lead, then connect the negative meter lead to the other two leads in turn. If both give a reading, then the positive meter lead is connected to the base, and the transistor is NPN. Try the positive meter lead on all three transistor leads until the other two leads give readings. If none do, then the transistor is PNP. Repeat the above procedure with the meter leads reversed. When you find a lead which, when connected to the negative meter lead, gives readings with both the others leads, that's the base, and the transistor is PNP.

If none of the leads gives readings with both of the others leads in either polarity, then the component is burned out or is not a junction transistor. It might be a regulator or a field effect transistor.

Now that you know the polarity, and which lead is the base, you can figure out which lead is the collector and which is the emitter. Connect the meter leads to the unknown leads of the transistor. If the transistor is NPN, wet your finger and use it to connect the base of the transistor to the positive lead of the meter. If you get a reading, then the negative lead of the meter is connected to the emitter. If you don't, reverse the meter leads and repeat. If you still don't get a reading, then the component is burned out or is not a junction transistor.

If the transistor is PNP, use a wet finger to connect the negative meter lead to the base. If you get a reading, then the positive lead of the meter is connected to the emitter.

What you're doing is using the tiny current which can flow across your wet finger to turn the transistor on and allow a large current to flow from emitter to collector. Connecting the leads with a wire would let a large current flow through just one of the junctions, and wouldn't tell you anything about which leads were the emitter and collector.

Now you can fire up your soldering iron, grab your meter, and start ripping transistors out of everything in sight, confident that you'll be able to use them to build your own switches, amplifiers, oscillators, and timers!


Feel free to E-mail me with questions. They'll help me improve this page.



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