How can you test a transistor?

Answer 1

Testing Bipolar NPN or PNP transistorsIf you have an ohmmeter or continuity tester, it's not hard to check a Bipolar transistor. First, set your ohmmeter on "Diode" mode (or turn it to the lowest Ohms range.) Identify the transistor's Base pin.

Next, for NPN transistors connect the ohmmeter's positive lead to the base. Briefly touch the other meter lead to the transistor's Collector, then to the transistor's Emitter. Both connections should show a low-hm's reading: a diode-type connection. Then, connect the meter's negative lead to the transistor Base, and again touch the other meter lead to each of the other transistor pins. The meter should show open circuit or infinite ohms on both pins, indicating reverse-biased diodes.

If the transistor is PNP, just reverse the meter leads to perform the above tests. Touch the meter's negative lead to the Base pin, then verify that the Emitter and Collector pins behave as "turned on" diodes. Touch the meter's positive lead to the Base, and verify that the Collector and Emitter are acting like turned-off diodes.

Testing for "transistor" function, locating the EmitterIf you use your wet fingers as a resistor, sometimes you can use your ohmmeter to check for amplifier function, and also detect which pin is the emitter.

First test for diode function in order to locate the transistor's Base pin. Connect your ohmmeter between the other two pins (no connection to Base, yet.) Wet your fingers and touch the Base at the same time as one of the other pins. Note the ohms reading if any. Then touch your fingers between Base and the other transistor pin. Was one ohms reading much lower than the other? If not, then reverse the ohmmeter leads. Then touch your fingers between Base and one pin, then Base and the other pin. When you find one ohmmeter reading which is much lower than all the others, then you've located the proper polarity for transistor amplification. During the lowest ohms reading, your wet fingers were connected between Base and Collector. The other transistor pin then has to be the Emitter. (You can also perform this whole test more reliably by using a 1K resistor in place of your wet fingers!)

Testing an FETIt's a bit more difficult, but FETs can be tested using an ohmmeter, a 9v battery, and wet fingers. (During low-humidity winter weather, when high static voltages are present, you might want to wear a grounding strap!)

First let's test for correct operation of a common N-channel depletion-mode MOSFET. Connect your ohmmeter between the FET's Source and Drain pins, with the positive meter lead on the Drain. While watching the ohms reading, touch the 9V battery's negative terminal to the Source or Drain, then use your wet fingers to briefly connect the battery's positive terminal to the transistor Gate pin. The ohms value between Source and Drain should become low. Remove the battery and briefly touch your wet fingers between Source, Drain, and Gate. The ohms reading should go high again. You've just applied and removed a charge to the floating Gate pin, switching the transistor on, then off again.

For testing a P-channel FET, just reverse all the polarities. Connect the ohmmeter to Source and Drain (positive meter lead to Source.) Use your wet fingers to touch the battery's negative terminal to the Gate pin (with battery positive touching Source or Drain.) That should make the ohms value go low.)

NOTE ON ALL OF THE ABOVE: I assume that the transistor is disconnected from any other circuit. If your transistor is soldered into a circuit board, most of these tests won't work.