Introduction: Test a Bi-polar Transistor (out of Circuit)

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first to…

You built a one transistor project and it worked great, but now it has stopped working. You decide the transistor may be faulty. But, you are not sure how to test it.

This Instructable is for testing a transistor after it has been removed from the circuit. When removing it from the circuit, always use a heat sink to protect the diode junctions from failure due to too much heat.

Pictured is an ordinary 2N2222 NPN low voltage bi-polar switching transistor. The order of pins from left to right is collector-base-emitter. The flat front provides the proper orientation for viewing the transistor. The order of the pins can vary, but the scheme used on this transistor is pretty common.

Step 1: Bias the Transistor for Testing

Use a 470 ohm resistor and a volt-ohmmeter with a diode check feature to check the transistor. As you can see a 470 ohm resistor has a yellow (4)-violet (7)-brown (x10) color band code.

The red lead from the meter connects to the positive socket on the meter. The black lead connects to the negative or common socket on the meter.

Normally, I just hold the resistor in one hand with the leads bent so I can touch two legs of the transistor at the same time. But, I needed one hand to operate the camera, so I used a breadboard to set up these photos. One lead of the resistor connects to the collector. The other lead of the resistor connects to the base. The positive lead (red) from the meter is connected to the collector. The negative or common lead (black) is connected to the emitter.

If this were a PNP transistor, rather than an NPN transistor, the red and black leads from the meter would be reversed in their positions.

Step 2: Turn on the Meter and Look for a Reading

Set the meter to the diode check position and turn the meter "on." If the transistor is good, there will be a reading similar to what you would expect across a diode junction. A word of caution: a weak or leaky transistor can show "good" with this test and still be faulty.

If you do not have a diode checker function on your meter, you can use the ohmmeter function, but the scale will have to be set to a very high range. I do not know why. It just works that way with my meter. Be careful, though. The reason you want a diode checker function is that it limits the current to the diode junctions in the transistor and protects them from overloads that could damage them. An ohmmeter could send too much current through the transistor and damage or destroy it.

I first became aware of this method of testing a bi-polar transistor through Using Your Meter by Alvis J. Evans. It was sold through Radio Shack and has a 1985 copyright.

Step 3: Testing Power Transistors

The same process for testing a small bi-polar transistor can be used for testing large power transistors. The metal case is the collector. The two pins are a bit above the center of the transistor, as you can visualize here. The left pin is the base. The right pin is the emitter. Use a 100 ohm resistor to bias the transistor instead of a 470 ohm resistor.