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
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
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.