How do I Test a Transistor?

T. L. Childree

You can test a transistor for functionality by performing a few simple procedures using a digital multi-meter. Most digital-type multi-meters are equipped with a diode test function that can be used to test a transistor. If the transistor is already connected to a circuit board, it will have to be removed from the board prior to being tested. An electronic transistor may be used within a circuit as an amplifier or as a switch. Regardless of its application, the procedure used to test a transistor is the same because all transistors basically function as two parallel diodes that share a common element.

Transistors are typically identified by a printed code, which often includes one or two letters followed by a series of numbers.
Transistors are typically identified by a printed code, which often includes one or two letters followed by a series of numbers.

Before you can begin the actual testing procedure, you will need to identify the type of transistor you are testing. Transistors that are known as positive-negative-positive (PNP) have two input terminals and one output terminal. A transistor that is negative-positive-negative (NPN) will have one input terminal and two output terminals. Both types of transistors have a total of three terminals, which are known as the base terminal, collector terminal, and emitter terminal.

A multimeter can be used to test a transistor.
A multimeter can be used to test a transistor.

The type of transistor along with the location and identity of its terminals is typically marked on the transistor’s outer packaging. If the transistor type is not marked on the packaging, you can perform a simple test with a multi-meter to make this determination. Determine the orientation of the transistor’s three terminals and connect the positive lead of the multi-meter to the base terminal of the transistor. Next, connect the negative lead of the meter to either the collector or emitter terminal of the transistor. If the multi- meter displays a reading above zero, then the transistor is an NPN type.

Transistors must be removed from the circuit board before testing.
Transistors must be removed from the circuit board before testing.

Once you have determined the transistor type and the orientation of its terminals, you are ready to begin the actual testing procedure. To test a transistor for functionality, you will need to turn the dial of the multi-meter to the diode setting. Next, connect the meter’s positive lead to the base terminal of the transistor. You should then touch the negative lead of the meter to the transistor’s collector terminal and check for resistance. Next, touch the negative lead to the emitter terminal and check for resistance. After you have completed this procedure, you will need to perform the complete test again with the negative lead connected to the base terminal of the transistor.

If the transistor is functional, the resistance reading from the first portion of the test will be very low and the reading from the second part will be very high. If the transistor is a PNP type, you will need to perform the first part of the test with the negative lead connected to the base terminal and the positive lead will be connected during the second portion. If the transistor is functional, the first reading will be high and the second will be low. Transistors generally tend to stop working suddenly rather than gradually. It is usually less expensive to replace a faulty transistor than to replace the circuit board itself.

Transistors are often soldered onto a circuit board, which makes it difficult to determine if a problem on the circuit board arises from a transistor or something else.
Transistors are often soldered onto a circuit board, which makes it difficult to determine if a problem on the circuit board arises from a transistor or something else.

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