How to Check a Transistor With a Digital MultiMeter

Keeping track of the components of an electrical circuit is important. You may want to know the voltage or current sent through resistors and other circuit elements for the sake of making sure they run with ease and safety. Various tools such as multimeters and ohmmeters are helpful for these purposes

For a diode test of transistors, you can be on the lookout for bad transistor symptoms. Transistors are used in diodes, circuit elements that let electricity flow through in only one direction. They're used in amplifying the electric current to a higher value.

They're engineered by sandwiching a thin slice of either n-type material between two bigger pieces of p-type material or p-type material between two bigger n-type pieces. In this setup, the p-type materials are positive due to an absence of electrons while n-type ones are negative due to an excess of electrons.

If you notice your circuit isn't providing results that are as effective as they could be, it may be time to test your transistor. Testing can help you figure out if the transistor is working as well as it could be. You'd use a multimeter, a digital device that measures various electric properties for circuit elements.

Transistor Testing Procedure

There are five steps to test a transistor in an electric circuit. The steps involve connecting the:

  1. Base to the emitter
  2. Base to collector
  3. Emitter to base
  4. Collector to base
  5. Collector to emitter

For an NPN transistor, the emitter is grounded with the collector at a voltage that the base controls. For a PNP design, the collector is grounded with the emitter at a voltage.

These methods of testing tell you if a transistor is shorted or opened for bipolar transistors. The transistor may still fluctuate in its performance, within a specific range, just as a result of how it was engineered.

To begin the transistor testing procedure, remove the transistor from the circuit itself. Take your multimeter and connect the positive lead to the base of the transistor. Then, connect the negative lead to the transistor's emitter.

At this point, check the reading on your multimeter. An NPN transistor that functions properly should show a voltage drop between 0.45 and 0.9 volts, and a PNP transistor should show an "over limit" message. Any signs on the multimeter that differ from these values could indicate bad transistor symptoms.

Then, connect the negative lead of the multimeter to the collector of the transistor; this is the "base to collector" step. As was the case with the previous step, an NPN transistor should have a voltage drop between 0.45 and 0.9 volts while a PNP one should be over the limit.

Switching the Readings

For the "emitter to base" step, connect the positive multimeter lead to the emitter and the negative to the base. In this case, the readings should be reversed. The NPN transistor should show an "over limit" message, and for the PNP, a voltage drop between 0.45 and 0.9 volts. Similarly, with the positive lead connected to the collector and the negative lead to the base, you should see the same results on the multimeter.

For the fifth and final step, connect the positive lead to the collector and the negative to the emitter. Both the the PNP and NPN designs should show "over limit" messages. Switch the leads with one another, and you should see the same messages.

It's also helpful to determine which lead corresponds to which in an unmarked transistor by looking at the voltage drops themselves and determining which ones correspond to which.

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About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.