4 Ways To Test A Transistor - WikiHow

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Terms of Use wikiHow is where trusted research and expert knowledge come together. Learn why people trust wikiHow How to Test a Transistor PDF download Download Article Explore this Article methods 1 Setting Up the Multimeter 2 Testing When You Know the Base, Emitter and Collector 3 Testing Without the Base, Emitter and Collector 4 Understanding Transistors + Show 1 more... - Show less... Other Sections Questions & Answers Tips and Warnings Things You'll Need Related Articles Expert Interview References Article Summary Co-authored by Richard Taylor

Last Updated: July 17, 2025 Approved

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This article was co-authored by Richard Taylor. Richard Taylor is an Electrician and Owner of R H Taylor Electrical Services based in Vero Beach, Florida. Richard has over 35 years of professional experience. He and his team offer electrical installation, re-wiring, and repair services. wikiHow marks an article as reader-approved once it receives enough positive feedback. This article received 12 testimonials and 81% of readers who voted found it helpful, earning it our reader-approved status. This article has been viewed 573,763 times.

A transistor is a semiconductor that allows current to flow through it under certain conditions, and cuts current off when other conditions are present. Transistors are commonly used as either a switch or a current amplifier. You can test a transistor with a multimeter that has a diode test function.[1]

Steps

Method 1 Method 1 of 4:

Setting Up the Multimeter

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  1. Step 1 Insert the probes into the multimeter. 1 Insert the probes into the multimeter. The black probe goes into the common terminal and the red probe goes into the terminal marked for testing diodes.[2]
  2. Step 2 Turn the selector knob to the diode test function. 2 Turn the selector knob to the diode test function.[3] Advertisement
  3. Step 3 Replace the probe tips with alligator clamps. 3 Replace the probe tips with alligator clamps.
    4. Advertisement
Method 2 Method 2 of 4:

Testing When You Know the Base, Emitter and Collector

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  1. Step 1 Determine which leads are the base, emitter and collector. 1 Determine which leads are the base, emitter and collector. The leads are round or flat wires extending from the bottom of the transistor. They may be labeled on some transistors or you may be able to determine which lead is the base by studying the circuit diagram.
  2. Step 2 Clamp the black probe to the base of the transistor. 2 Clamp the black probe to the base of the transistor.[4]
  3. Step 3 Touch the red probe to the emitter. 3 Touch the red probe to the emitter. Read the display on the multimeter and note whether the resistance is high or low.[5]
  4. Step 4 Move the red probe to the collector. 4 Move the red probe to the collector. The display should give the same reading as when you touched the probe to the emitter.
  5. Step 5 Remove the black probe and clamp the red probe to the base. 5 Remove the black probe and clamp the red probe to the base.
  6. Step 6 Touch the black probe to the emitter and collector. 6 Touch the black probe to the emitter and collector. Compare the reading on the multimeter's display to the readings you got previously.
    • If the previous readings were both high and the current readings are both low, the transistor is good.
    • If the previous readings were both low and the current readings are both high, the transistor is good.
    • If both readings you receive with the red probe are not the same, both readings with the black probe are not the same, or the readings don't change when switching probes, the transistor is bad.
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Method 3 Method 3 of 4:

Testing Without the Base, Emitter and Collector

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  1. Step 1 Clamp the black probe to 1 of the leads of the transistor. 1 Clamp the black probe to 1 of the leads of the transistor.
  2. Step 2 Touch the red probe to each of the other 2 leads. 2 Touch the red probe to each of the other 2 leads.
    • If the display shows high resistance when each of the leads are touched, you have found the base (and you have a good NPN transistor).
    • If the display shows 2 different readings for the other 2 leads, clamp the black probe to another lead and repeat the test.
    • After clamping the black probe to each of the 3 leads, if you don't get the same high resistance reading when touching the other 2 leads with the red probe, you either have a bad transistor or a PNP transistor.
  3. Step 3 Remove the black probe and clamp the red probe to 1 of the leads. 3 Remove the black probe and clamp the red probe to 1 of the leads.
  4. Step 4 Touch the black probe to each of the other 2 leads. 4 Touch the black probe to each of the other 2 leads.
    • If the display shows high resistance when each of the leads are touched, you have found the base (and you have a good PNP transistor).
    • If the display shows 2 different readings for the other 2 leads, clamp the red probe to another lead and repeat the test.
    • After clamping the red probe to each of the 3 leads, if you don't get the same high resistance reading when touching the other 2 leads with the black probe, you have a bad PNP transistor.
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Method 4 Method 4 of 4:

Understanding Transistors

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  1. Step 1 A transistor is basically 2 diodes that share one end. 1 A transistor is basically 2 diodes that share one end. The shared end is called the base and the other 2 ends are called the emitter and collector.[6]
    • The collector accepts an input current from the circuit, but it can't send the current through the transistor until allowed to by the base.
    • The emitter sends a current out into the circuit, but only if the base allows the collector to pass the current through the transistor to the emitter.
    • The base acts like a gate. When a small current is applied to the base, the gate opens and a large current can flow from the collector to the emitter.
  2. Step 2 Transistors may operate by junctions or field effects, but both come in two basic types. 2 Transistors may operate by junctions or field effects, but both come in two basic types.[7]
    • An NPN transistor uses a positive semiconductor material (P-type) for the base and negative semiconductor material (N-type) for the collector and emitter. On a circuit diagram, an NPN transistor shows an emitter with the arrow pointing out ("Never Points iN" aids remembering).
    • A PNP transistor uses an N-type material for the base and P-type material for the emitter and collector. The PNP transistor show an emitter with the arrow pointing in ("Points In Permanently" is the memorizer).
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Community Q&A

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  • Question How do I differentiate between transistors? Community Answer Community Answer By clamping the black probe to one of the three leads. If the multimeter displays high resistance when each lead touched, the transistor is NPN and is good. The reverse of the above conditions is PNP transistor. Thanks! We're glad this was helpful. Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 11 Helpful 33
  • Question How can I tell the difference between a collector and an emitter? Community Answer Community Answer A collector accepts input current from the circuit. An emitter send current out into the circuit. Thanks! We're glad this was helpful. Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 45 Helpful 50
  • Question Is there any difference between the resistance value of base emitter and base collector? Community Answer Community Answer Yes - voltage drop is a little higher for the base to emitter than for the base to collector. Thanks! We're glad this was helpful. Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 11 Helpful 21
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Tips

  • You can also test a transistor in a circuit with a 6 volt power source and 2 lamps, which are small light bulbs. Or you connect the multimeter across the emitter and collector, then short the collector to the base. Thanks Helpful 0 Not Helpful 2
Submit a Tip All tip submissions are carefully reviewed before being published Name Please provide your name and last initial Submit Thanks for submitting a tip for review! Advertisement

Things You'll Need

  • Multimeter with leads
  • Alligator clips
  • Transistor

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Expert Interview

Thanks for reading our article! If you’d like to learn more about transistors, check out our in-depth interview with Richard Taylor.

References

  1. ↑ Richard Taylor. Electrician. Expert Interview
  2. ↑ https://learn.sparkfun.com/tutorials/how-to-use-a-multimeter/all
  3. ↑ https://learn.sparkfun.com/tutorials/how-to-use-a-multimeter/all
  4. ↑ https://sciencing.com/diagnose-circuit-board-bad-transistor-8049011.html
  5. ↑ https://sciencing.com/diagnose-circuit-board-bad-transistor-8049011.html
  6. ↑ https://www.explainthatstuff.com/howtransistorswork.html
  7. ↑ https://www.allaboutcircuits.com/textbook/semiconductors/chpt-5/junction-field-effect-transistors-jfet/

About This Article

Richard Taylor Co-authored by: Richard Taylor Electrician This article was co-authored by Richard Taylor. Richard Taylor is an Electrician and Owner of R H Taylor Electrical Services based in Vero Beach, Florida. Richard has over 35 years of professional experience. He and his team offer electrical installation, re-wiring, and repair services. This article has been viewed 573,763 times. 18 votes - 81% Co-authors: 28 Updated: July 17, 2025 Views: 573,763 Categories: Electrical and Electronic Circuits Article SummaryX

To test your transistor, first clamp the black probe of a multimeter to the transistor’s base. Then, touch the red probe to the emitter and read the display to see if the resistance is high or low. Next, move the red probe to the collector, and check that the reading is the same as it was before. Once you have those readings, remove the black probe from your transistor and clamp the red probe to the base. You can then touch the black probe to the emitter and collector and look at the new readings. Your transistor is good if your previous readings were both high and the current readings are both low, or the opposite is true. To learn how to test your transistor when you don’t know the base, emitter, and collector, keep reading! Did this summary help you?YesNo

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Yes No Advertisement Cookies make wikiHow better. By continuing to use our site, you agree to our cookie policy. Richard Taylor Co-authored by: Richard Taylor Electrician Co-authors: 28 Updated: July 17, 2025 Views: 573,763 81% of readers found this article helpful. 18 votes - 81% Click a star to add your vote Bobby S.

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"This knowledge will enable me to fix my electronics or, at worst, teach me how to properly test each component to..." more Tonuan Tokaka

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"Well, thanks very much for providing a good article, so I really appreciated your help, but I have a bit of..." more Anonymous

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"It really helps and now I can test my transistor and identify what type it is with base collector and it's..." more Chimdesa Asebe

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