Transistor test: FQP30N06L N-Channel MOSFET transistors in a breadboard

The Only Transistor Test You Need (Simple Multimeter Method)

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In this transistor test guide, we show you how to perform a transistor test using a multimeter. We cover both BJT (NPN and PNP) and MOSFET transistors, and also introduce other common types like JFETs and Darlington transistors. We explain when to use a transistor tester, and how to recognize a faulty transistor.

Checking a transistor with a multimeter might sound intimidating, but it’s actually a simple and valuable skill for anyone getting started with electronics. Whether you’re troubleshooting a circuit or learning how transistors work, knowing how to test a transistor using a multimeter can save time and frustration.

Table of Contents

Why You Should Know How to Test a Transistor

Whether you’re troubleshooting a circuit or salvaging components, knowing how to perform a transistor test is a valuable skill. This guide is designed for beginners and walks you through testing different types of transistors—like NPN, PNP, N-Channel MOSFETs, and P-Channel MOSFETs—using a multimeter or a transistor tester.

You’ll also learn the basics of checking a transistor for faults and how to use different methods to identify faulty parts, whether you’re using a simple multimeter or a dedicated transistor tester.

What Is a Transistor?

A transistor is a three-terminal electronic component used to amplify signals or act as a switch. It’s one of the most essential components in modern electronics and is used in everything from computers and radios to power supplies and motor drivers.

There are several types of transistors, including:

  • BJT (Bipolar Junction Transistor) – Comes in NPN and PNP types.
  • MOSFET (Metal Oxide Semiconductor Field Effect Transistor) – Includes N-Channel and P-Channel versions.
  • JFET (Junction Field Effect Transistor) – N-Channel or P-Channel transistors commonly used in analog applications.
  • Darlington Transistor – A high-gain transistor pair in one package.
  • IGBT (Insulated-Gate Bipolar Transistor) – Used in high-power switching applications.

Tools You’ll Need to Test a Transistor

  • Digital Multimeter (with diode mode) – see our diode test article for more information
  • Transistor (to test)
  • Optional: Transistor tester / component tester

How Transistors Fail (And What to Look For)

Transistors aren’t indestructible—over time, they can fail due to excessive heat, electrical overstress, or simple aging. One of the most common causes of failure is applying voltage or current that exceeds the device’s ratings. Voltage spikes, in particular, can punch through the internal junctions instantly.

When a transistor fails, it usually does so in one of two ways:

  • Short circuit – Often between collector and emitter, but sometimes between all three pins.
  • Open circuit – Where the transistor no longer conducts between key terminals like base-emitter or collector-emitter.

You can often detect a bad transistor with a simple multimeter test, but here are a few physical signs to watch for:

  • Burn marks or discoloration on the casing
  • Cracked or melted housing
  • A transistor that feels unusually hot to the touch (after the circuit is safely powered off)

Safety Note: Never touch components while the circuit is powered, especially if you’re working near mains electricity. Always switch off and unplug the equipment before testing or handling parts.

Tips for Testing Transistors

  • Always remove the transistor from the circuit before testing.
  • Use the diode test mode on your multimeter.
  • Know the transistor’s pinout – look up the datasheet if needed.
  • Compare readings with a known-good transistor.
Examples of multimeters set to the diode mode for a transistor test
Examples of Multimeters Set to the Diode Mode for a Transistor Test

Testing a BJT Transistor Using a Multimeter

BJTs have three pins: collector (C), base (B), and emitter (E). Here’s how to check an NPN or PNP transistor using a multimeter.

First get the datasheet for the transistor. This is necessary because different transistors that have the same package can have different pin numbering. The first part of a datasheet for a PN2222A NPN transistor is shown in the following image. As can be seen in the image, the transistor is housed in a TO-92 package. When looking at the flat face of this transistor, the pin numbering is Emitter, Base, and Collector from left to right.

First part of a transistor datasheet
The First Part of a Transistor Datasheet Showing the Transistor Pin Numbering

NPN Transistor Test

  1. Set the multimeter to diode mode.
  2. Connect the positive probe to the base and the negative probe to the collector – You should see a reading around 0.6V to 0.7V. This is like testing a forward biased diode between B and C – see the NPN transistor diagram below.
  3. Move the negative probe to the emitter – Similar reading (~0.6V to 0.7V). This is like testing a forward biased diode between B and E – see the NPN transistor diagram below.
  4. Reverse the probes – You should see no reading, which is usually displayed as OL on the multimeter. That is: Test the reverse biased diode between C and B. Then test the reverse biased diode between E and B. Again, see the NPN transistor diagram below.

If you get a short (0V) or open (no voltage drop) reading in both directions, the transistor is faulty.

Diode test mode selected on a multimeter
Diode Test Setting on a Multimeter

To test an NPN transistor, look at the transistor as two diodes with their anodes connected that represent the transistor base (B). The following image illustrates this concept. Now test these two ‘diodes’ using the diode test. This is the test described in the steps above. Just make sure you select the correct pins for emitter, base and collector for your particular NPN transistor part. Do a forward and reverse test for each ‘diode’ in the transistor.

NPN transistor test diagram
NPN Transistor Test Diagram Showing that the Test is Like Testing Two Diodes

PNP Transistor Test

  1. Again, set the multimeter to diode mode.
  2. Connect the negative probe to the base, and the positive probe to the collector – Reading around 0.6V to 0.7V.
  3. Move the positive probe to the emitter – Similar reading.
  4. Reverse the probes – No reading should be seen – usually OL.

If you get a short (0V) or open (no voltage drop) reading in both directions, the transistor is faulty.

As with the NPN transistor, the PNP transistor can be looked at as two diodes connected together. In the case of a PNP transistor, the base of the transistor is where the two diode cathodes are joined. The following image illustrates this connection. Just test these two ‘diodes’ using the diode test.

PNP transistor test circuit
PNP Transistor Test Circuit Diagram

Common MOSFET Failures and Testing Tips

Field Effect Transistors (FETs), especially MOSFETs, can handle large currents with minimal heat when used correctly—but they can be sensitive to voltage spikes and static discharge.

Here are a few key points to know:

  • Gate Drive Voltage Matters: MOSFETs typically require 2V to 5V between Gate and Source to fully turn on. If the gate voltage is too low, the MOSFET may partially turn on, resulting in overheating.
  • Low On-Resistance: When fully turned on, a MOSFET has a very low resistance between Drain and Source (just a few milliohms). This allows high current flow with minimal voltage drop.
  • Static Sensitivity: Power MOSFETs, especially those in TO-220 packages, can be damaged by electrostatic discharge (ESD). Avoid touching the Gate directly and handle them with care when unprotected.
  • Typical Failure Mode: When a MOSFET fails, it often becomes shorted internally. In many cases, all three terminals (Gate, Drain, and Source) may read as connected (shorted) when checked with a multimeter in diode mode. This is a quick way to confirm a dead MOSFET.
  • Replacing a MOSFET: Always match the replacement part’s voltage and current ratings. Also check the gate threshold voltage (Vgs(th)) to ensure your circuit can fully turn the new MOSFET on.

Understanding these failure modes can help you troubleshoot power electronics more effectively, and prevent future damage by choosing and using the right components.

How to Check a MOSFET Using a Multimeter

MOSFETs are different from BJTs and require slightly different testing. MOSFET testing using a multimeter for N-channel and P-channel devices follows.

N-Channel MOSFET Transistor Test

  1. Identify the Drain (D), Source (S), and Gate (G).
  2. Set the multimeter to diode mode.
  3. Touch the positive probe to Drain, and negative to Source – You should see a diode drop.
  4. Charge the gate by briefly touching the positive probe to Gate and the negative to Source – Now the Drain to Source will show continuity (on).
  5. Discharge the gate (short Gate to Source) – Drain to Source should now show open.

P-Channel MOSFET Transistor Test

Follow similar steps, but reverse the polarity of the probes.

IGBT Transistor Test

IGBTs combine the input characteristics of a MOSFET with the output of a BJT. Testing them is similar to N-Channel MOSFETs:

  1. Identify the Gate (G), Collector (C), and Emitter (E).
  2. Apply the same test steps as an N-Channel MOSFET.

Using a Transistor Tester

If you have a transistor tester multimeter or a dedicated component tester, you can simply insert the transistor and read the type, gain (hFE), and pinout on the screen. This tool simplifies checking a transistor of any type, especially useful for beginners.

Other Types of Transistors Worth Knowing

Besides BJTs and MOSFETs, there are several other transistor types you may encounter as you dive deeper into electronics:

  • JFETs (Junction Field Effect Transistors) – Analog-friendly transistors that are normally on and controlled by gate voltage. To test, measure resistance between drain and source, then apply reverse voltage to the gate. A good JFET will show reduced conduction with gate bias.
  • Darlington Transistors – Paired BJTs in one package for higher current gain. Can be tested like BJTs, but the forward voltage drop across base-emitter will be around 1.2V to 1.4V.
  • Phototransistors – Light-sensitive transistors used in sensors. You can shine a flashlight and measure collector-emitter conduction.
  • Unijunction Transistors (UJTs) – Used in triggering and timing circuits. Less common in beginner electronics but worth knowing.

Each type may have a slightly different testing method, so always consult a datasheet or use a transistor tester for quick identification.

Final Thoughts on How to do a Transistor Test

Learning how to check a transistor with a multimeter is a great skill to build your confidence with electronics. Whether you’re testing a small NPN transistor or troubleshooting an N-Channel MOSFET in a power circuit, these methods can help identify faulty components and keep your projects running smoothly.