How to Test a MOSFET

By John Papiewski
You can test a MOSFET with a digital multimeter.

If your stereo has just died, a bad MOSFET may be the culprit. Many high-powered stereos in 2011 use them to handle the large currents in the amplifier’s output stages. Luckily, a simple test will tell you if the MOSFET’s good or not. Some digital multimeters have a diode test mode that puts 3 to 4 volts across the device you’re testing. If the MOSFET is working, the multimeter’s voltage will switch it on and off. A dud MOSFET won’t respond to the voltage.

Clip the anti-static wrist strap’s ground connector to a solid electrical ground, such as a metal cold-water pipe or conduit. Slip the strap onto your wrist.

Identify which of the MOSFET’s three leads are its source, gate and drain. Refer to the MOSFET’s data sheet, if necessary.

Turn the digital multimeter on and set its selector knob for the diode test mode.

Grasp the MOSFET by the case. Avoid touching the leads. Connect the negative multimeter probe to the source lead. Touch the positive meter probe to the gate lead.

Move the positive meter probe to the drain lead. Read the multimeter’s display. It should indicate a low resistance.

Keep the positive probe at the drain lead. Grasp the source and gate with your fingers. The meter should now show a high resistance reading. If it doesn’t, replace the MOSFET.

Things Needed

  • Anti-static wrist strap
  • MOSFET transistor
  • Data sheet for MOSFET transistor
  • Digital multimeter with diode test mode

Tip

The term MOSFET is short for "metal oxide semiconductor field-effect transistor." They have metal oxide junctions instead of silicon junctions, as other kinds of transistors have. You can find them in computers, power systems and amplifiers.

Warning

Make sure you're testing either a loose MOSFET or one in a circuit that has had the power turned off. Never attempt to service electronic equipment while it's plugged into an outlet.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."