How to Test a MOSFET with a Multimeter

MOSFETs are common in power supplies, motor drives and switching circuits. A failed power MOSFET often develops a drain-source short, but a multimeter can also reveal gate damage and body-diode behavior.

Confirm the pinout first

Identify gate, drain and source from the exact device datasheet. Similar packages can use different arrangements, and an incorrect assumption can make a good device appear faulty.

Check drain to source

With the circuit unpowered, look for an abnormally low resistance between drain and source. Remember that a MOSFET normally includes a body diode, so diode-mode behavior is directional.

Check gate isolation

The gate is insulated and should not normally show a low-resistance path to source or drain. A clear gate short is a strong indication of failure.

Gate charge can change the reading

A meter can charge the gate enough to switch some MOSFETs temporarily. If drain-source behavior changes during testing, discharge the gate to source and repeat the measurement in a controlled way.

In-circuit readings need context

Gate resistors, driver circuits, snubbers and parallel MOSFETs can affect results. Isolation or comparison with a known-good channel may be required.