MOSFET Transistor and Diode Checks

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How-To Check a MOSFET Transistor

Standard MOSFET Transistor
Standard MOSFET Transistor

  • G ==> Gate
  • D ==> Drain
  • S ==> Source

1: Check that your transistor is not connected to your circuit and turn on the multimeter.

2: Connect the multimeter's negative lead to S. While holding the transistor, be sure to only grip it by its plastic casing so as not to interfere with the reading.

3: Touch the multimeter's positive lead to G to charge the internal capacitor. The MOSFET is now "on".

4: Move the multimeters's positive lead to D. You should see a low reading (<1 Ohm) on the multimeter. This verifies that the MOSFET is on and that it is a conductive device.

5: With everything still connected (negative to S & positive to D) touch your finger across all three pins, discharging the gate. You should get a high reading (>50 kOhms). This indicates that the MOSFET is now non-conductive.

If at any point in the above steps you lose contact between either of the multimeter leads and the transistor, touch your finger across all three pins and start over. If you got through all the steps and got the expected readings, then your MOSFET transistor is functioning.

How-To Check a Diode

Standard Diode
Standard Diode

1: Ensure that the diode is either completely disconnected from the circuit board or connected only on one side.

2: Connect the multimeter's negative lead to one end of the diode and the positive lead to the other end. Take note of the resistance reading on the multimeter.

3: Reverse the leads and take note of the new reading on the multimeter. If the two readings are close to each other in value, then the diode is bad. An ideal diode has a resistance higher than 50 kOhms in one direction and gives an "Overload" reading on the multimeter when connected in the opposite direction. This indicates high resistance one way and a short the other way, which is exactly what a functioning diode is supposed to be.

Standard Diode Diagram
Standard diode diagrammed in both orientations described in the How-To above