Checking power MOSFET with simple tester

Having lots of second hand power mosfets? These interesting electronic components are very useful in hobby constructing. It replaces relays and simple transistors. But how to test them quickly?
All you need is simple tester. Almost every tester have buzzer for circuit continuity testing and in same configuration you can test diodes. The multimeter LCD show voltage dropout on diode. Now look at schematic diagram of N-channel mosfet.
N-channel power mosfet
You can see diode in drain-source path. Lets check it:
mosfet internal diode
You can see 0.5V dropout on internal Schottky diode. In reverse there must be nothing if mosfet is open.
Reverse internal diode
My multimeter shows “1” if open circuit.
Now we must check the gate. There must be no connection in any other pin and in any other direction.
But here comes most interesting part. When we connect wires to gate, in one case we open device and in other we close. If power mosfet is good quality, the residual charge of testing leave the device open. So touch the gate and check the source- drain path.
open mosfet
It showing short circuit, as device is open ant the resistance is only fractions of ohms. Discharge the gate, and the device must close. P-channel devices work in same way, only polarity of gate is reversed.

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6 Responses to Checking power MOSFET with simple tester

  1. Seth Amankrah says:

    i hve a fet in my 50 ins plasma Lg Tv with number 30F124 can you help me its characteristic or symbol

  2. Maybe GT30F124? It is IGBT made by Toshiba for PDP (plasma display panel).
    Vces=300V, Ic(pulsed)=200A, Vce(@120A)=2.3V , in TO220 package.

  3. Michael says:

    Thank you! Very helpful!

  4. wathiq says:

    I am currently writing up a PhD thesis. I am researching USVs. I came across an interesting image from the following website
    [MOSFET schematic]

    I wish to use include this image in my thesis. Could you kindly provide me with a better resolution image which can be reproduced in print. The image will be referenced to you in the PhD. If you have a standard format to reference your image kindly provide me the same.

    Many Thanks,


    Plymouth university

  5. William Fernandes says:

    4 pin Power MOSFET on Bigger heatsink of Power supply Board for Chassis MJ17AS of Samsung syncmaster 710 N, 17 inch LCD Monitor has blown up and I do not have the No. of the MOSFET to replace it, how can I get the No. to replace it, is it possible for you to help me out of this situation.

  6. Are you sure it is mosfet? There is no 4 pin mosfets in this device.
    Please show photo of your device.

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