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mercoledì 22 luglio 2015

How to easily test an N-CHANNEL or P-CHANNEL MOSFET using just a multimeter

You should have noticed that most multimeters come with a function to test BJT transistors, but none has a function to test the today's more common MOSFETs

Here are the steps to test an N-CHANNEL mosfet with just a multimeter ( applies to P-CHANNEL too, just swap + and - ), of course the MOSFET has to be disconnected completely.

  1. Make sure you are not wearing shoes and so that you don't have any electrostatic charge, FETs are very delicate to ESD.
  2. Now touch with your finger both Gate and Source terminals, this will make sure that the gate is uncharged
  3. Put your multimeter in diode/continuity test mode ( beep mode )
  4. Touch with the negative lead the source pin, and with positive lead the drain, it should not beep and show > 999 ohm, if there's current flowing, after making sure again that gate is discharged, the MOSFET is damaged.
  5. Touch with the negative lead the source pin and with positive lead the gate pin, no current should flow, if current flows it means that the oxide layer between gate and substrate or N junctions has been broken by a gate-source overvoltage or ESD, trash the mosfet if that is the case.
  6. Now WITHOUT touching the gate terminal at all, touch the drain pin with positive lead and source pin with negative lead, you should get the multimeter to beep and a very low resistance reading
Finally, before trashing any mosfets, make sure that the method works with most of them using your multimeter ( there may be some multimeters that use too low voltage in continuity test mode and thus not reaching Vt threshold of the mosfet ).
To avoid ESD damage always store your MOSFETS with all pins joined together using aluminum foil or copper strands.

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