How to test MOSFET's


100 kW
May 24, 2006
How I found a bad FET:

"(From: Bruce (

You can get a pretty good idea about the condition of a MOSFET with some quick & simple bench tests. The first thing you can do with a meter is measure the parasitic substrate diode that connects the drain to the source. In an NMOS part, this diode's cathode will be at the drain, and the anode at the source. It will meter out similar to any conventional diode in both fwd / reverse directions. You can see this diode in the schematic representation of the FET in some databooks and a few schematics. The FET should show infinite resistance, gate - source and gate - drain. If it does not, then the gate oxide may be blown."

There's a lot more you can do then that, but it's all I had to do to find it.

I had found out what pins did what by searching mouser for the part #, then opening up a datasheet from one of the compatible matches.

They were still on the board connected to other stuff when I first tested them, so they didn't react as in the above description. However I was still able to find one that was way off the mark. After removing it from the board, I tested it again and confirmed that it's bad.

I did find a warning regard dead FETs and controllers:

"When MOSFETS fail they often go short-circuit drain-to-gate. This can put the drain voltage back onto the gate where ofcourse it feeds (via the gate resistors) into the drive circuitry, prossibly blowing that section. It will also get to any other paralleled MosFet gates, blowing them also.
So, if the MosFets are deceased, check the drivers as well! This fact is probably the best reason for adding a source-gate zener diode; zeners fail short circuit and a properly connected zener can limit the damage in a failure! You can also add subminiature gate resistors -- which tend to fail open-circuit (like a fuse) under this overload, disconnecting the dud MosFet's gate."

In other words, the driver on my controller is probably blown...

Still I read this:

"Dying MOSFETS often emit flames or blow-out, even more so in hobby built electronics projects. What that means is that a defective unit can usually be spotted visually. They show a burned hole or 'something black' somewhere. I have seen them alot especially in ups's which can have as many as 8 or more mosfets in parallel. I always replace all of them if a couple are defective plus the drivers."

Mine looks no different then the others...


Staff member
Dec 31, 2006
California Bay Area, USA
I agree with the part about the gate drivers blowing. I've seen that happen repeatedly. Using tiny gate resistors works like a charm.
I usually get the FETs to blow up real good so they're easy to spot.
No question about this one:


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10 kW
May 25, 2006
i have repaired lots of controllers with blown fets, and there are a couple of things i have learned. first is that if the fet gate to drain is shorted it will usually blow the driver. in the crystalytes this is an ir2101, an 8 pin smt that is easy to get. blown drivers will usually show high voltage on the control inputs which are isolated from the controller chip by resistors.

in the crystalytes they have 10 ohm sm0805 resistors in series with the gates. i have found that fets that have blown can stress these resistors changing the value dramatically, which will negatively affect the ability of the fets to share current. i replace the 10 ohm units with a stacked pair of 20 ohm.