Testing MOSFET's and "low" readings

[Solder Boy]

Did some research on testing MOSFET's.. trying to determine if the one I am working with is a dud or not. My DVM had a Diode setting. Followed the instructions I found on the Net and I read that I should get a "low" reading.

Not sure how low is low... the reading I got was around 975.

Thanks!

[severach]

Re: Testing MOSFET's and "low" readings

Many low voltage MOSFETs have such a low Rds(on) that they appear to almost be a short when turned on. Your DVM must be able to tell the difference between 0.010 voltage drop and 0.001 voltage drop. Sadly some cannot though they display enough digits to pretend that they do.

A MOSFET will always conduct in one direction between drain and source from the body diode. 0.975 voltage drop sounds like a body diode on a high voltage mosfet.

The gate is effectively a capacitor and can be made to stay on by charging it up and disconnecting. The mosfet may appear to be shorted. If the short appears and disappears as you flip the polarity on the gate then the mosfet is working. Your finger is enough to drain the gate and make the mosfet stop conducting.