ODD FAILURE, just shows anything can happen

[Seekonk]

I have a FET switch that is driven remotely from about 100 feet.  This has been running for years and yesterday it wouldn't turn off. It is a 100A FET that only sees about 1A max.  I removed power and the FET measured near zero ohms.  Then I checked the gate voltage, that was almost zero V.   I measured the FET again and it showed a couple meg D-S.  It is a simple circuit.  12V switch goes to 47K at the FET gate with a .03uF for filtering whatever RF may come from that 100 feet of wire.  Even have a 18V zener for spike protection. The gate drain of resistor is a new 300K 1W metal film from a good manufacturer rated for 2KV pulse duty.  I used this one watt because it was handy and I got thousands of them.  Sometimes I just cut the leads off for jumpers and throw away the body.  When I measured the resistance it was totally open. The meter load was enough to drop it to zero.  A 100 meg and the circuit still would have worked.  We never had this resistor fail in production. Just shows it can be the last thing you would expect.

[german77]

It looks to me, that vibration was the main cause of failure.

[Seekonk]

It is bolted to the wall of my power shed. Nothing vibrates, just electronics. A 1W resistor is mechanically pretty robust.

[Relayer]

Hello Seekonk,
What is the maximum Gate to Source voltage rating of the MOSFET you're using?
The problem with measuring FET's and MOSFET's is that they have some residual
charge still in them due to parasitic capacitance.
So when measuring them, you need to short all the leads together to eliminate
this problem, then you can measure the MOSFET with better accuracy.
I have to agree that the Gate/Drain resistor should have been more robust, but it
would really depend on what currents and loads its subjected to, as I don't know
what your circuit looks like.
Regards,
Relayer

[thm_w]

Do you have a microscope to inspect the resistor?

[Seekonk]

I've just bridged a resistor across it. I should pull it out and look with a microscope.  From the data sheet you can see this is quite a robust resistor, RSF1A.

https://www.alliedelec.com/m/d/0f55ae409ed1593e9b9313a6f8a907d3.pdf

[german77]

Bad luck then. I never have gotten a bad resistor even Chinese ones. I have burned 0.47 ohm resistors like the one you had and they always fail open with no visible damage. That's expected to happen when you pass a 10A load. In your case it could be defective and time just finished it's work.

[james_s]

For a while in the late 90s and early 2000's I saw quite a few bad resistors, often just completely open circuit. Seems like most of these were in CRT monitors and TVs though not in particularly high voltage areas. I remember thinking it was odd at the time, and then after about 5 years I don't recall seeing another bad resistor that wasn't obviously burned.