On 11/11/2013 4:38 AM, Jay wrote:
Hello,

I am having a high failure rate on a 2N7000 FET that drives a relay coil.
Please see the attached jpeg. This gate drive is coming from a 5 volt
PIC18F452 pin 5 (RD7/PSP7) Any thoughts greatly appreciated. Note that it
will run for quite awhile, perhaps 20 to 30 thousand cycles before failing
entirely shorted Drain to Source.

Thanks.

What do you see when you put a voltage probe and a current probe on the
drain
and look at 'em with a scope...initially and some time later near failure?
A digital scope with multiplication capability will make the job easier.

Thermal cycling of the chip can kill it even way below max average
dissipation.

What's the gate resistor for?
If you're driving it from low impedance, you don't need it.
If you do need it, it's too big. Miller capacitance may be what's
killing you.