On Mon, 11 Nov 2013 08:16:16 -0700 Jim Thompson
wrote in Message id:
:

On Mon, 11 Nov 2013 07:38:52 -0500, 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.

The PIC18F452 pin 5 (RD7/PSP7), is that high-side-only drive? If so,
the 10K G-S pull-down is inadequate for quick turn-off, and the 2N7000
dissipates to death.

Or it could be the other way around and the PIC can't raise the gate
voltage quickly enough due to its large capacitance. Likewise a slow
death due to dissipation.

...Jim Thompson

Hi Jim,

Thanks. I think your second thought may be right. I took a scope capture
of the gate at turn on. (Attached)
Think that could be the culprit? Any ideas to get rid of it?

To address Tim's question:
The total trace length including wire is about 6" or so between the PIC
and the FET. 4" of that is on a ribbon cable between two connectors.
The diode is at the coil, not the FET.

Turn off looks fine, no ringing or anything...

Thanks everybody for your suggestions.