Bob Blick wrote:
> On Sun, Jan 8, 2012, at 04:36 PM, Carl Denk wrote:
> > I have a TIL157 NPN Darlington photo transistor controlling an IRF9530=
=20
> > Power Mosfet, that is controlling a 1" cube auto relay with typically=20
> > 133 ma coil. The circuit may be powered up for an extended period of=20
> > time, but generally less than 20 minutes. Here's a brief description: I=
=20
> > do have a small JPG of the circuit, just send me a private to attach to=
..
> >=20
> > The circuit has been in use for 6 years, and this is the 2nd time I=20
> > needed to replace the Mosfet. There has been a rubber (??) insulator=20
> > with a machine screw to tie the mosfet down to the PCB as heat sink.=20
> > This time the PCB was black/brown color in the mosfet area - extreme=20
> > heat. The PCB is entirely Thru-the -hole.
>=20
> I guess something is causing the mosfet to get a couple of volts of
> gate, and it sits warming until it goes pop.

That was my first thought, too, but with a 90-ohm relay limiting the curren=
t,
there would only be about 0.5 watts dissipated in the MOSFET, worst case.

I would look for external transients, such as nearby lightning or an
accidental short to line voltage.

-- Dave Tweed
--=20
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