I guess something is causing the mosfet to get a couple of volts of
gate, and it sits warming until it goes pop.

Perhaps you don't have enough drive on the optocoupler (the transfer
ratio goes down by half over a few years).

Or perhaps there is moisture/dew/dust?

Bob

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 TIL157
> >     Emitter > ground
> >     Base > N.C.
> >     Colllector > Mosfet G, with .01mfd to ground, and 100k ohm to + 12V=
DC  (Note this a a lead/acid batter and 14 VDC common)
> >
> > The Mosfet
> >     S > 12 VDC
> >     G > From the TIL157
> >     D > to the relay coil with surge diode across the coil. Coil ~ 133m=
a
> >
> >
>=20
>=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 would be happy if someone could review things and comment. I would=20
> desire to keep the PCB's, cost is little priority since this is one=20
> board with several spares.

--=20
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--=20
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