Fred wrote:

> I have done some minor controls of products that have been UL,
> CSA, CCC approved and CE Marked. The controls did not include
> safety shutdown circuits. I was wondering how difficult it would
> be to have the safety shutdown controls on the PIC and get it
> approved by the agencies. The product would be a welding power
> supply. The safety circuit cuts the bias supply and opens a
> contactor relay.
> The shutdown is caused by excessive output voltage, or over
> temperature. I save some cost by putting this in code instead of
> analog comparators.
>
> Any shared experiences are appreciated.


No liability is taken as a result of this email and I don't confess to be a
safety expert ;-)
With that said, they will be looking for a circuit that fails safe.  When
you set this thing on fire or smash it up, it must cut off the danger to the
user or environment.  The relay or set of contacts will only switch on in
the case of all the correct operating conditions.  Use of a WDT should be
implemented.  If it were me, I wouldn't do this with a PIC.  I would use a
thyristor across the power supply with a fuse. When the device is triggered
by the over voltage the fuse blows.  The over temp part can be done with a
thermal fuse.  The two fuses supply your relay.

> I save some cost by putting this in code instead of
> analog comparators.

Not if it makes the circuit less reliable and causes someone harm.  Safety
shouldn't come down to cost, although in the real world...

Brian.

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