On Thu, 22 Apr 2004 16:34:18 +0100, you wrote:

>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...

Comparators are cheap, especially compared to a welding power supply - I =
would be inclined to do
this in hardware.

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