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Subject: Re: A protection problem
Date: Wed, 08 Jan 2003 22:59:12 +0100
NNTP-Posting-Date: Wed, 08 Jan 2003 22:59:12 MET
X-Received-Date: Wed, 08 Jan 2003 22:59:13 MET (news01.chello.no)
John Larkin wrote:
> It will be hard to find a relay that will break 600 VDC, especially
> when the current is a zillion amps. I recently had an emotionally
> scarring experience at 200 VDC using relays... damn near physically
> scarring, too.
So use a make that only makes if the input is within range.
Even better, it must be possible to use a FET to power the thing through. Drive
that FET only when input voltage is safe.
FET with zener to ground to limit gate voltage, and high R to V+, and higher R
Transistor/FET from teh gate to ground. This transistor must be driven on
rapidly if the input voltage is too high.
Make it a FET again, same zener and R to ground, but also a zener in series with
the R to V+, so this one conducts at overvoltage only.
This second FET can be replaced with a thyristor, the triggering of which can be
accellerated with a clever R/C network in series with the zener to V+.
This would leave the protection active until the 600 volts were removed.
Add a second path with C to gorund etc and you can make this sticky: device
doesn't work for an hour after abuse.