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From: jeh@dcs.simpact.com
Newsgroups: sci.electronics
Subject: Re: wanted: device to turn brownouts into blackouts
Message-ID: <1991Apr16.154857.2280@dcs.simpact.com>
Date: 16 Apr 91 22:48:57 GMT
References: <1991Apr15.180755.19771@odin.corp.sgi.com>
Distribution: na
Organization: Simpact Associates, San Diego CA
Lines: 46

In article <1991Apr15.180755.19771@odin.corp.sgi.com>,
 nelson@bolyard.wpd.sgi.com (Nelson Bolyard) writes:
> This morning at 6:19 AM PDT, a power line went down in the city of
> Milpitas, causing a brownout condition that lasted for 58 minutes
> before the power went out completely.  During the brownout, we got 60
> volts AC out of the wall sockets.  This did nasty things to my
> computer equipment, and to the compressor motors in my fridge and
> freezer, and to the furnace blower.  Motors just don't like low
> voltage.  [...]
> So I'm looking for a device that I can hook into the power line for my
> computer equipment that will cut all power off completely if the power
> quality drops below some threshold (say 90% of normal), and will remain
> shut off until the power has been restored to normal for at least a
> minute or so (to guard against blinking and surges).

I asked about this once and was told that you can get circuit breakers
that include under- and over-voltage tripping.  

Over-voltage?  Yes, that was the original reason for my query.  A capacitor
somewhere in the northern San Diego county area shorted out awhile back, 
resulting in about 200 homes being fed with something like 180 volts instead
of 117.  Naturally, most things that lack an old-fashioned off-on switch, the
kind that actually disconnects the unit from the wall, were fried.  This
includes anything that can be turned on via remote control or built-in timer,
(most modern TVs and all VCRs), clocks and clock radios, lamps with built-in
touch-sensitive dimmers, BSR home automation modules, garage door openers, 
etc., etc.  SDG&E ended up being liable for damages in the large-fractional-
megabuck range.  (They got off lucky; the overvoltage could easily have 
burned down several homes.) 

DIY couldn't be too hard.  Run a tiny power transformer off the line voltage,
rectify and minimally filter the output, and feed the result to a couple of IC
op-amp comparators, one for low and one for high. The comparators drive a relay
or whatever else you want to cut the power off. You need some hysteresis to
avoid stuttering in case the AC hangs near the trip point for a while. 

But for stuff like this, I, for one, want to go buy something off the shelf and
have a licensed contractor install it.  It might not be built any better than
what I could do myself, but that way if it shorts out and burns the house down,
I can sue someone else instead of being on the hook myself.  No way will the
insurance company pay off if there is even a hint that a fire was caused by
homebrew power-control devices. 

	--- Jamie Hanrahan (x1116), Simpact Associates, San Diego CA
Internet:  jeh@dcs.simpact.com, or if that fails, jeh@crash.cts.com
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