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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!ihlpg!fish
From: fish@ihlpg.UUCP (Bob Fishell)
Newsgroups: net.analog
Subject: Re: AC current monitor
Message-ID: <344@ihlpg.UUCP>
Date: Thu, 4-Apr-85 13:29:59 EST
Article-I.D.: ihlpg.344
Posted: Thu Apr  4 13:29:59 1985
Date-Received: Fri, 5-Apr-85 04:05:43 EST
References: <5139@fortune.UUCP> <9482@brl-tgr.ARPA> <167@ski.UUCP>
Distribution: net
Organization: AT&T Bell Laboratories
Lines: 45

> > >      I have a tough question here.  I need a circuit to indicate
> > > the presence of a load on an AC line. Wait, it gets more difficult.
> > > The load can vary from 100 watts to 1000 watts, and the output 
> > > signal must be a TTL level. 

> A second technique was to use a transformer in series with the load to
> act as an isolated current transformer.  I belive they used a 120 to
> 6.3 v. power transformer.  Current from the load was passed through
> the transformer's SECONDARY (the 6.3 v. side). 

***     AC  T              YOUR     AGE ***

Following this discussion has gotten me interested in such circuits,
and I have, indeed, put one together that does the trick, using an
ordinary power transformer, as the excerpt above suggests.  I had
an old 3 amp power transformer laying around, so I thought I'd
experiment with a circuit wherein the transformer's 6.3 v coil is
in series with the load:

	             ,--------
AC	---------:||(        ^
line	--:      )||(        |
	  |      )||(      Vsec
	  |      )||(        |
	  |    --'||(	     v
	  |    |     `--------
	  `--load
       
Vsec, open circuit, is fairly high with a 100-watt bulb serving as
the load, but a 150-ohm resistor pulls it down to a level suitable
for driving CMOS.  Voltage drop accross the primary (the 6.3v coil)
is negligible, and there's no heat buildup after several minutes
of operation.

The complete circuit full-wave rectifies Vsec and uses it to
drive a power transistor which, in turn picks up a relay that
controls another 110v circuit.  A 500-ohm pot controls the
base current so that the relay is picked up when the load
current (and hence Vsec) reaches a desired level.  I use it
to control power to my TV set, which lacks remote control, by
the power supplied to my VCR, which has a wireless remote.
/_\_

				Bob Fishell
				ihnp4!ihlpg!fish