Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site ski.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!whuxl!whuxlm!harpo!decvax!genrad!panda!talcott!harvard!seismo!hao!hplabs!intelca!qantel!dual!ptsfa!ski!dr
From: dr@ski.UUCP (David Robins)
Newsgroups: net.analog
Subject: Re: AC current monitor
Message-ID: <167@ski.UUCP>
Date: Wed, 27-Mar-85 14:29:04 EST
Article-I.D.: ski.167
Posted: Wed Mar 27 14:29:04 1985
Date-Received: Wed, 3-Apr-85 03:26:09 EST
References: <5139@fortune.UUCP> <9482@brl-tgr.ARPA>
Distribution: net
Organization: Smith-Kettlewell Institute, S.F., CA.
Lines: 50

> >      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.  Also, it cannot drop the 110 volt
> > AC line by very much.  One more thing: the ground of whatever 
> > circuit is used will be tied to the neutral line of the AC, so complete 
> > isolation is not possible.
> > 
> How about building a current sensor by running (one side of) the cord
> through a coil.  You could then sample the output with a comparator
> circuit.  This is how my PDU in the computer room works.
> 
> -Ron

You can also do two other things. 

Popular Electronics, and Radio-Electronics have had circuits for
stereo system power switches.

One techniques is to run the load current through a pair of diodes
(large enough to carry the current) in parallel , but reversed.
What this does is to show the diode voltage drop (about 1 volt or so)
on bot h halves of the AC cycle, whenever the load is on.
This voltage is therefore 0 when off, and about 1 volt over most of
the 360 degree sine-wave cycle.  A differential-input sensor picked up
this voltage and used it to close a relay, to power the rest of the
stereo system when the turntable was turned on.

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).  Since the load
resistance dropped the voltage, the transformer saw little voltage.
This current in the secondary created a voltage on the primary, which
was then sensed.  The secondary current rating of the transformer
should be high enough to carry your load current.

I like the first solution better, because the voltage to be sensed is
not related to the load current, in the case of the varying loads you
referred to.

If you need references to the articles, I *THINK* I cut them out of
the magazines, and can dig them up.  Send me mail if you need them.

-- 
David Robins, M.D.; Smith-Kettlewell Institute of Visual Sciences
2232 Webster St; San Francisco CA 94115
415/561-1705
			{ucbvax,dual,sun}!twg!ski!dr
			dual!ptsfa!ski!dr