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From: jpexg@mit-hermes.ARPA (John Purbrick)
Newsgroups: net.analog
Subject: Re: Re: AC current monitor
Message-ID: <2338@mit-hermes.ARPA>
Date: Mon, 25-Mar-85 10:42:44 EST
Article-I.D.: mit-herm.2338
Posted: Mon Mar 25 10:42:44 1985
Date-Received: Thu, 28-Mar-85 06:13:20 EST
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Organization: The MIT AI Lab, Cambridge, MA
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The toroid is the way to go. But you don't need to cut it--just pass one lead
(not both!) of the AC line through the hole, and wrap "some number" of turns
of wire around the toroid then lead the ends off to the inputs of an op amp
or better still an instrumentation amplifier. You will have created a crude
transformer, where a magnetic field in the toroid will be induced by current
in the ac line, then coupled into the "secondary" winding. Turning the output
of the amplifier into a ttl level is left as an exercise for the reader. But
I'm with the folks who get nervous as all hell about connecting in any way to
the ac line! What if the damn thing ever gets plugged in backward?


A more complicated version of this circuit would involve sawing a slot in the 
toroid and slipping a hall-effect (magnetic field) sensor into it, like the 
Sprague UGN3501T--that way you get a voltage output. I've done this and it
works pretty well with both AC and DC.