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From: zben@umd5.UUCP
Newsgroups: net.analog
Subject: Re: AC current monitor
Message-ID: <446@umd5.UUCP>
Date: Sun, 24-Mar-85 16:52:23 EST
Article-I.D.: umd5.446
Posted: Sun Mar 24 16:52:23 1985
Date-Received: Thu, 28-Mar-85 00:04:12 EST
References: <5139@fortune.UUCP> <445@umd5.UUCP>
Reply-To: zben@umd5.UUCP (Ben Cranston)
Distribution: net
Organization: U of Md, CSC, College Park, Md
Lines: 32
Summary: Shooting from hip considered dangerous (corrections) 

Oops, I forgot that LEDs don't shine at all until you overcome the band-gap
energy (which is .7 volts for silicon).  Relays don't have these kinds of
problems... :-)

Well, if you make the shunt big enough to drop .7 volts at 1 amp, it will
drop 7 volts at 10 amps, and that is probably too much.  The only other
approach I can suggest is to find an old filament transformer (remember
those?) of 10 amps capacity (maybe 5 or 6 volts) and use it backwards as
a current sensing transformer.  Something like:

-----------------------()()()()()()()()()()--------------------
Line                    ==================   
                          ()()()()()()()
                          |            |
                           sense here

With the 5 volt side in series with the load and some kind of sensor on
the 110 volt side.  Problem areas:

Secondary, being designed for 5 volts, might not have 110 volts worth of
insulation to the case of the transformer.  This would be a bad failure
mode...

Impedance of the sensor circuit would be reflected back into the line
circuit by the turns ratio of the transformer.  You should make this
impedance as small as possible.

A note on removing 60 cycle chatter - use TTL level to continuously
retrigger a 74121 multivibrator set up to maybe 1/10 to 1 second time
constant.  Its a one chip solution.
-- 
Ben Cranston  ...{seismo!umcp-cs,ihnp4!rlgvax}!cvl!umd5!zben  zben@umd2.ARPA