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From: "Bill Sloman"
Subject: Re: opto isolators
Date: Fri, 8 Nov 2002 01:15:49 +0100
Organization: Planet Internet
References: <3DC9BE19.1980C7C0@webaccess.net> <firstname.lastname@example.org>
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"Geraldo Sazias" wrote in message
> "Chuck Simmons" wrote in message
> Some of the specs I've seen led me to believe that the linearity of a
> optocoupler is far less than can be achieved with transformers. Is this
You can achieve quite remarkable accuracy with carefully wound
transformers - the two coils of a bifilar wound transfomer can be equal to 1
part in a billion, and bundle-wound ratio transformers typically offer a
ten or more windings that are identical to one part in ten million.
I built a dead cheap isolating amplifier a long time ago (1975) around an
RM8 cored transfomer with three equal-turn coils - the primary was used to
drive flux arond the core, one secondary was used to monitor the voltage
induced and control the rate of change of current, and the other secondary
drove the isolated output. Despite the fact that the amplifiers involved
were off-set trimmed uA748's - not my choice - we couldn't measure any error
over the 2V to 10V range required, looking at the output with a 6-digit DVM,
which was impressive, if slightly frustrating - this meant that we'd
overshot the spec by a couple of orders of magnitude, and nobody wanted to
waste time finding out what the actual errors were.
The transformer would have cost about a dollar or so. It wasn't a ratio
transformer, or anything like it - just one multilayer winding in heavyish
copper wire, and two single layer windings in 0.1mm wire, which was as fine
as I could wind. The coil winders could wind 0.05mm wire without breaking
it, but they were busy.
Bill Sloman, Nijmegen
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