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From: email@example.com (Bill Sloman)
Subject: Re: generating calibration voltage
Date: 8 Jan 2003 15:46:13 -0800
References: <3E1C54DA.3DE2AC41@rica.net> <firstname.lastname@example.org>
NNTP-Posting-Date: 8 Jan 2003 23:46:13 GMT
email@example.com (Tom Torfs) wrote in message news:...
> Hello Bill,
> > For the rest of the circuit, Vishay's MPM SOT-23 100:1 dividers come
> > to mind (Farnell order code 309-8552 and $6,77 in small quantities).
> Thanks for that excellent tip! I already have a 2.5V reference voltage
> in the circuit. If I put such a 100:1 divider followed by a 50:1
> divider, giving me a 5000:1 reduction, I end up with a 500uV voltage.
> I thought about doing this with a regular resistive divider but
> thought the tolerances would not allow it, but with these babies it
> seems quite possible.
They really are very nice. If you can justify buying 150 you can get
Vishay to make them with more or less arbitrary resistances, provided
the ratio is less than 300:1 (PRA 135).
If you can use a grounded calibration source, it does seem to be the
way to go.
Pity.An RM4 or an EP7 core would work. You can get away with swinging
the flux density from +150mT to -150mT and be pretty sure that the
core won't saturate.
The RM4 core has a minimum cross section of 11mm^2, and the EP7,
So V.s can be as high as 0.3 times 8.5^-6 = 2.55^-6
Since s is 16.7 milliseconds, V can be as high as 153uV per turn, so
you can get away with a four turn output coil.
The primary and the feed-back coil would need to be wound with the
finest wire you could manage. I can manage 42 SWG - 0.1mm diameter oer
the copper, 0.125mm in diameter over the insulating enamel. Spehro
Pefany claims to be able wind 48 swg without breaking it, and the
Cambridge Instruments winding shop routinely used 46 swg, which is
roughly what I'd expect from a pro coil winder.
John Woodgate has claimed to know people who can wind even finer wire
- the name was probably Rumpelstiltskin.
Details of the cores and formers are available from
The technical definitions are exceedingly helpful - seems to be the
old Siemens soft magnetics materials catalog that I loved and lost
back in 1978 and Tony Williams e-mailed to me and Win Hill and a few
privileged others over the weekend.
Tony Williams' minature soft-iron toroids could take quite a lot more
magnetic flux before they'd saturate, but the cross sectional area
might be a bit low.
I suspect he has a few toroids out of a magnetic core memory ...
Bill Sloman, Nijmegen
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