From: John Woodgate
Subject: Re: Don't do this! Just don't!
Date: Sun, 19 Jan 2003 21:44:07 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 19 Jan 2003 22:03:16 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that John Larkin wrote (in <8s2m2vcp39h4ofvpkjthranc9tqaia246m@
4ax.com>) about 'Don't do this! Just don't!', on Sun, 19 Jan 2003:
>OK, every once in a while we need to have a trimpot crank in a little
>bit of DC offset into an opamp or something, and there are those nice
>+-15 or whatever power rails handy. But I see so many presumably
>intelligent engineers do something like this:
> 100r O <-------- small offset
>This is a *very*bad*idea*. I recent sold a bunch of
>amplifier/discriminator boxes because somebody else did this, among
>other dumb things.
>So quit doing this. Just quit it.
It was so common in the 80s that Bourns made a model OT-1 trim pot that
has a centre-tapped track. You ground the centre-tap in the above
circuit, which makes it work. However, Bourns, in their infinite wisdom,
sent me a 50 kohm part as a sample, instead of the 1 kohm that I asked
for. I still have the sample. One day, I might find a use for it. AFAIK,
the product was withdrawn after a short interval.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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