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From: C.D.H.Williams@exeter.ac.uk (Charles DH Williams)
Subject: Re: Quickie Thermal Probe
Date: Thu, 12 Dec 2002 23:26:36 +0000
Organization: speaking for myself...
References: <3DF8FDBE.34F2@Spam.Bots> <3DF93617.2159@Spam.Bots>
NNTP-Posting-Date: 12 Dec 2002 23:29:38 GMT
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In article <3DF93617.2159@Spam.Bots>, Mike Monett wrote:
> Thanks, Charles. A plain 4W bulb measures about 350 - 450 ohms and
> increases just by holding it in your hand. It might be a bit too fragile
> to poke in dark corners, but a grain of wheat bulb would be easier
> to protect and might work also. I just measured one at 44.5 ohms, and it
> also increased when I touched it briefly. This approach would use a bit
> more battery power and would need an external amplifier.
I think you may have missed how the dc-substitution bridge aspect works.
If you don't like the tungsten filament use a thermistor instead. The
filament doesn't even need to glow in this cicuit, you select the bridge
resistors so it's held at a constant temperature say 100degC above ambient
by the feedback. So, the 'series' resistance in the same arm needs to be
about 30% bigger than the filament resistance at room temperature. Use the
smallest lamp you can lay your hands on. The circuit will operate with a
dc offset of a few volts, and minute drafts will cause big changes in
This type of circuit works really well, honest!
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