Subject: Re: Quickie Thermal Probe
Date: Thu, 12 Dec 2002 17:50:10 -0800
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I'll second that.
I use #382 bulbs just cause they're small and I
Use a bridge and break the glass on one.
Fragile, but really sensitive.
Get extra bulbs. It takes some practice to break
glass without breaking the filament.
Charles DH Williams wrote:
> 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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