From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Quickie Thermal Probe
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Date: Thu, 12 Dec 2002 17:46:24 -0800
NNTP-Posting-Date: Thu, 12 Dec 2002 17:46:25 EST
Organization: Bell Sympatico
John Larkin wrote:
> transistors should work fine, but personally I wouldn't literally use
> a diffamp; I'd rather each transistor be privately biased at a
> constant current, which would avoid thermal runaway issues. You could
> use, essentially, two emitter followers, bases connected, and use an
> opamp to nab the differential emitter voltages.
> Sure, a SOT-23 has a high theta j-a (like, 350 k/w) and a shorter
> thermal time constant. I'd go for 100C or so idle temperature and get
> lots of signal.
> I did a thing like this once. We took it to the park and hung it way
> out a car window to calibrate it.
I can see it now - one to drive, one to hold it out the window, and one
to record the speedo and meter readings. And onlookers shaking their
heads in amazement!
Your approach sounds good, and probably worked very well. I'm thinking of
a much smaller temperature rise in the sense transistor - perhaps several
degrees above ambient. Hopefully this will allow it to sense small
changes in temperature as well as air currents.
I'm not actually trying to measure wind velocity. The problem is mold
spores coming up from the basement. I have carefully sealed all the floor
trim and other cracks in the appartment, shut off the heating system and
sealed the vents, sealed the entry holes for the pipes in the kitchen and
bathroom, and sealed the light fixtures and wall switches. So there's no
great rush of air, just a constant seepage. This allows spores to enter
the appartment that make me very ill.
I felt some suspected areas with my finger but could not feel anything.
There are other places too difficult to reach, which is why I'm looking
for a sensitive probe. A diffamp looks promising, and from your
description the SOT-23 seems a bit better than a TO-92.