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whit3rd whit3rd is offline
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Default Thermistor Puzzle

On Monday, June 1, 2020 at 8:12:45 PM UTC-7, Paul Hovnanian P.E. wrote:
Given all this free time, I'm fixing stuff around the house.

I have a Brookstone Grill Alert. Stick a probe into a chunk of meat, plug
the cable (2.5mm phone plug) into the box and it happily monitors my steak
on the barbecue. Except it stopped working with unreasonably high readings
(150F reading at room temp). ...

It appears to expect an NTC thermistor with 180K ohms giving an 80 F reading
and 60K ohms reading 125F. But then I pick up the old (broken?) probe and
measure it. At room temp (65F) it has a 220K resistance. Holding the probe
in my hands drops its resistance to 140K. Seems to be OK. I wiggle the cord,
expecting an intermittent short (low resistance giving a high temp reading).
Nope. It's fine.

It might be that there's some oscillation at the sense amplifier; the impedance of
the probe wiring might be the issue.

If so, good news; that means it's not gonna need a new probe. Bad news,
you have an oscillating consumer device to debug.