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Adrian Tuddenham[_2_] Adrian Tuddenham[_2_] is offline
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Default Thermistor Puzzle

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). Wisdom on the 'Net says that the probe is
probably broken. But the replacement part (Brookstone) is no longer
available. No problem. I'll plug my decade resistance box into the grill
alert, start cranking in some resistance values and figure out the
thermistor curve. Then find another probe.

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. Maybe it's something wrong with the plug/jack interface. So
I open the box, unsolder a lead from the jack and measure the probe
resistance plugged in at the back of the jack. It reads 220K. Hook it all
back up and it's still wonky (probe gives a 150F reading sitting on my
desk). But if I plug my decade box in to the grill alert and dial it to 220K
ohms, the thing reads room temp.

The Grill Alert appears to just not like that probe. What have I missed?

Could it possibly be a semiconductor probe that uses the forward voltage
drop of a diode?

~ Adrian Tuddenham ~
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