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Scientech Calorimeters and Negative Resistance

The Lightning Stalker

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I am currently studying the manual for the 362 which was so kindly provided over on PL
hoping to build an LPM around my 038-0101 "calorimeter". The thing that got my attention
was the preamp section. I couldn't seem to figure out why there is a power darlington
stage after the op amp. About ready to give up, I started reading from the beginning,
(which I should've done in the first place) and there it was, a warning about not shorting the
leads because the 362 presents a negative resistance to the calorimeter. So I traced out
the preamp again and sure enough, it's a negative impedance converter.



So it is actually designed to force power back into the sensor. I'm thinking that these are
TEC sensors and the meter is cooling the sensor down again to get a faster response. It
may even be key to getting an accurate measurement.

I'm wondering if anyone else knows anything about this because none of the LPM preamp
circuits in the DIY meters perform this function.

As a side note, it seems that the art of manual writing is long lost. In this day and age of
online PDF manuals, there is a distinct lack of in-depth information, weather negligent or
intentional. Everyone is so greedy with their secret sauce and afraid to share any real
information. Instead of building with quality that makes people want to buy, it's like
everyone is trying to deplete imaginary competitors of the smallest tidbits of real
information. Anyway, that is just my impression. You each have your own, I'm sure.
 

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Hemlock_Mike

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I had to get out my book -- I have 2 of these 362 meters.
The sensor is a TEC and the neg resistance may be to establish
a reference temp. I worked with a gas sensor once which used
the same principal to maintain a cathode temp based on heater
resistance.
HMike
 

LSRFAQ

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May 8, 2009
Messages
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Google:

Self Balancing Bridge RF Power Meter.

Steve
 




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