Re: Converting a NTC voltage into temperature in Celcius

In message <879d5bd8-a857-e5f8-e9a5-f3c004fbb937@electrooptical.net>, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes

On 2025-04-08 05:54, Jean-Pierre Coulon wrote:

The resistance vs temperature relation is well known. But we are using one with an industrial controller that sends 100 uA into it and provides the corresponding voltage.
 Are there any cheap converters on the market, to convert this voltage into a nice linear voltage vs temperature relation ? Then there there  are many voltage-to-display converters.
 Bye,
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The usual method of approximate linearization is to put a carefully chosen resistor in parallel, which may or may not be good enough depending on what you're doing.  (See e.g.
<https://circuitcellar.com/resources/quickbits/ntc-thermistor-linearizat
ion-2/>.)
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It doesn't help resolution on the high temperature end, but it does prevent the ADC from railing at the low temperature end.
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A positive resistance works with an NTC, because its resistanance vs temperature curve is concave upward.  A metal RTD's characteristic is concave downward, so you need a negative resistance for the job. Because its nonlinearity is smooth and gentle, you can do an amazingly good job that way--theoretically under 1K error from -100 to +150C iirc, and much closer over narrower ranges.  (I designed a couple of those in my misspent youth.)
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You wouldn't bother nowadays, since it's going into an ADC anyway, and code is much cheaper than op amps.
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Cheers
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Phil Hobbs
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I tried to do that in the 1970s to control the gain of an APD with temperature. I ended up using a diode as a temperature sensor as I couldn't get it linear enough.
Later on I wanted a non-linear control for another application.We tried OP amps with piece-wise shaping circuits.  We concluded however the best way to do it was a PIC with a  built in ADC and DAC  and a lookup table as you suggest. That might be the  cheapest way for the OP
  I've also used AD590s which are  pretty good.
Brian
--
Brian Howie