Re: Converting a NTC voltage into temperature in Celcius

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,
 

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-linearization-2/>.)
It doesn't help resolution on the high temperature end, but it does prevent the ADC from railing at the low temperature end.
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.)
You wouldn't bother nowadays, since it's going into an ADC anyway, and code is much cheaper than op amps.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com