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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Converting a NTC voltage into temperature in Celcius
Date: Thu, 10 Apr 2025 13:38:28 -0000 (UTC)
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brian <nospam@b-howie.co.uk> wrote:
> 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,
>>> 
>> 
>> 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/>.)
>> 
>> 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
>> 
> 
> 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

 Linear-mode APDs are much tougher, I agree.  For one thing, the
nonlinearity is much stronger.  

For another, the high bias voltage leads to a lot of dissipation which
makes the die temperature fail to track the board temperature. And of
course currents are less convenient to compensate than resistances. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics