Re: Converting a NTC voltage into temperature in Celcius

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