# Temperature measurement

Discussion in 'Electronic Basics' started by Abbie, Nov 24, 2003.

1. ### AbbieGuest

Can someone recommend the simplest circuit for temperature
measurement with a passive thermistor. I also need the theory
and the mathematics. I recall that I need two clibration points
and two variable resistors.
thanks
Rex

2. ### Rich WebbGuest

The simplest would probably just be a voltage divider with the
thermistor as the lower (ground side) resistor. A starting point would
be to use an upper (Vcc side) resistor that's around 1/2 the nominal
value of the thermistor, giving you a voltage range from about 2/3 of
Vcc down towards ground as the thermistor heats up.

Measure the voltage, calculate the thermistor resistance as
Rt = R1 * Vm / (Vcc - Vm). Look up or calculate the temperature as a
function of the resistance. That can range from a simple linear
approximation, to a two- or three-segment linear approximation, all the
way up to a Steinhart-Hart approximation (google for it).

3. ### SurajGuest

HI!
I did a similar project by interfacing a 10 K NTC to an 8 Bit ADC a
couple of years ago. Literally learnt everything off this group, even the
thermistors were sent by somebody on this group!!
I implemented the Steinhart's approx in C++. If you need some of the helpful
posts I got, I'll send them to you.

Suraj

4. ### Bob MastaGuest

If you are not absolutely locked into using a thermistor,
consider using an ordinary junction diode instead.
The forward resistance is extremely linear over a wide
temperature range. The only caveat is that each unit
needs individual calibration (ice water and boiling
water, etc). So this approach is probably better for
a one-off project than a production item. But it
eliminates the need for linearity correction, which
can be a problem with thermistors.

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

5. ### BaphometGuest

"forward resistance is extremely linear"? Did you mean forward voltage drop?

6. ### Bob MastaGuest

The foward voltage drop is only linear with temperature when it is
passing a constant current, hence my use of "resistance".
It's not that resistance *as such* is linear with temperature,
but at any given measuring current it is. You just can't
compare measurements taken with different currents.

Incidentally, the linear range for this technique is reputed
to go down to a few degrees Kelvin. On the high end it
is limited by melting of the device or its contacts, but
I imagine it should be about as good as the original
device rating.

Bob Masta  