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Topic: Convert the resistance of a PT100 to a voltage signal (Read 4480 times) previous topic - next topic

hansibull

Mar 26, 2015, 09:48 pm Last Edit: Mar 26, 2015, 09:52 pm by hansibull
Hei! I'm quite new to OPamps (never actually used them in my own application yet), but I got a mission where I'lll have to use one. I want to convert the resisance in the PT100 to a voltage signal. This device is going to be a standalone device, that you can hook up an Arduino to, or an IO device for PC logging.

Here are the requirements:
20 degC on the PT100 (107.7928 Ω) --> 1v out
50 degC on the PT100 (119.3951 Ω) --> 5v out
At least 0.5 degC accuracy


I've never actually used OPamps before, so If someone could provide a circuit and an explanation, I'd be deeply thankful. :)

Here is a Temperature vs Resistance table for the PT100
MightyCore -  ATmega1284, mega644, mega324, mega164, mega32, mega16, mega8535
Github.com/MCUdude/MightyCore

MiniCore - ATmega8, mega48, mega88, mega168, mega328
Github.com/MCUdude/MiniCore

knut_ny

As the PT100 is 100 Ohm, selfheating may cause trouble.
The easy way :
connect PT100 (connected to 0V)  series with 470 Ohm (to 5V)
Split voltage will vary from .933 to 1.013V
This voltage must be amplified 50 times (49.96)
selfheating effect is 25mW (may be problem)
This voltage is passed to an amplifier.
select Rin and Rf  to match 50 times.
Alternative is to add an instrumentation ampl.
Adjust pot to get 1V@20C
-------------
please do the math.. This was 'in a hurry'
error in picture.  amp shall be TS912. (several alternatives exists)
Ny

hansibull

#2
Mar 26, 2015, 11:56 pm Last Edit: Mar 27, 2015, 12:00 am by hansibull
Yes, self heating may be a problem. This sensor is supposed to be used in a custom made air heater (I'm helping out a teacher at my school). This sensor is going to be read by LabVIEW, through a 12 bit ADC.

Is there possible to use a bigger resistor in series with the PT100? And the school got a bunch of INA128 instrumentation amplifiers, if thats easier to use that ordinary OPamps

EDIT:
I found this circuit online. It does exactly what I want, except the temperature range is wrong.
How can the math be done to make this match my specs? :)


MightyCore -  ATmega1284, mega644, mega324, mega164, mega32, mega16, mega8535
Github.com/MCUdude/MightyCore

MiniCore - ATmega8, mega48, mega88, mega168, mega328
Github.com/MCUdude/MiniCore

knut_ny

I used excel in MY calcs.
Try 1K series to reduce to <4mW
Amplifier must do 85.5 times
If you have to: use low voltage for the wheatstone br.  Arduino has  a 3V3 pin. Reduce heat more than 75%
....
that sketch doen't match your original wish..
Ny

knut_ny

use the instr.amp (INA...)
Take a look at the datasheet..its similar to separate opamps.
find suitable resistor combination.
...
'same shit..new wrapping'
Ny

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