# PT100 Thermo Probe, 4.7K pullup ADC. problem.

Good evening gentlemen,

First thing is please go easy its my first post here and I don’t know all the politics that may or may not exist within this forum.

Incas you are not familiar here is the data table for the PT100 : http://www.micropik.com/PDF/pt100.pdf

The circuit this is being integrated into is the RAMPS 1.4 Board (used in open source 3d printing)
Pull up resistor is 4.7k
Control voltage is 5V

I first selected a temperature I wanted to find the ADC value for. Say 10degrees. I found the average resistance at 10C to be 103.9 ohms. (from the data table).
I took this number ,103.9 ohms and added it to the resitance of the pullup , 4.7k.
Using 4803.9 ohms. I found the current in the ‘circuit’ (5v) 0.00991493 amps.
From this , 0.00991493 amps. I found the voltage that must be going through the temperature prob ( = 0.00991493 amps x 103.9 ohm)
This gives me 1.034028039 volts. From my understanding the ADC value is a ratio between 1.034028039v/5v = /1023.
This gave me an ADC value of 22 (rounded to nearest hole number).

Can someone please confirm that the methodology / math is correct?
If it is, This does not give me nearly the accuracy I require. Would it be correct to say that changing the 4.7k pullup to a much lower resistance would help?
Currently going from 22 ADC to 23 ADC is a 10 degree shift.

The circuit is , to the best of my knowledge much like what member upuptothesky posted in this 2009 thread:

except his 10k thermistor is my PT100
and his R10K is 4.7K in my setup.

Comments and help most definitely welcome. If I’m no where near the right track please point me in the correct direction. IV done a lot of research to this point . (I am NOT an electrical guy.)

Steven.

5V/4803.9=0.001040 Amps. Not sure how you got your figure?

0.001040820 x 103.9 = 0.108141 Volts [u]across[/u] thermistor

ADC Val=( 0.108141/5 ) * 1023=22.12 or 22 rounded off. I do see your problem of resolution.

ADC value I get from [u]your[/u] final calc is, 211.5621367794, or 212 rounded to integer.

Do not get series resistor too low as you will get the current through the thermistor high enough to dissipate significant power in the thermistor. That is the thermistor will get hot due to the sense current you are providing and not the environment.

Try google for PT100 to microcontroller and see what happens, you will probably have to use an opamp to help scale the characteristics to what you want.

Do you have any particular application in mind, as there are other better and easier themperature sensors that interface with microcontrollers

Hope this helps, Tom