# PT100 millivolts to Temperature

So i am sensing a PT100 sensor which when connected to my circuit gives equivalent millivolts. I have provided constant current source of 1mA for it. So when its 100 ohms, output is 100mV, for 150 ohms it gives 150 mV. Total range is 48 ohms(48mV) to 280 ohms(280 mV). So in short I have to convert these ohms/millivolts to its equivalent temperature. Any ideas how should i convert and show it on my lcd display?

Thank You

Any ideas how should i convert

Look at the data sheet for the sensor. It should show an equation, or graph, showing the relationship between voltage and temperature, or between resistance and temperature.

That's roughly -150C to +500C.
A ~650C span, and only ~232mV difference.
The best Arduino can do is measure with 1.1volt Aref enabled. ~1mV per A/D value. ~2.5C resolution.
If you want better (e.g. ~0.5C/step), you have to amplify with an opamp before sending it to the Arduino. Or use an external A/D.

If you can use a ~3.5mA CC source (absolute value is not important), things are a bit better.
Then 500C will have an A/D value of ~1000 with 1.1volt Aref enabled.
Try some simple code first that prints the A/D value of the sensor with 1.1volt Aref enabled.
https://www.arduino.cc/en/Reference/AnalogReference
Once you have the A/D values, then converting that to temp should be easy.
Leo..

I am using Mcp3551 adc. I am getting adc values. Is there any formula or anything from which i can convert?thx

Kelvin, Celcius, Fahrenheit?
AFAIK that chip needs an external Aref.
What Aref voltage are you using.
Did you try/write any code?
Post it.
Did you enter "MCP3551" in the search box on top of this page.
Did you see this:

Leo..

Wawa:
Kelvin, Celcius, Fahrenheit?
AFAIK that chip needs an external Aref.
What Aref voltage are you using.
Did you try/write any code?
Post it.
Did you enter "MCP3551" in the search box on top of this page.
Did you see this:
http://forum.arduino.cc/index.php?topic=164643.0
Leo..

1. Celcius
2. Using REF3020 = 2.048 Ref to MCP3551
3. Not written any code. Just displaying the Ohms(millivolts in my case)
4. I searched but it is not according to my application.
5. I read that but as I said, I am already getting millivolts which was converted from ohms using constant 1mA current source(LM334).

Raw code to display ohms. Eg. if its 100 ohms, vin will be 100 mv. Below is just a snippet from my program.

``````if (rtd == HIGH)
{
lcd.setCursor(0, 0);
lcd.print("Ohms: ");
lcd.setCursor(0, 1);
lcd.print(vin,1);
//delay(500);
}
``````