RTD to ADC

[slepax]

Hello all,

I am trying to use an RTD as an input to an AVR ATmega8 to measure temperature. I searched the net for information but I am still confused.

Just a brief explanation first about what my project is ..

I want to convert an old bread making machine into a coffee roaster. I only want to keep the heating element and the circulating arm, then control the whole roasting process via micro-controller.

I have been experimenting a bit with micro-controllers, reading the analogue value and implementing the control software is easy. I am having problems though converting the signal coming from my PRT100 RTD.

I searched the net in and out and it seem Wheatstone bridge is the way to go. I will use the two-wire configuration because the distance in my case is very short, so no need to compensate for lead interference (and accuracy is not big issue).

**broken link removed**

This is what I do know:
* If I calculated this right, at 0 deg C my RTD should give me 100 ohm, and at 300 deg C I should get 217.25 ohm. Not a big span in terms of resistance values.
* My ADC is 10 bit, which means I can measure changes of 0.0049v (5v/1023 = 0.0049).

What I don't understand is how to limit the 10bit reading to my temperature range, assuming I don't need the full -200C to +800C range.

How do I decide what values R1 to R3 should be? I played around with some numbers but the span from minimum to maximum value in the ADC input (V0) is always very small something like 0.5v rather than the full 5v range I have as reference for the micro-controller .

Sorry if that's too long, and thanks for reading so far.

Any help deeply appreciated!

Thanks,
Ronen

 

[rosscopico0]

Hi there Slepax,
I too am fairly new to RTD's.

From what I have learned so far, wheatstone bridges can be expensive and troublesome as you require high precision resistors and they may need to be matched.

I am currently working on a high accuracy/precision temp interface to an RTD 1000 to be installed in a commercial espresso machine grouphead.

For your application, I think you will need a constant current source, 1mA or less, to prevent self heating and consequent errors, a voltage divider, a reference voltage to narrow your range of interest and an opamp set up as a difference amplifier.

I have been playing around on 5Spice simulation and have developed the cct attached.
I am starting to suspect this may not be accurate enough for me (I want 0.1C accuracy and resolution), but it could be useful to you.

Good luck and happy roasting!

 

[Jon Chandler]

Have you considered a different sensor, such as the Dallas DS18B20? It's self-contained, pre-calibrated, and uses a digital signal to transmit data.

Here's an example in Swordfish Basic but I suspect there are Arduino commands to read One-wire devices.