Connecting a 4-wire RTD temperature sensor to MCBSTM32EXL Board

lisa2 · ‎2012-11-22

Posted on November 22, 2012 at 12:37

Hi,

I am trying to configure my MCBSTM32EXL board to read from an external temperature sensor. I think I have most the code sorted, including reading from the ADC. However, my knowledge of electronics is weak and I cannot figure how I should be connecting the RTD to the board.

I have a 4-wire RTD sensor. If I understand correctly, these 4 wires constitute two loops; one of which is the ''excitation'' channel which needs to be connected to pins through which a current is running, and the other is the ''channel'' channel which needs to be connected to the pin the ADC is going to read. Have I got that right???

Also, I understand that 5V of power is coming in as my power supply via USB from my PC. This is then stepped down to 3.3V on the board. Excuse my naivete, but I connect my scopemeter to some pins and ground and I get 3.31V and nearly nothing when I measure at other pins so, I'm assuming some pins get power and some don't.

So, is it that I'm connecting a red lead to a pin with 3.31V and its white compatriot to ground, and then the other red lead to a pin without power and its white ''other half'' to ground? And then try to read the upowered pin via the ADC? Can a hurt the RTD by messing around with different connection scenarios?

Thanks in advance for any enlightenment and/or pointing me to any helpful literature.

Lisa

frankmeyer9 · ‎2012-11-22

Posted on November 22, 2012 at 13:05

A suggested reading is ''OpAmps for everone'', which treats the topic of interfacing an ADC with the analogue world rather extensive.

Andrew Neil · ‎2012-11-22

Posted on November 22, 2012 at 14:45

The 4-wire arrangement is a very common & well-established technique for using this kind of sensor - so there is a lot of literature out there describing the principles & practice of using them, and giving examples.

Definitely worth a bit of googling...

lisa2 · ‎2012-11-26

Posted on November 26, 2012 at 17:30

Thanks fm for the link. Looks fairly extensive.

I have been Googling of course but am just that bit inexperienced so hard for me to make the leap from schematics to what I actually have or made need to add on/to my board. Anyway, I have the RTD working after a fashion - not accurately but good enough for my purposes at the moment.

It seems in the long run, I need to find a constant current source and add a small resistor for the outside loop, and then connect the inside loop to my board where there is no power source.

Thanks both for you input.

Lisa