How to maintain constant current across a 4-wire RTD?

[mav1234]

I'm looking for a circuit (& parts) recommendation to maintain a constant current across an RTD (changing resistance with temperature)???

See attached image of an RTD. I understand that I have to maintain a constant current even with R(RTD) changing constantly with temperature - how do I do that?

 

[IanP]

I[exc] has to be generated by a constant current source.
You can use voltage regulators as constant-current regulators, see:
http://www.reuk.co.uk/LM317-Current-Calculator.htm
or, as RTDs require quite small constant currents, you may use opamps, see:
**broken link removed**
:wink:
IanP

 

[Avla 0652]

fyi,
many of modern converters allow "ratiometric" connection, i.e. they can use RTD excitation current as a reference,
so it is not necessary to maintain a constant current across an RTD or constant voltage.

for example, AD7781 can work in ratiometric mode:

In this circuit, the voltage across 0.1%25ppm/°C resistor is used as a reference voltage.
I hope it helps.

 

[krixen]

a simple op-amp circuit should do the trick, when you want constant current across a resistor that can change due to temperature, it is sometimes best not to use other resistors because in application (vs theory) it can cause voltage changes. An op-amp will hold the current no matter what kind of load you hook up to the output of it, so it would seem that it would be the best bet for you. You can change the feedback & input resistors to change the current and it will not affect the load

 

[crutschow]

A precision constant-current source can be made with a TL431 shunt regulator as show in Figure 35 of the data sheet.

Edit: For good current stability Rcl should be a film resistor with a low temperature coefficient.

 

[mav1234]

Found a good (at least looks like it) circuit with the derivation of the result on Page 3 of this App Note: https://ww1.microchip.com/downloads/en/appnotes/00687c.pdf