Hi folks, I had another post on a circuit with constant current but it turns out I was wrong and what I actually need is to do is to put power through an RTD which causes it to heat up (and thus increase its resistance, nominally 100ohms at zero centigrade which rises to about 125 ohms at 70 centigrade).
When air blows over the RTD it causes cooling of the RTD and so its temperature (and resistance) lowers, the circuit then increases the current going through it to bring its temp and resistance back up.
The measurement of how much extra power (current) it takes to keep the temp of the RTD is a measure of the air flow.
I have done some measurements and it takes about 80-150mW of power to heat the RTD up to 60-70 degrees which seems like a good temp to run it at.
I have got a circuit here that I think should work but I can't get it to...I really suck at this so any help anyone can give me would be great.

 

One helpful addition is a measurement of ambient temperature to normalize the hot wire's response - http://electronicdesign.com/article...s-and-a-microcontroller-simplify-anemome.aspx

You can find a lot of material under the term "hot wire anemometry".

 

Hello,

On this page you will find several circuits for thermal anemometers with their differences :
http://www.fonema.se/anemom/anemom.html

Bertus

 

One helpful addition is a measurement of ambient temperature to normalize the hot wire's response - http://electronicdesign.com/article...s-and-a-microcontroller-simplify-anemome.aspx

You can find a lot of material under the term "hot wire anemometry".

Yes, I plan on adding offset adjustment based on background temperature. I was just trying to get the basic feedback circuit running correctly first. Thanks for the link.

 

Hello,

On this page you will find several circuits for thermal anemometers with their differences :
http://www.fonema.se/anemom/anemom.html

Bertus

Thanks a lot thats very helpful!