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Temperature Probes?
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Re: Temperature Probes?

by peck on Wed Apr 01, 2009 10:52 pm

Ok, so close here (and completely realizing that thermistors are NOT the way to go, but want to make it work as learning exercise).

So my circuit it like this one:
Image

Where my resistor is 250k Ohms, which leads to a reading on Analog 0 of 550 at room temperature.

I need to combine two function to change that into temp, first one is determinining resistance from voltage.
Va = Voltage on analog 0 * (1023/5)
R = fixed resistor
T = Thermistor
V = Input voltage

Va = V*(T/(R-T))
which moves around to T = ((Va/V)*R/(1-(Va/V))

Works great at room temp, but the function goes backwards and tells me that the resistance is increasing with temp, which doesnt make sense at all since Ive measured it with the DMM and the analog pin is reading higher as well, so maybe I'm using the wrong function?

Also using the RT = RTze^B(1/T - 1/Tz) function to find the temp after that, seems my B is 4121, but need to get the above function to verify if thats correct enough or if theres some fudging needed.

Thanks,
peck
 
Posts: 17
Joined: Sat Mar 28, 2009 7:03 pm

Re: Temperature Probes?

by peck on Wed Apr 01, 2009 11:27 pm

Figured it out, had my equation backwards. I was solving for the closest resistor, but I had changed my circuit around, seems to work reasonably well now.

Off a little at the top end but pretty accurate before that.
peck
 
Posts: 17
Joined: Sat Mar 28, 2009 7:03 pm

Please be positive and constructive with your questions and comments.