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# Temperature vs. Output Voltage watch

1. I built a potential divider circuit with an NTC thermistor and a fixed 10k resistor, and measured output voltage across the thermistor. However-> I have straight line graphs (negative correlation) for alll temp values between 10-50 degrees celsius, and I thought I would get a non-linear graph? Have I gone wrong? Can anyone point me in the right direction?

Thanks!
2. It depends a lot on the actual thermistor. (What is its resistance at 25 deg?)
Although the resistance-temperature variation is a curve, it can approximate to a straight line between a small range of temperatures. 10 - 50 is quite a small range!
There is a graph here
http://www.sensorsmag.com/sensors/te...i-characte-811
(scroll down a little)
that shows this.
Although your range is a bit low (compared with the graph), you can see that the line would be more or less straight over that range for some NTCs.
3. Thanks, but I'm measuring voltage against temp, not resistance.
4. The voltage depends on the resistance. V=IR
What does your circuit look like?
Are you keeping I constant, or keeping the applied V constant?
Finally, what is the aim of the experiment? Is it investigate how resistance varies with temperature? If not; what?
Lots of questions, sorry.
5. Its OK- thanks for your help.
I described my circuit in the original post. I'm using a 6V battery.
The aim of the exp. is to calibrate a thermistor between the temps of 22-29 degrees celsius, but I was taking general readings between 10-50 to begin with.
6. If it's calibration then you just need the graph of something against temperature for the device. For the small range of temperature you are dealing with, the graph could be a straight line. It depends, as I said, on the thermistor. You have a 10k resister in the circuit with it, so it depends on how the resistance of the thermistor at 25 deg compares with that.
So no, don't worry about the line being straight.
7. (Original post by Stonebridge)
If it's calibration then you just need the graph of something against temperature for the device. For the small range of temperature you are dealing with, the graph could be a straight line. It depends, as I said, on the thermistor. You have a 10k resister in the circuit with it, so it depends on how the resistance of the thermistor at 25 deg compares with that.
So no, don't worry about the line being straight.
OK, thankyou!

So it doesn't matter whether I plot temperature against voltage or if I convert voltage to resistance and plot against resistance?

(Rep for you btw.
8. It depends how you go on to use your data.
The reason for calibrating the thermistor is that you can then go on to use it to measure temperature. If you measure the voltage across it at that range of temperatures, then when you use it later as a "thermometer", you just measure the voltage at the unknown temperature, and work out what that temperature is. Just make sure you keep the battery voltage and the other resistor the same value.
If your graph is a straight line, you can work out the unknown temperature by simple proportion. (Or reading the value off the graph). If it isn't a straight line, you have to do it by reading off the graph.
Good luck.

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