i am facing big problems using a pt100 thermistor. this device has 3 wires, two red and a blue one. The two red wires are connected, i.e. using the beeper on the multimeter it beeps. I suppose that the way this works is this: apply a tension on red and blue wires, and the difference with respect to the initial voltage, is to be correlated with room temperature, I.e. the thermistor reduces the voltage applied. Right?
The second device of my configuration is an analog input module, capable of reading 0-5volt inputs (among other things), with 12 bit resolution. I wrote a simple driver that converts tensions to two bytes, used a transformer on it and aknowledged that it works! fine.
What about putting the two together? I want to read the output of the pt100 device using this module, but I fail. I cannot understand why, applying a tension on the red and blue wires of the pt100 thermistor, all i get is a very very small variation of the initial voltage(~4 mV) and no further variation on increasing diminishing heat on the pt100 device. What is wrong? am i using them wrong? did i connect them wrong? i tried putting 1,5V, 3V etc but i noticed no significant difference....
Dear JF, thank you for you reply. it is exactly what you have described in your post. a platinum resistance thermometer.
I think I understand now that this device works with a very low voltage, but i have no idea on exactly what voltage it is needed. What do i have to do in order to read successfully the small voltage variations that are provoked by the temperature variations?
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The usual approach is to buy an already built meter like:
http://www.omega.com/ppt/pptsc.asp?ref=DP460-T&Nav=temm07
If you want to build your own then it gets complicated.
What do you want to do?
JF
I would like to read with my 0-5V analog input device the values of the thermometer. Applying a very small voltage on the two wires of the thermometer ( 18 mV ) and heating it, the voltage between the two wires of the termometer increases. Problem is that i cannot get a temperature value ( a resistance value ) just from reading the voltage change, or can I?
we find that a 100 ohm sensor with an alpha of 0.00385 exhibits a resistance of 100.00 ohms at 0C and a resistance of 138.51 ohms at 100C
That's a difference of 38.51 ohms, and if you want to use an analog input device to indicate zero volts at 0C and 5V at 100C, then we must convert that 38.51 ohm resistance change into a 5V change in voltage.
To start with, we must excite the RTD electrically, but we mustn't force too much current through it or it'll self-heat to the point where its accuracy will be degraded.
Assuming that 1mA is OK and that we have a 10V excitation supply, we can start like this:
10V Vin | [R1] | +--->Vout | [RTD]R2 | 0V
With 1mA through the RTD at 0C, we'll have, as Vout:
Vout = Irtd R2 = 0.001A * 100R = 0.1V
Now, in order to limit the current to 1mA, we must calculate the value of R1:
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