strange thermistor? (resistance decreases w/ temp)

[nicola]

Hello,
I'm trying to build a temp sensor with a thermistor available in the workshop kit (a 4k7 thermistor), and I'm getting a strange behavior: the resistance seems to decrease with temperature!

my circuit is very simple:

5v----<thermistor>----x----<10k res>---ground
                                 |
                             analog pin 0

I'm measuring Vx at the "x" with analog pin 0, between the thermistor and the 10k resistor.
What I get is: if I increase the temperature, the adc (and Vx) is increasing.
=> Rx (the resistence of the thermistor) is decreasing, according to:
Rx = R*(V - Vx)/Vx
(where V=5v and R=10kohm)

I've tested the circuit replacing the thermistor with resistors of known values plugged into 5v, and the behaviour is correct: the bigger the resistor, the lower is the voltage measured at "x".

Any suggestions? What am I getting wrong???

thanks.

[retrolefty]

Well a data sheet for the thermistor should verify if it has a negative or positive temperature coefficient. You can always swap the position the thermistor and the fixed resistor to reverse the action.

Lefty

[nicola]

thanks, so it's possible that a thermistor has a negative coefficient to temperature?
I've always thought that resistence always increases with temp, that's why I found all this so strange.

I don't know how to find datasheets for this thermistor; the name on the list of components in the workshop kit only says "4,7k ohm thermistor"...

nici

[retrolefty]

" so it's possible that a thermistor has a negative coefficient to temperature?"

NTC or PTC as explained here:  

http://en.wikipedia.org/wiki/Thermistor

I guess without a data sheet you could try and classify your device by measuring it's resistance in ice water and boiling water and at a few points in between comparing it with a glass thermometer. However I don't believe thermistors are very linear over their measurement range.

Good luck

Lefty


Lefty

[nicola]

thanks so much; that wiki page is very clear.
I assume I'll have to sample values and try to infer the coefficients...

nici

[liuzengqiang]

Metal's resistance increases with temperature since metalic ions shake more rapidly at higher temperature and electrons get more resistance going through the metal.

Semiconductor's resistance decreases with temperature since higher temperature helps release more electrons to conduct. Your thermistor is probably a doped semiconductor. The 4.7K probably means its resistance is 4.7K at 25deg C.