Help understanding PTC specs

[Simon]

Hi guys, I'm trying to select a PTC resistor for an application, I need a slight variation in value for temperature changes. I've looked yp parts on Farnell but none of them have a specific R/C value. How are PTC's characterized then ? how to i understand wehat they will do so that in my circuit I can predict the result ?

[Zero999]

I'm not sure what you're asking.

Most of the PTC thermistors have a clear temperature vs resistance graph on the datasheet.

http://uk.farnell.com/epcos/b59701a70a62/thermistor-ptc-smc/dp/3876690
http://www.farnell.com/datasheets/7181.pdf

why does it need to be PTC? The most accurate temperature sensors are NTC and it's pretty easy to make the output voltage positive with a positive coefficient by exchanging the places in the potential divider.

[Simon]

it needs to be positive because I want to use it to bias the output of a current sensing circuit, the current sensor is the MAX4172 which outputs a certain current for a certain voltage in (sensed from a series resistor), so normally the current output is just one resistor with a current through it proportional to the current under examination, you choose the resistor value to get the output voltage range you want it. I was thinking of putting something in series with the output resistor so that temperature would alter the reading and allow more current when cold and less when hot. I was going to use a diode but tests on a 1N4001 showed that the coefficient is negative

[Simon]

and this is a very iluminating spec sheet: http://www.farnell.com/datasheets/357013.pdf

I'm after through hole stuff as the SMD ones are too tricky for me at the moment