Reading a thermistor - optimal range

[pjrebordao]

In my current project i have to read the value of a thermistor. Cold it has a resistance of about 100ohm, fully warm gets to 50ohm.
If i use the recommended circuit from manual 3, i use a 10k resistor connected to ground in series with the thermistor.
However, this gives a rather limited range of values returned by readadc (about 50-60 units).
Can i use a different value for the resistor to increase this range, while keeping this thermistor ?

 

[inglewoodpete]

For the maximum range of output voltage values, the resistor should equal the mid-range value of the thermistor. However, this will depend on the ratings of the thermistor. What is the maximum rated voltage/current (power dissipation) of the thermistor?

 

[pjrebordao]

So i should make this resistor 50ohm for a total of about 150ohm in series.
I dont have the specs for the thermistor, but it's an engine temperature sensor rated for 6-24V

 

[Goeytex]

A thermistor with such low resistance is not an ideal choice for a simple temperature measurement project. It will be much easier with a 10K thermistor.
Shall we assume that your thermistor is rated at 100 ohms for a temperature of 25C ?

Can i use a different value for the resistor to increase this range, while keeping this thermistor ?

Maybe ... Consider what will happen if you reduce the pullup resistor to 100 ohms. At 25C there will be current of 25ma. So the thermistor will have to dissipate 62.5 mW. This is how much the thermistor will self-heat due to current. You will need the datasheet to determine how much error this will be. But it is substantial.

To use a low value thermistor and not suffer the effects of self-hearing the circuit resistance must increased and the signal amplified. This complicates things.

Is there a reason that you cannot use a 10K thermistor ?

 

[Technical]

In my current project i have to read the value of a thermistor. Cold it has a resistance of about 100ohm, fully warm gets to 50ohm.

Doesn't sounds like a thermistor at all, self heating would make this very inaccurate? Can you use a different sensor?

 

[pjrebordao]

The application is to measure an engine operating temperature as part of an ECU. For obvious reasons i would like to use a readily available off the shelf temperature sensor.
The values i gave are for the existing sensor connected to a temp gauge. In the meantime, i searched and realized that more modern sensors tipically used in ECU driven engines, have a much higher resistance - as much as several Ks when cold...so i'm gonna get one of those.

 

[rossko57]

Self-heating effects on a sensor in good thermal contact with gallons of circulating coolant are going to be negligible.

But yes, if you can find a modern higher-value sensor of the right thread fitting etc. it will be easier to work with, and in all likelihood more linear output.

 

[MFB]

I have used the AD590 temperature 2-pin sensor ideal for automotive applications. It has a linear current output that can drive metres of (even unscreened) cable without interference problems and can be easily interfaced to a PICAXE analog input by adding a pull-down resistor. Will however need a bit of simple code for offset and scaling.