Temp readings and voltage input

[Acetronics2]

Hey Ace,

The Carbsmart uses a 100K thermistor (supposedly a ntsa0wf104fe1b0). I have a spare and was thinking of using it with a pic to do some temp recordings to see how my temps are going.

I just need some advise on the circuit design. I was going to use a 5V regulator to supply the pic and ref for the thermistor. Also use a 100k resistor as a voltage divider as my analog voltage input to the pic. Is this enough?

Also, the thermistor has a current rating of 0.14ma @25C. Does that mean max current draw through the ntc over the entire temp range or what?

See attached circuit.

Hi, Squib

as always ... it relies on what you want to do !

your scheme works fine if you just want to switch a relay ( a home thermostat generally works between 17 and 23 °C !!! ) or run an alarm ... NTCs perfectly can be used on limited temp ranges where you use a linear or 2nd order approx.

now, you also can use a retrive table ( You did it with Excel, it seems ... ) and linearize between points: look at Microchip AN's !!!

The main problem is NTC's often come with 10 or 20% nominal value tolerance ... so, it's not possible to get something precise without any calibration ( my home thermostat or my car temp indicator offer you to correct the read temp by adding or substracting 0 to 5 degrees ... simplest and easiest way !!! )


For your thermistor current rating 0.14 ma ^2 x 100 000 = 2 mW ...

let's suppose it's the maximum current not to disturb the thermistor temp. by self heating !!!



Now, ... once more, the sensor will be chosen from the temp range and the use you will do with the output ...

trying to know a model engine head temp @ +/- 1°C is a nonsense ... as you won't be able to place the sensor very close to the combustion chamber ... you just measure "something indicative ... " and the same measurement, with cold ot hot weather do not show ... not negligible ... temp changes at the "critical" point !!!

Alain