Measure temperature with thermistor and PIC internal ADC

[Futterama]

Hello forum,

I would like some help on this project: I wish to measure a temperature, range from 0°C to 150°C.

The temperature probe should be a small thermistor (perhaps NTC), and I wish to use a PIC12F683 with internal 10-bit AD-converter and 256byte EEPROM.
The PIC will run from 5V, and I have some precision voltage references for this project (MAX4050 4.096V 0.1% reference and Zetex ZR431)

The overall precision should be +/-1°C.

I plan on putting the thermistor on a hot surface (150°C) and read the ADC value of the PIC for every degree as the hot surface cools down, and note these values, and program them into the EEPROM of the PIC for lookup when operating.

But I think I need some help with the circuit. I've read on this forum that 2.5k is tha largest acceptable impedance for the PIC ADC to keep ADC accuracy...

So, I need help on choosing the right thermistor, and building the circuit.

Thanks.

Regards,
Futterama

 

[Nigel Goodwin]

Hello forum,

I would like some help on this project: I wish to measure a temperature, range from 0°C to 150°C.

The temperature probe should be a small thermistor (perhaps NTC), and I wish to use a PIC12F683 with internal 10-bit AD-converter and 256byte EEPROM.
The PIC will run from 5V, and I have some precision voltage references for this project (MAX4050 4.096V 0.1% reference and Zetex ZR431)

The overall precision should be +/-1°C.

I plan on putting the thermistor on a hot surface (150°C) and read the ADC value of the PIC for every degree as the hot surface cools down, and note these values, and program them into the EEPROM of the PIC for lookup when operating.

But I think I need some help with the circuit. I've read on this forum that 2.5k is tha largest acceptable impedance for the PIC ADC to keep ADC accuracy...

So, I need help on choosing the right thermistor, and building the circuit.

Use an opamp to buffer the thermistor - check my tutorials to see what I used in my analogue input tutorial.

You need to be VERY! aware that thermistors are no where near linear!, you would require a look-up table to convert the reading from the thermistor to an accurate temperature - and you would probably need to calibrate it against an accurate thermometer.

Far easier to use one of the modern IC sensors, they are available with linear outputs (so many mV per degree), or with direct digital data outputs, where you can simply read the temperature direcly (no analogue conversion required).

 

[Mike - K8LH]

...

 

[Futterama]

Nigel, as I wrote, I will use a lookup table, cause I know a thermister is not linear. I will measure the temperature with either a cooking thermometer, or with my thermometer-probe for my multimeter, or perhaps I could borrow some more precise equipment.

Regarding the IC's, I would love a serial output IC, but I haven't found any that can go to 150°C and that can meet the space requirements. Take a look at this image (click to enlarge), there is only 1.5mm for the black thermistor.

**broken link removed**

But Nigel, perhaps you know about a small linear or serial IC that is useful?
Actually, the +/-1°C is only nessasary at low temperatures, above 100°C, +/-2°C or +/-3°C would be acceptable.


Mike, K8LH, what does your circuit do?

/Futterama

 

[Oznog]

Oh! You're looking to measure Cylinder Head Temperature. Looks like an RC car too.

This is usually done with a thermocouple, not a thermistor, primarily because the temp is generally too hot for a thermistor. Actually there are thermistors that can go as high as 150C but this is the extreme upper edge of the range.

Cylinder head temp is typically measured with a Type J thermcouple. You will need a low offset op amp to measure it accurately. Generally you don't need to buy a thermocouple probe, just Type J wire. Thermocouples create a voltage based on the differential between the hot and cold junction sides, so for an accurate reading you still need to use a thermistor (or temp sensor) to get an absolute temperature for the cold junction side rather than assuming the cold junction (where the thermocouple is soldered to the amplifier inputs) is 70F or whatever. That looks like an RC car so it can easily be much hotter or colder.

If that's indeed an RC car CHT we're looking at I should mention 1 deg C is hardly necessary, will be very difficult to achieve, and probably pointless. In use some areas will be several deg C hotter than others. Really 5 deg C should be sufficient and easily achievable.

 

[Futterama]

Yes, it is an engine for a RC car, and the thing with the thermocouple sounds like a lot of trouble :?

I will look for an IC again I think...