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  1. #11
    Fanatical Contributor Tim's Avatar
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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    Thanks Les

    One thing looking at the parts and doing a sim or 2. I need an amplifier to get a usable signal. 10 bits over 0-5v with the signal only covering 1v in range is not an option.


    Tim
    Tim

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  3. #12
    Fanatical Contributor top204's Avatar
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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    You can try using the internal reference voltage of the ADC and reduce it as close to 1V as possible. This acts as a form of amplifier for the ADC. Then change the cQuanta constant to use the reference voltage divided by the resolution instead of the VDD voltage. Also, a lot of the devices have built in op-amps that can be used as a non-inverting amplifier with a couple of resistors.

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  5. #13
    Prolific Poster See_Mos's Avatar
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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    +/-2C should not be too difficult using something like Farnell 116 4823 NTC in a potential divider arrangement , especially if you can ignore the out of range readings.

    What is your supply voltage? Which PIC device?
    My RAM is failing

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    Fanatical Contributor Tim's Avatar
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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    Hi

    I have that part already. The problem I'm worried about is when I run a sim in Labcenter with a 100k PTC device and a 100k on the other side in the divider and look at the voltage ranges I get

    5V/0V rails

    160oC = 1.886v
    159oC = 1.890v
    ..
    100oC = 2.139v

    So at the top end 1c is = 0.004v

    The pic I might use is PIC18F25K22 with no vref I get 0.004v per step. If I use an external Vref_ and Vref- it may help. But I think it would be better to get an opamp to cover say 1.8v to 2.7v but not sure how.

    Thanks

    Tim
    Tim

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  9. #15
    Prolific Poster See_Mos's Avatar
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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    Hi Tim, the device is NTC so the resistance drops as temperature increases. Looking at the data sheet at 100c the resistance is 6773 ohms and at 200c it is 636.7. If the NTC is connected to VSS and the other resistor to VDD then using a fixed resistor of 4K the voltage should be 1.86 at 100c and 4.3 at 200c. I picked 4K as a random value but by changing the value you can move the range to improve the span.
    Last edited by See_Mos; 28th May 2019 at 20:39.
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    Fanatical Contributor Tim's Avatar
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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    Hi See_Mos

    I did the maths and yup it looks good. I can set the Internal Vref to 4.2v and that gives me the best range and resolution.

    Many thanks it looks doable!

    Tim
    Tim

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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    Tim, one problem you may encounter is the lack of precision of the internal voltage reference. Perhaps consider an external voltage ref.
    George.

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    Default Re: Thermistor Steinhart-Hart linearisation procedure for Proton BASIC

    Now I would have simply used a temperature divider and use any standard temperature measuring component
    Fanie

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  17. #19
    Junior Member Fred Philpott's Avatar
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    Default Re: Temperature sensing 100-160c

    I have come in a bit late on this, so apologies if I have missed anything, but what about using a silicon diode like a 1N4148. I use them frequently for thermocouple ambient compensation and cheap sensors for things like water baths. They will give you approx. 650mV at 25C and a signal of -2mV/C so 160C will give you a change of -320mV at 1mA. This is within the range of a 1.024V reference and will give you a resolution of 0.5C per bit on a 10 bit ADC. If you stick to one manufacturer the tolerance between devices is usually pretty good and a single point calibration should be sufficient in production. Can't get much cheaper than that. I haven't used them that high but they should handle it.

    Just checked the GE data sheet. Maximum junction temperature 175C.
    Last edited by Fred Philpott; 9th June 2019 at 11:56.

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