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Thermostat reliability - hardware threshold vs. software threshold

Aaron Ridinger
Intellectual 490 points
Other Parts Discussed in Thread: TMP101, LM56, LM57, ADS1015

Hello,

I am working on a design that will automatically turn a heater on if the ambient temp is below -20degC and turn the heater off if the temp rises above -20degC.  Accuracy and hysteresis are not critical parameters, but long term operation and reliability are.

Which is more reliable and robust: a hardware programmed threshold device such as the LM56, or a software programmed threshold device such as something similar to the TMP101?  

Thanks,

Aaron

  • 0
    TI__Mastermind 37080 points

    Hi Aaron,

    Thank you for your inquiries on Temperature Sensor Products. Based on your application description, I would go for the LM56 or LM57 (programmable trip point). The benefit is that you can direct connect the output pins to the heater where digital temperature sensor requires to reprogram the temperature whenever there is power down, and for some device it’s bigger die and it might require additional discrete components.

    I just want to clarify that you must set a threshold in the range of 1-5°C so that it won’t oscillate during at -20°C.

    If you have further question, please feel free to post again.

    Aaron Heng

  • 0 in reply to Aaron Heng
    Intellectual 490 points

    Hi Aaron,

    Thanks for your response.  In general the system will be running continuously so there will not be many power downs where the sensor would have to be reprogrammed.  However, in the  times it will be powered down, the extra step of programming the threshold causes more complexity and I guess could possibly be a failure point.  So I will use the LM56 for heater control, and then connect the Vtemp output to an ADC (ADS1015) for monitoring and reporting ambient temp. 

    I'm not sure what you mean by setting a threshold in the range of 1-5°C.  I understand that the LM56 has built in hysteresis of 5°C, so it shouldn't oscillate.  Is my understanding correct? I will only use VT1 to set the threshold and OUT1 for the heater control.  

    Thanks,

    Aaron

  • 0 in reply to Aaron Ridinger
    TI__Mastermind 37080 points

    Hi Aaron,

    For LM56, you do not need to worry the threshold. You are absolutely correct. I'm sorry for any confusion.

    Aaron

  • 0 in reply to Aaron Heng
    Intellectual 490 points

    Hi Aaron,

    No problem.  Thanks for your help.

    Aaron