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What Thermistor Mister?

Discussion in 'General Electronics Discussion' started by Raven Luni, Nov 14, 2012.

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  1. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    Thermistors: Never used the things before and dont understand their operation other than that their resistance varies with temperature.

    I've got a project in mind involving heat pads and I want some basic temperature regulation but nothing fancy - just something along the lines of switching off a transistor above a certain temperature set by a pot.

    What values would I be looking for? Everything I've seen so far just has them listed like resistors with a fixed value and no information about its properties.

    And why are values at the extremes such as 100K and 10ohm much cheaper than the mid range values (10K, 30K etc)?
     
    Last edited: Nov 14, 2012
  2. john monks

    john monks

    693
    2
    Mar 9, 2012
    There are positive temperature coefficient thermistors and negative temperature coefficient thermistors. The most common are negative coefficient, that is the resistance goes down as the temperature increases.
    Thermistors are typically used with and operational amplifier.
    The resistance you see on the package is typically the value you get around 25 degrees C.
    A thermistor that gives 100kohm at 25 degrees C will typically be around 33.6kohm at 50 degrees C and 351kohm at 0 degrees C.
    Thermistors are typically used in some form of a bridge circuit.
    I don't know why the price differentials.
    Exactly what kind of a project do you have in mind?
     
  3. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    Ah - that makes sense - thanks.

    As for the project, I saw these heat pads recently (http://www.coolcomponents.co.uk/catalog/heating-5x10cm-p-1024.html) and thought they would be ideal for some of the smaller animals at the wildlife rescue place where I volunteer. I rekon a basic thermostat would be enough to control it - comparator fed by a thermistor and a pot, maybe an additional trimpot for fine tuning or adjusting hysteresis and output to a power transistor like a TIP41 (apparently it draws about 0.8A @ 5V)
     
  4. john monks

    john monks

    693
    2
    Mar 9, 2012
    I think you are on the right track.
    The TIP41 can be driven from another transistor, perhaps a 2N3904, in a darlington arrangement that is driven from an operational amplifier, maybe a 741 used as a comparator circuit. You might be able to find a micro power operational amplifier or comparator so that your device can run off a battery.

    Is this going to be a lighting circuit or something for a camera?
     
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