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  1. #1
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    How do you check a thermistor?


    Air conditioners have thermistors all over controlling the different parts.....

    I would appreciate if anyone can advise how to check them. I know we have to check the resistance across the terminals.


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  2. #2
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    Re: How do you check a thermistor?

    Hello Aiyub.

    Different a/c manufacturers use different value thermistors with their units.

    You need information from the manufacturer of the resistance at certain temps for the thermistors used on their units.

    Make a slurry of ice and water in a cup place the thermistor in this and check on the charts resistance at 0 deg c.

    You could use a thermometer in a glass of water alongside the thermistor.

    In my experience they usually open circuit.

    (Especially when someones dog chews it off the side of the unit.)

  3. #3
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    Re: How do you check a thermistor?

    Thermistors used in AC systems are NTC devices (negative temperature Co-efficient) this means that the temperature and resistance are inverse to each other. It is never a linear value, so you will need in each case the manufacturer?s co-efficient chart or table. It should be in their manuals

    Some use the same value gradient for all the sensors on their machines, but don?t assume that they all do.

    Sensors have a centre value. Usually this is the resistance at a fixed temperature as a reference. For example 25 degrees at 10kohm. But it could vary according to the sensor range.

    To check one you need to take two or three accurate readings of temperature against resistance along the curve.
    You definitely need a good electronic thermometer and a good Multimeter.

    If there is any variance, ditch the sensor. Usually the values are more compressed at the lower temperature end and this is where you need the greatest accuracy.

    Some sensors have a habit of being erratic due to water getting through the seal.
    ________
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    Last edited by Argus; 07-02-2011 at 09:23 AM.

  4. #4
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    Re: How do you check a thermistor?

    Another good tip is to take a reading at the given(ambient) temp, say 20C and see how that relates to the manufacturers chart. With the meter connected, hold the thermister (to warm it up) and watch your meter to see if it responds as per the chart. When the thermister stops warming (at around 30C), you should get a feeling if it is responding correctly or erratically.

    You do need the chart though no matter how you test the sensor (unless it is open circuit or short circuit )
    I'm back on the Pale

  5. #5
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    Re: How do you check a thermistor?

    If I'm not mistaken, thermistors have no polarity so if you check the current/volts/resistance using the ohms formulas you can pretty well nail the resistance to a "T" using a good thermometer and multi meter.

    Check the I, R and V going to the thermistor. Remove the thermistor (probably shouldn't run the A/C while it's not connected), and check the resistance through the terminator at different temperatures. You can probably get a rough quadratic equation (non-linear change over temperature fluctuations) and apply the ohms formulas to determine how the resistance effects current and voltage drops across the thermostat. It's a whole lot more math, but if you don't have the tech specs, it can be done I believe.

  6. #6
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    Re: How do you check a thermistor?

    Auto correct on iPhone changed thermistors to "terminator" and "thermostat". Hope that clears things up...

  7. #7
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    Re: How do you check a thermistor?

    Quote Originally Posted by TimH View Post
    (probably shouldn't run the A/C while it's not connected) it can be done I believe.
    Timothy

    twice in one day... your hilarious...Thankyou

    R's chillerman
    If the World did not Suck, We would all fall off !

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