thermistors

Discussion in 'General Electronics Chat' started by cv64, Jul 16, 2010.

  1. cv64

    Thread Starter New Member

    Jul 16, 2010
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    Hi,
    I'm new to this site. I have a honeywell thermistat. It's display is off/blank. I've spent a good part of the morning trying to find info for this product. I've been wanting a schematic on line. Can't find one. Troubleshooting tips suggest checking the fuse. There is no "F" ref des on the pca. There is a "RT" (thermistor). I've tried finding some info on the subject including the post "working of thermistor" by "hbkamit". The only way to test it is to use an external heat source (stove top). Is this really the only way?

    Thank you
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    pictures speaks a thousand words
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    No, you don't need a stove top to test a thermistor. The resistance of a thermistor in a room thermostat will change significantly with a change from room temperature (say 80 degrees F) to fingertip temperature (say 95 degrees F). Just put your hot little fingertips around the tip of the thermistor; the resistance will decrease if it is an NTC (negative temperature coefficient) thermistor and increase if is a PTC. (NTC is most likely.)

    It's also not likely that the thermistor has failed, especially if the symptom is no display.
     
  4. cv64

    Thread Starter New Member

    Jul 16, 2010
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    I did the test the thermistor appears to be working. Is there someplace where I can get the schematic?
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    1. Google the part number on the thermostat or any other numbers you see on the PCB.

    2. Contact the manufacturer.

    3. Contact your local service provider.

    Or, you can take some closeup photos of both sides of the PCB, post them here and someone may be able to spot the fuse for you.
     
  6. Wendy

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    Mar 24, 2008
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    Generally it is considered cheaper to replace these units than to fix them. While I wouldn't recommend it for everyone, I've personally replace mine twice over the 26 years I've owned my house.

    Are you sure your batteries aren't dead? That is the symptoms I've had over the years.
     
  7. cv64

    Thread Starter New Member

    Jul 16, 2010
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    The reason this has happened is we lost power to the house and when power came back on the unit doesn't have a display. The fan works. If the thermistor was supposed to work to prevent this failure, because there is no fuse. Where then is the most probable cause the main ic that connects to the led?
     
  8. cv64

    Thread Starter New Member

    Jul 16, 2010
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    this unit is one without batteries
     
  9. Wendy

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    Mar 24, 2008
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    The thermistor is a temperature sensor, it is extremely doubtful this is the problem. It is a resistor whose resistance varies with temperature.

    More than likely something solid state and complex died during a power spike.

    No batteries in either the base or the part you took off the wall? Unusual, but then I usually get setback units.
     
  10. cv64

    Thread Starter New Member

    Jul 16, 2010
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    I was able to find this part number CAR5UT43 it has honeywell brand on it. I type it into google and it gives me one link. It then takes me to the americanhvacparts.com web page. Or TAYSTAT340 with trane brand on it.
    I hope this will help
     
  11. tracecom

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    Apr 16, 2010
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    In the troubleshooting section of the manual, I think the reference to replacing the fuse refers to the fuse in the fusebox, not a fuse in the thermostat. Of course, you probably have a breaker (instead of a fuse) and a breakerbox (instead of a fusebox.) If the breaker is not tripped that supplies power to the thermostat, I think the thermostat is toast.
     
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