Differential temperature switch

Discussion in 'General Electronics Chat' started by Jaguarjoe, Jun 29, 2010.

  1. Jaguarjoe

    Thread Starter Active Member

    Apr 7, 2010
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    Hi guys, I'm going to make a temperature switch that will shut off my central air unit when the outdoor temperature is lower than the indoor temperature. My plan is to use a pair of LM34 temperature sensors feeding an op-amp difference amplifier with a gain of 10 or so. This would feed an LM311 comparator which would then control my output relay. A 78L05 and a wall wart will power the whole thing.
    I have a few questions-
    Is there anything cheaper/better than the old LM34?
    I can use a PIC instead of the LM311 but I think taht would be overkill.
    Any problem using an OP90 for the op-amp?

    Thanks in advance for your input.
     
  2. Dx3

    Member

    Jun 19, 2010
    87
    7
    1) probably not
    2) true
    3) It would be a lot easier on me if you would provide the datasheet for the OP90
     
  3. Jaguarjoe

    Thread Starter Active Member

    Apr 7, 2010
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  4. Dx3

    Member

    Jun 19, 2010
    87
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    Yes, the OP90 looks like it will work.
     
  5. dsp_redux

    Active Member

    Apr 11, 2009
    182
    5
    Maybe a TC1047A will also fit for a temperature sensor, if that's what you are looking for.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    971
    You might want to perform some real tests first.. It can take HOURS for the inside temps to stabilize to outdoor temps and they can actually increase pretty quickly. Solar loading,stored heat,etc...
    I've thought about that too, but here in NC if the outdoor temp drops below my indoor temp during the night it will still be pretty uncomfortable if you shut the AC off. Your AC also does a good job of reducing humidity too which can be just as uncomfortable as the heat.
    Now if you can shut off the AC, open the windows and turn on some fans and the dehumidifier that might be better...Much larger task though.. :)
     
  7. Dx3

    Member

    Jun 19, 2010
    87
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    mcgyvr seems to be saying that the job can get complicated rather quickly. Now that you've had some time to think about it, we will be available for the questions that will present themselves.
     
  8. Jaguarjoe

    Thread Starter Active Member

    Apr 7, 2010
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    We can stand a little bit of heat and humidity if the A/C was inhibited. At least 5 degrees for sure. I can manually try this with two thermometers and the A/C off switch on the T-stat. That'll get me target values.
    To make things easy, I decided not to go with 1970's analog approach (how I grew up). Instead I'll use a pair of DS18B20 digital sensors and a small uP. This gives me much better control over set points, hysteresis and is much simpler.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    As an HVAC professional, I can tell you that these are available, very inexpensively, in the consumer market. They are called OATs. Outdoor air temp sensor.
    http://www.specsensors.com/oat.asp

    You use it with a relay to open and close the circuit.

    You should check your existing thermostat to see if it has an OAT function. This will make your life much easier.

    Many ACs have an Outdoor Ambient Air Temp Shutoff to be sure your AC doesn't operate in temperatures under ~50F.

    But, running a line of 2-wire from the stat to the outdoors is a pretty easy way to go.

    http://www.pexsupply.com/Honeywell-T775-SENS-OAT-1097-ohm-Outdoor-Air-Temperature-Sensor
     
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