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  1. #1

    Thermostat Test Circuit

    Hi All,

    I am building a circuit to test a thermostat (completely disconnected from any other HVAC equipment). I've done work around my house, but I am not an HVAC professional. I'm a computer science researcher and looking at the new wireless/internet controllable thermostats to review from a computer science point of view.

    My plan is to hook up a 24VAC power supply to the thermostat and then I want little LEDs to turn on when the thermostat calls for blower or compressor, etc... My plan is to wire in a diode --> capacitor ---> LED as the "load" for each item (blower, heat, ac) to accomplish this. It seems simple, but I thought I'd check here for two things:
    1. Does a test device like this already exist I can just buy for $10-20?
    2. Is there anything else for the load circuit I'm missing. I'm not sure of the load the thermostat expects... to be safe I'm thinking I should add in a 75ohm resistor to limit the current.

    Let me know your thoughts and if I'm missing something obvious to people in the know.

    (Also, let me know if this isn't the right forum for my topic. It was my best guess! :-)


  2. #2
    Join Date
    Dec 2007
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  3. #3
    Join Date
    Jun 2011
    Central Indiana
    Quote Originally Posted by danrckville View Post
    Hi All,
    1. Does a test device like this already exist I can just buy for $10-20?

    This can even be used for lots more than your wild goose chase.
    The only true knowledge is the pursuit of knowledge

  4. #4
    Join Date
    Oct 2004
    Central WA
    It might work for some...

    Many thermostats use conventional wiring conections and micro relays to control the equipment. Some, however, are of a "power stealing" design where a small current is constantly trickled through an unused load to support itself. I wonder if running a diode as a load and essentially rectifying that trickle current would cause problems?

    As far as limiting the current with a resistor, I can't imagine your LEDs will draw that much. Typically thermostats should be able to handle 1.5 amps as might be seen on a larger electric heating system.

    Another trend in the industry is communicating thermostats where no switching is done at the thermostat itself - power and com wires only, and sometimes power only and wireless. Some communicate to an interface module that is hooked up at the furnace with conventional connections, but oem's are designing stats to communicate directly with the control boards on the equipment.

    The last thing to consider is configuration. Depending on what kind of testing and reviewing, once you hook the thing up it needs to be told what it is - heat pump, conventional gas, dual-fuel, electric, forced air, boiler -- and then all of the control options that go along with them. With the exception of some "home depot" models, the manuals are written for technicians, and some industry knowledge will be required at that point.

  5. #5
    Thanks! When you say they use conventional wiring and micro-relays. Do you mean in the thermostat? How I've understood (which could be very wrong!) was that the low voltage wires (24VAC) are going into/out of the thermostat and at the air handler/furnace unit there are relays there to turn on/off the line voltage parts (blower, etc...). I didn't think the thermostat would have any relays in it directly. If so, why would they do that? The first one I'm looking is using the C wire to power the thermostat itself. Is that indicating the power stealing design you mention? I did wonder if rectifying the power was going to cause a problem.

    Thanks for the answer about the LED!

    I have seen some of the versions with a unit and power/com only in the thermostat. If my initial reviews go well, I'll look into those also.

    I have configured a few, and with the setup I'm talking about (where the actual configuration doesn't matter much since it isn't hooked up to a real HVAC unit) I should be okay. The main parts I'm interested in really are how it connects to the internet and what types of issues may arise there.

    Thanks for your help!

  6. #6
    Join Date
    Jul 2009
    I would suggest you buy a couple electronics books and learn basic electricity and how circuits work. R doesn't have to be 24vAC on the newer stats it can be a mVDC to a couple dc batteries if you want; as long as the stat is powered by it's own battery pack. This is because a digital stat has it's own power circuit that just switches a bunch of internal micro relays. Even a mechanical stat can be tested this way. A basic book will help you understand what you need to do. Breadboard, 2xAA battery pack, LED's, Resistors is all you need for a simple test circuit. As far as hooking up to the internet each device will be different and recommend reading the manual of the specific stat.

  7. #7
    Thanks for the advice. I do have a very good understanding of electronics and circuits, just not thermostats. I just want to make sure my understanding of thermostats is sufficient.

    The stat I'm using doesn't have a battery pack and requires the C wire to work (thus the need for 24VAC). I don't think it can take DC directly without modifying the internals.


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