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Reducing Voltage with a Resistor

  1. Jan 23, 2013 #1
    I have been advised by my thermostat manufacturer that I need to reduce the voltage to prevent the O/B terminal on my thermostat from blowing again. The current voltage reading to power my thermostat is 28 volts and it needs to be closer to 24. I assume this is possible with resisters, but I don't know where to begin. If possible what type of resistor do I need to buy? Thanks.
  2. jcsd
  3. Jan 23, 2013 #2
    volts=I*r now the only info you supplied is the type of circuit and the current voltage.
    thankfully most thermocouples are 4-20 ma. If this one has higher this calc will be off.

    based at the high temp sensitivity 20 ma I get 1200 ohm resistor. as its low amp a standard 5% precision low amp resistor will work.

    edit just noticed that you didn't specify thermocouple lol. without the supply current I cannot give an exact value. one trick to try is to use a potentiometer 5k should work place in series to the 0/B dial the pot till desired voltage then remove from circuit and read the resistance on the pot.
    Last edited: Jan 23, 2013
  4. Jan 23, 2013 #3
    One other thing to check, furnaces take 120 volt of power from your service panel so in the furnace there should be a 120 to 24 volt converter. some of them have a variable potentiometer on them. If so the 24 volt converter may just need to be trimmed.

    Edit: I was thinking about this further it almost sounds like the thermostat or cabling is drawing too much current. This can show up as higher voltage. Disconnect the 24 volt line at the source meter it out without a load from the regulator to confirm.
    Last edited: Jan 23, 2013
  5. Jan 23, 2013 #4

    jim hardy

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    28 volts sounds suspicious to me. 24 volts is industry standard in this country.

    There is a step-down transformer on your furnace that makes 24 volts for the thermostat.

    It might be designed for 120 volt, 240 volt, or 208 volt input.

    IF your transformer is made for 208 volts input and your furnace feeds it 240 volts, it'll put out about 27.8 volts instead of 24.
    The transformer will also run hot to the touch.

    Watch this video, pay special attention to the last minute, and then look at your furnace:

    Make sure you have the right transformer in your furnace and it's set to the right tap. A 208 volt transformer operated at 240 will burn up in a few seasons.

    The transformers cost about twenty bucks at my local hardware store.

    http://www.grainger.com/Grainger/ecatalog/N-1z0dnhq [Broken]

    Don't mess around with resistors - fix it right.
    Because Murphy's Law says a shortcut will ALWAYS cause you trouble , and at at the worst possible time.

    old jim
    Last edited by a moderator: May 6, 2017
  6. Jan 24, 2013 #5
    This is what I am working with:


  7. Jan 24, 2013 #6

    jim hardy

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    well that looks to me like single phase 240 volt.

    Find the transformer, it'll look like those in the Grainger link and that youtube video.

    Read its nameplate and check its output voltage..

    If it is making 24 volts and you have 28 at thermostat, something elase is wrong.
  8. Jan 24, 2013 #7
    I looks like I possibly have two of them? Also noticed that the top one says 230v.

  9. Jan 24, 2013 #8

    jim hardy

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    top looks like a transformer, bottom like a relay.

    Now you need to see whether it's producing 24 or 28 volts.

    Also - housepower is nominal 240 but can run 253 here in US .

    253/230 X 24 = 26.4 , so your transformer would be within its rights to deliver 26.4 volts if your electric company's voltage is at its high limit that day. Maybe even a little more if it is lightly loaded.......

    Can you safely read incoming voltage and transformer output within a short time, like a minute ?

    It might be that you need a 240 volt transformer.

    or perhaps you need a more robust thermostat.
  10. Jan 25, 2013 #9
    Incoming voltage is 246-247.
    Output is 27.8 which is the same at the thermostat.
    So I am thinking that I do need a different transformer (240) to get the the output closer to 24. Your recommendations? Thanks for all of your help.
    Last edited: Jan 25, 2013
  11. Jan 25, 2013 #10

    jim hardy

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    Well in an ideal world, a 230 to 24 volt transformer with 247 on its input would put out 25.78 volts.
    You are getting two volts more, almost 8% more, which suggests your transformer is lightly loaded.

    Before picking one i'd want to know its load.

    You should be safe if you get one rated and marked for 240 input.
    If you can't determine load on it, pick one that weighs no more and not a lot less, maybe 2/3.
    Feel the one that's in there - it should be pretty warm with that much voltage on it.
    A 240 volt one should run noticeably cooler.

    I realize this is a physics forum and i apologize if i'm making it into DIY board.
    People learn by doing and it's an old instinct to help.

    Will improve.

    old jim
  12. Jan 26, 2013 #11
    My guess is that the thermostat design is defective.
    A thermostat should work at voltages higher than 28 volt.
    Try a different brand thermostat.
    What brand thermostat are you using?
  13. Jan 28, 2013 #12
  14. Jan 28, 2013 #13

    jim hardy

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