How to reduce from 28VAC to 20VAC

Discussion in 'General Electronics Chat' started by Big Al MA, Jan 31, 2013.

  1. Big Al MA

    Thread Starter New Member

    Jan 31, 2013
    I'm installing a Filtrete 3M50 thermostat in my home. It offers wifi connection to internet and thus I can control the thermostat from anywhere. Wifi feature requires power, to run the small radio transceiver. Manufacturer indicates in manual that 12-24VAC is required. I have a Taco SR504 switching relay with 24VAC output terminals, but the actual output voltage is 28.3VAC, measured with my digital multimeter. Thought it would work, but unit beeps upon turning on 24VAC source. I contacted manufacturer (Filtrete/3M) at, and they indicate the reason for the beeping is that the input voltage is too high, and they told me that the voltage should be between 16 and 24 VAC.

    I'm thinking that I could use a resistor on one leg of the supply to drop the voltage to 20 volts. The manufacturer tells me that with the radio on, the current is 250 milliamps.

    If i wanted to have a voltage drop of 8 volts (from 28 V to 20 V), can I use Ohm's law to calculate the required resistance? 8V divided by 0.25A = 32 Ohms. Also Power would be 0.25A x 20 V = 5 Watts?

    Is it correct then to search for a resistor which has a power rating of at least 5 watts, and which is rated at 32 Ohms?

    Is there a more accurate way to do this using my multimeter, that is, instead of using current figures supplied by the manufacturer, use actual current values, etc....

    And, then, can I just solder the resister to the end of one leg of the voltage supply wire (the red wire, connected at the "C" or common terminal)?

    Thanks for any help!

  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Big Al.

    Depending on your volt meter, it may be measuring peak voltage instead of RMS voltage. I am pretty sure 3M is referring to RMS voltage and 28.3 peak x 0.707 is exactly 20. You may not have a problem with input voltage but rather a problem with your thermostat.

    What brand and model volt meter are you using?
  3. Big Al MA

    Thread Starter New Member

    Jan 31, 2013
    My volt meter is a Radio Shack Auto-Range Digital Multimeter 22-163

    Not sure if there isn't a problem with the thermostat, but it is new and when powered by batteries only (without 24VAC source) the display is just fine. It only starts beeping when the 28 VAC power source is applied. I am operating under a present assumption that the thermostat is OK, and that I need to reduce the input voltage.

    I'm not sure how to definitively tell whether the multimeter I have reads in true RMS or peak. I checked on line to try and find a manual, and I found one, but it makes no indication of this. It does indicate that the maximum voltage is 750 VAC RMS.

    However, what I gather from researching online, is that my multimeter is not likely to be a True RMS meter, but rather measures the average (rectified) voltage. It most likely DOES NOT measure peak voltage. ASSuming that I am dealing with a true sine wave, this is not likely to amount to much of any difference.

    Any thoughts on how I can accomplish this?
    Last edited: Jan 31, 2013
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    More likely the multimeter is measuring RMS.

    I took 2 minutes and found this AC to AC 12V 400mA and a cheaper 500mA at Jameco. This 18V with 2200mA is also from them matches the 16 to 24 Volt suggestion with current to spare, but it is likely to waste some watts of power.

    Note that Jameco stocks are very fluid.

    Those links will die within a few days or months - but the actual stock will be replaced with similar items almost as quickly.

    If you really want to use your 28 Volt...

    Yes you could use a dropping resistor but you would also need to have a shunt in parallel with the thermostat to provide a reliable current draw.

    Your thermostat will very likely have times where it powers down and draws almost zero current because it is done for now and waiting before it does anything else.

    At those times your thermostat will see 28VAC again because the dropping resistor will not be dropping anything.

    You need a Shunt resistor that draws at least 50% of the 250 mA that the thermostat draws at max. 100% more than the 250 mA is better for purposes of regulation.

    Say 24VAC wanting 375mA to 500mA, the shunt needs to be maybe 47 Ohms or 56 Ohms. That needs to to be a 15 Watt Power resistor!

    For the 56 Ohm shunt you would want a 10 Ohm 5 Watt dropping resistor.

    That dropping resistor will drop about 4 Volts when the thermostat is powered down, and over 6 volts when the Thermostat is most demanding.

    If you went for the minimal shunt that I would recommend, a 150Ohm 4 Watt shunt, with a 27 Ohm 5 Watt dropping resistor, your thermostat at full draw will pull the voltage across it down to 13VAC.

    The waste of power is pretty horrible.
    Big Al MA likes this.
  5. #12


    Nov 30, 2010
    It is normal for the air conditioner transformers to run high on voltage. 28 out of a 24 volt transformer is something I see every week. One way to do this is to get a standard air conditioning transformer and connect 120 VAC to the 208V primary tap. That will get you about 14 to 16 volts AC, but...everything else in the air conditioner needs 24 volts to run and those parts will not be happy.

    or, you could get a transformer rated at 5 volts and wire the secondary to buck the 24 volt transformer voltage down from 28 to 23 volts.

    I suspect that something else is wrong because no design engineer would make a thermostat that asks for 24 volts and refuses to work with 28 volts, all the while connected to an air conditioner that needs 24 volts...or would he?

    The longer I think about this, the more I think your wireless transmitter is either broken, or a poor design, or miswired in some way.
  6. crutschow


    Mar 14, 2008
    No commercial meter I know of measures peak voltage. Most commonly they measure the average value and mathematically convert that to the RMS of a sinewave. True RMS meters measure and display the true RMS value of any waveform (as long as it falls within the frequency and crest factor limits of the meter).
  7. Big Al MA

    Thread Starter New Member

    Jan 31, 2013
    Potato Pudding, I think I understand the concept of a shunt, but I'm not sure. Two resistors in parallel, right?

    I'm hearing your conclusion. The waste of power is pretty horrible. Is there a better way to do what I want to do (more efficient?). Transformer?
  8. MrChips


    Oct 2, 2009
    You only need to drop about 5V. Hence a 22Ω 2W resistor should work.
  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    If your thermostat always drew 250mA then the 32 Ohm resistor would be fine.

    You have to assume it will draw much less as in nearly 0mA quite often and when it does your 32Ohm resistor would drop 0V and your thermostat will complain that it is getting 28Volts across it.

    You solve that by putting a shunt in parallel with the thermostat and the shunt will always draw some amount of current, and now your dropping resistor will always drop at least 4Volts.

    If you want to use the minimum 32 Ohm resistor you could also replace the shunt with 24Volts of zeners which would give you a 24 Volt peak voltage trap wave.
    You would very likely have to make your Zener total from a whole lot of lower value zeners in series to make an equivalent wattage unless you can find some metal cased stud mount diodes.

    The better way to do this whole power supply is to get a correctly rated wall wart transformer. As I suggested earlier, check Jameco and you can find several for under $10.
    Last edited: Feb 1, 2013
    Big Al MA likes this.
  10. Big Al MA

    Thread Starter New Member

    Jan 31, 2013
    Thanks Potato Pudding. I understand the problem of energy loss, and I agree with your conclusion. I will get a wall transformer. How long can I run the output wires from the wall transformer? It typically comes with 6' wires, but I'm thinking I'd install the transformer in the basement, and run the output wires longer, maybe even via the existing thermostat wire....
  11. t06afre

    AAC Fanatic!

    May 11, 2009
    Have you asked what the firm behind the Taco SR504 switching relay say about this. Maybe you can get another module if it is new
  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    Really, that made me think and look at it again.

    I think the problem might be that your thermostats are just powered down, and drawing no current because they are seeing too high a voltage. They are seeing too high a voltage because they are drawing no current and the Taco SR504 switching relay is unloaded. Transformers are often designed to work as a dropping resistor. (The core saturates and limits output.)

    When transformers are unloaded they read high.

    You might only need to shunt about 20mA to 100mA.

    Put a 1k 0.5Watt resistor in parallel with the thermostat.
    See how that changes the voltage reading.