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Simple, or so I thought. Creating circuit to transform 24vac control to 12vdc.

Discussion in 'Power Electronics' started by Quagmire, May 15, 2021.

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  1. Quagmire

    Quagmire

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    May 15, 2021
    Hello everyone, thank you for having this forum!

    I am attempting to develop a circuit to allow the use of a home thermostat (24vac control) in an RV (12vdc control) and I guess I could be at the problem as to why there aren't retail options to do this.

    I have a 24vac wall transformer (putting out 26) and (3) 24vac to 12vdc converters. Keeping this away from RV wiring or thermostat, I'm just checking the outputs by jumping the ac hot wire directly and using the multimeter and 12vdc leds to read.

    I have the C wire going to one end of each converters ac input, the other ac input is an open wire that I touch the hot wire to, this is the connection the thermostat will perform.

    The dc output of each converter share a common ground connection (blue wire), with positive output as an open wire for reading (yellow, green, white wire).

    I must be too new to upload a pic of wiring, sorry for the text layout.

    What is happening that I did not expect, is when I touch the hot ac wire to any of the three ac input connections, it powers all three outputs, not just the corresponding output to the input that I applied the hot wire to.

    This behavior will translate to heat, cool and fan coming on together when any of them is called for.

    I do not understand how when touching the hot to one of the converter inputs, it powers the other 2 converters as well despite the hot wire not touching their inputs (remaining an open wire on one ac input end, but connected to the C wire on the other end.

    Any thoughts and thanks in advance?
     
  2. Kiwi

    Kiwi

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    Jan 28, 2013
    Please draw a diagram for us.
    Cheers.
     
  3. crutschow

    crutschow

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    May 7, 2021
    To understand what you are doing please post a complete circuit diagram of all you parts and connections.
    If you have 24Vac, what is the purpose of the 24Vac to 12Vdc converters (and why three)?
    Don't you have 12V available?

    You might consider a battery operated home thermostat.
    I used one to control the low voltage thermocouple valve on a fireplace, which required no other power.
    It also had a cooling control (which I, of course, didn't use).
     
  4. Quagmire

    Quagmire

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    May 15, 2021
    wiringA.jpg [​IMG]
     
  5. Quagmire

    Quagmire

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    May 15, 2021
    Thanks folks, here we go. This is all that is currently connected. The space between the R wire and Y, G, & W would be the thermostat controlling AC compressor (Y), AC Fan (G) and Gas Heater (W). The orange blocks are 24vac to 12vdc converters, because an RV control system is a coach battery (DC) and home thermostats are AC. So I need to convert each thermostats 24vac signal into a 12vdc signal to turn on RV equipment.
     
    Last edited: May 15, 2021
  6. Quagmire

    Quagmire

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    May 15, 2021
    What is happening, is touching R to W will also create a DC output from Y & G converters (or any combination) without Y or G connected to R. I'm at a loss how all three converters can put out 12vdc even when only one of them is connected to the R wire.
    In example there is 26vac at the W converter inputs, but not at Y & G converters, yet they all output the 12vdc.
     
    Last edited: May 15, 2021
  7. Quagmire

    Quagmire

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    May 15, 2021
    I'm trying to go beyond the bland RV thermostat market and use a home wifi and alexa enabled programmable thermostat. There is a DC controlled RV wifi thermostat in the works I read, but is super expensive and ridiculously calls for a $5 monthly subscription to operate, good luck with that ;)
     
    Last edited: May 15, 2021
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Can you post a link to these so called "24vac to 12vdc" converters?
    Also show a photo of the thermostats you are using and if possible any manual/specs on them as well.
    Usual problem uploading photos is they are too large, resize any to around 200kb.
     
  9. Martaine2005

    Martaine2005

    3,408
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    May 12, 2015
    Your diagram in post #4 is correct and should work how you want it to.
    However, with AC input and possibly very cheap buck converters, you may get some leakage to the outputs.
    Testing with an LED may not be sufficient. You might need a minimum load.
    Try connecting a vehicle 12V bulb to each output and see if you still get the same results.

    Martin
     
  10. Quagmire

    Quagmire

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    May 15, 2021
    Thanks Martin, I have put a load on each. I've since put the circuit to the system and it behaves as described running all 3 units at same time, not as expected by circuit design.
    It's not leakage voltage unfortunately, it's the full 12v even when one of its ac inputs is not connected. Like the worse possible magic trick, yet you have to wonder how the magician did it.
    Somehow it does need to be the converters, but how???
     
  11. crutschow

    crutschow

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    May 7, 2021
    Can you give us information on exactly what converters you are using?
     
  12. Quagmire

    Quagmire

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    May 15, 2021
  13. Bluejets

    Bluejets

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    Oct 5, 2014
    Cause possibly somewhere in the complete circuit diagram which you do not show.

    There have already been a couple of requests.

    It has to be feedback somewhere , perhaps in the way you have the outputs connected.

    Bear in mind the units were made for a one of isolated load.

    A start would be to add a load to each rather than use a high impedance meter which is likely to give false readings.

    It's possible you may have to add a steering diode.

    Did you try to common the positive and switching the negative instead...??
     
    Last edited: May 16, 2021
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