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

#### Quagmire

May 15, 2021
9
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?

#### Kiwi

Jan 28, 2013
469
Please draw a diagram for us.
Cheers.

#### crutschow

May 7, 2021
834
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).

May 15, 2021
9

#### Quagmire

May 15, 2021
9
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.

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#### Quagmire

May 15, 2021
9
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.

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#### Quagmire

May 15, 2021
9
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

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#### Bluejets

Oct 5, 2014
6,862
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.

#### Martaine2005

May 12, 2015
4,903
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

#### Quagmire

May 15, 2021
9
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???

#### crutschow

May 7, 2021
834
Can you give us information on exactly what converters you are using?

#### Bluejets

Oct 5, 2014
6,862
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...??

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#### Kc7svi

Oct 3, 2022
1
Isolation would be best accomplished with mechanical relays. 24vac coils but contacts carry tiny amounts of current. I'm designing a solution for my RV. The recently upgraded coleman heatpump/t-stat is nice but no auto change-over or programmability. Plus the propane furnace is on a separate 12v fuse, I'd like to restore power fusing to be in line with the original dometic.

I'll pass along a diagram when I draw it up. I can here looking to see if the puzzle had already been solved.

#### ezevalentin1994

Jul 6, 2023
1
Have you got an update on this? I am trying to do the same thing and your diagram looks correct. I am picking up my thermostat tomorrow. I don't want that new one made for RV’s it's too expensive.

#### AnalogKid

Jun 10, 2015
2,862
The TS was last here over two years ago. Better to start your own thread, and reference this one with a link.

ak

#### Quagmire

May 15, 2021
9
Ezevaletine, I wasn't able to isolate the AC from somehow getting to all inputs, abandoned for now and maybe another day.

If u find some good solution, probably better made converters, I tried 2 Chinese brands, pls let us know in this or your thread, thank you.

#### Delta Prime

Jul 29, 2020
1,807
Don't be a stranger Quagmire we miss you.

#### Quagmire

May 15, 2021
9
Was a matter of time. Been hearing things for a bit, but someone has finally put out a 12v RV programmable thermostat with wifi/bluetooth.

It checks all the functionality boxes very well, but 3rd party app is what has me hesitating.

This is the model that would replace my RVComfort digital/non programmable model. Fully programmable with up to 4 different temps per day. Smart phone app works locally over bluetooth or distance of wifi and in lies my only hesitation.

I'd prefer to buy one Alexa enabled for security so I don't have every tom, dick and harry's app on my phone for every device. This is a poor consumer approach for ingenuity as Micro-Air deserves some cred for brining to market, but hey, already invested in Alexa, might as we'll keep its security.

Winter is coming, I may change my mind for the excellent functionality, to be fair, I don't see anything that causes me worry over the company app, just a preference of my smart phone use.

As a recap, my circuit design was to change the power supply for an Alexa enabled home thermostat with a C-wire to 12v. Micro-Air did all the work, but left out the Alexa for their app.

#### kellys_eye

Jun 25, 2010
6,497
I'd prefer to buy one Alexa enabled for security
Bwahahahah!

The only time you have real security is when you aren't wired into the net.

Not that I'm mocking your application - some good work, kudos. But please don't use the word 'Alexa' in the same sentence as 'security'.

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