Powering an ESP32 through R-C terminals of electric furnace

[darmani]

Hi everyone,

This is a follow-up question/issue to my project from Dec23-Jan24 where I made my own thermostat from and ESP32-4channelRelay-DHT11. I’m not an electrician or a native english speaker so the terms I’ll use will be wrong

Powering an ESP32 through C-Wire

Hello everyone, Hating cloud stuff, I’m very happy that I started playing with HA in the last week. My first project would be to control my electric furnace with the scheduler component/card and take full advantage of dynamic pricing we have here. My issues are with powering the ESP32 through the C-Wire of the furnace using a stepdown (24V DC to 5V DC). There must be something I’m not understanding, how would I connect the negative terminal of the converter back to the furnace? [thermostat V…

I ended up using this device to power the ESP32, converting 28VAC to 5VDC, connected both input+ wires to the R and C terminals of the furnace and the output+/- to a USB cable

It works fine, for about 24-48h, then the module just stops supplying power to the ESP32. It seems the furnace supplies 27.9VAC but it jumps up to 30VAC from time to time, making this device trip?

I can’t find a comparable device with a wider or more appropriate range (20-40VAC to 5VDC lets say)

Is there a cheap solution to this? something I can put in series to limit the current input to the converter? a different power supply? My last solution will be to use a 5V wall wart and a nearby socket but I would really like to power the ESP32 through the furnace terminals

Thanks for the help!

[Zixxorb]

You may have to design your own circuit or look into a buck converter at least.

[darmani]

I would not go as far as making my own electronic design.

As far as buck converters go, the only ones I find are DC-DC, the only low current AC-DC converter I could find was the one I got

[darmani]

I have found this, i think it would be good?

5-30VAC to DC 3.3-33VDC buck converter?

[squirtbrnr]

check your furnace transformer. What is the label on the side say it is rated for? 27.9VAC on the output makes me think it’s a very cheap transformer and/or could be faulty.

[Karosm]

@darmani

Is there a cheap solution to this? something I can put in series to limit the current input to the converter?

Current limiting doesn’t help you if problem is voltage. Your furnace doesn’t have mains voltage anywhere in it’s terminals??

[darmani]

1 yellow wire goes to the 24V relay, the other one goes to the R terminal I tap into for the thermostat

Also reading on other forums and such, seems like lot of people get 28v on R-C so I dont think its faulty, but I’m no HVAC tech

[darmani]

What do you mean mains voltage? the R terminal should provide low voltage to send to the others to activate heater lvl1-2, fan etc

[Karosm]

I see 240 on your image. Take that to 240VAC to 5VDC converter. Ready made like your previous one or just connect it to wires of wall wart…

I mean 40VAC to 5VDC is very uncommon, 240VAC to 5VDC you find in dollar store…

[darmani]

True just tested a few leads and I could get 240V to a converter or a 100-240VAC to 5VDC wall wart, got plenty of those lying around. It would require fishing a new wire up to the thermostat. I’ll probably try the 30VAC buck converter I linked earlier but I’ll keep your solution in mind, thanks!