Powering an Arduino Mini Pro from a central air 24V.

Thread Starter

pdavis68

Joined Nov 27, 2013
23
I'm thinking about building a little nest style thermostat (but a little different). I want it wi-fi enabled and I'd prefer to not have to use batteries since wifi will probably eat up some.
I know the 24V line is used to turn on the heat, cold and fan. I don't know how dirty that 24V gets, but I was wondering if it's feasible to use a small 1VA transformer (115V/36V which would give me about 24V/7.5V and then rectify that to about 6.1V.

I'm guessing there's enough juice in that 24V line to run an arduino with wifi. Will the power be too dirty? Is there not enough power in the line?
 

Yaakov

Joined Jan 27, 2019
1,612
I'm thinking about building a little nest style thermostat (but a little different). I want it wi-fi enabled and I'd prefer to not have to use batteries since wifi will probably eat up some.
I know the 24V line is used to turn on the heat, cold and fan. I don't know how dirty that 24V gets, but I was wondering if it's feasible to use a small 1VA transformer (115V/36V which would give me about 24V/7.5V and then rectify that to about 6.1V.

I'm guessing there's enough juice in that 24V line to run an arduino with wifi. Will the power be too dirty? Is there not enough power in the line?
You should use a bridge rectifier and the usual filtering, into a buck converter.
 

Raymond Genovese

Joined Mar 5, 2016
1,658
I'm guessing there's enough juice in that 24V line to run an arduino with wifi. Will the power be too dirty? Is there not enough power in the line?
I don't think you should necessarily make that assumption. See https://electronics.stackexchange.com/questions/261607/what-is-the-current-capability-of-the-common-c-wire-for-house-thermostats for example.

If you can only get 250-300 mA after the rectifier and regulation, you are going to have difficulty...an ESP 8266 will want more than that for brief amounts of time. You could investigate further and get a better idea.
 

Thread Starter

pdavis68

Joined Nov 27, 2013
23
Check the transformer in the HVAC, it normally has a rating on it, also the size of the iron core is an indication.
Max.
Thanks for the info (and sorry for the delay in response). Checking the rating on the transformer did the trick. 40VA. More than enough to power a little wifi board.

In fact, a Raspberry Pi Zero W (built in wifi) running all out uses about 230mA. Another 150mA or so for a 16x12 LCD module and I should be good to go.


Thank you so much!
 
Last edited:

dendad

Joined Feb 20, 2016
3,261
One thing to check, if you are just rectifying the 24VAC for DC into a regulator for your Pi or whatever, there is a possibility of a short if you connect the Pi's 0V to the aircon supply common, depending on how the aircon supply is wired. It may well be ok, but check it before hand.
See if you can find a circuit of the existing system.

It may be easier to run a buck converter from the existing DC supply.
 

mvas

Joined Jun 19, 2017
538
If you use a Bridge Rectifier between the HVAC 24 Volt AC Power Supply and the Pi power then you cannot ever connect HVAC Common to the Pi Common. The Black Wire is typically 24 AC Common for HVAC. All I/O interfaces between Pi and any 24 Volt AC powered relay must be electrically isolated by a set of relay contacts or an SSR or equiv.

This is the color code used by my Thermostat ...
https://i0.wp.com/highperformancehvac.com/wp-content/uploads/2006/05/typical-thermostat-wire-color-diagram.jpg
 
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