Can you use a full bridge rectifier off of 1 AC line?

I am making an arduino powered HVAC controller. In most HVAC controllers, there is a red, black, and control wires for the system. it seems like my system was installed without the common wire.

it seems like for nest, whenever the system is running, it gets some power and feeds it to its batteries. I was thinking I could do something like this. Regardless, can I power a full bridge rectifier with a single AC line?

You will need to provide a schematic. HVAC controllers are almost always 24VAC and a neutral is not required.

A full-wave bridge rectifier is intended to be connected to an AC source.

I don't have a schematic. I know that HVAC controllers are 24VAC. what I am asking is wether or not I can power a full bridge rectifier with 1 AC line. I will be using an Arduino to control the HVAC system. Also, the other components and in this case, battery, will need the dc voltage from the rectifier

Can you use a full bridge rectifier off of 1 AC line


Reason one = you need to reduce the line voltage else you will end up with 170 DC :boom: :zap:

Reason two = you need to isolate your Arduino from the mains line. Isolation in this case is provided by a transformer. A transformer converse AC current into magnetic energy then converts the magnetic energy to a lower voltage (practically no loss.
What is gained is there is not electrical connection between the lower voltage and the mains. This makes it safe for you and I to work with in our Arduino Projects.

Reason three = Such a system will likely damage your HVAC controller.

I suggest you use a "wall wart" aka a plug in power supply like the one that charges your phone.

I don't follow the OP with "one AC line". Draw a picture, Kizsh.
"Line" doesn't mean HV, though it's customary that "line" refers to line voltage, then yes, the line needs to be isolated through a transformer. But, I've never seen an HVAC controller that is run from line voltage.

There are two possible solutions, pull out the existing wire and pull in a new wire with the correct number of wires, a few spares will not hurt. The second option is to pull in the common wire. The common will be available at the controlled unit.

Guess I forgot to clarify. My HVAC controller runs off of 24VAC. I will be using a transformer/voltage regulator at the output of the rectifier to step the voltage down.

As you can see in the image, I have the control wires, (white, yellow, green, and red). the C wire is missing. What I will be doing is I will be putting the red line across the rectifier. then, I will use a voltage regulator/transformer to step down the voltage to an acceptable level.

A 2-wire thermostat turns on the furnace by shorting the two wires together. While the furnace is running you probably can't steal any power from the wire.

When the furnace is off, you MAY be able to steal some power from the wire without making the furnace think it is time to turn on. Experiment! Put an AC Voltmeter across the wire to see if you get 24VAC. Then put a potentiometer across the wire in series with and an AC Ammeter. Turn the resistance down slowly until the furnace turns on. The current reading at that point is more than you can safely draw. That will give you an idea of how much power you can steal.

I was looking at the actual furnace, not the thermostat. there seems to be a Common wire routed to the thermostat. From other documentation, I have seen that the C wire is used to provide continuous voltage. The red wire has current when the system is "on" That wire is not present in the thermostat terminals. it is probably inside my wall. but if it isn't there, I want to be able to power my controller off of just the red wire

it doesn't seem like my circuit has a ground, though. I tested a few of the wires, and they all read at 2-3 volts, no load, just reading the voltage

So you're NOT talking about a 2-wire thermostat.

How many wires are there in the cable from the thermostat to the furnace?

What are the colors of the wires?

If you use a bridge rectifier to power your Arduino the voltage will be between 28 and 33 Vdc. But you loose your reference to the 24Vac. Poor explanation but I don't know what you have in mind and how you plan to connect it.

From your wiring I'm guessing you have a A/C + furnace or a Heatpump etc.

What is your goal?

What is your goal?

My goal with this is to make an arduino switch on and off my HVAC system. I will be using a relay for this, so all I need to do is put the wires in the correct terminals.
It will be connected to a "base station" so that I can get more accurate temperature controls, and some extra functionality.

all I need to know is if I can power a full bridge rectifier from a single 24VAC wire

Wire Name What it does
green fan
white heat
yellow compressor
red 24VAC

This is the connection diagram. There might be a "common" wire somewhere inside the wall.

Yes, as long as you don't directly connect the Arduino to the HVAC. You can use relays (which are electrically isolated contacts).

You will need some capacitors on the output and likely a buck converter (28VDC to 5VDC).

You can't power anything from just one wire.
Here is what I think you have (not my drawing).

The "C" wire is the transformer secondary common, what you are calling neutral- which it is not. Without the common wire, the 24VAC on the red wire is useless to you.

If you have a common wire, then the red-common will provide 24VAC which you can use to power the Arduino. There's plenty of 24VAC to 5VDC modules online:

I am not sure what circuit I have, but the diagram seems to line up.

Without the common wire, the 24VAC on the red wire is useless to you.

Why so?

My understanding is that the rectifier only allows current on a single phase. a full bridge rectifier allows it on both phases, not at the same time. I don't really see a problem with using 1 wire because it will satisfy the 2 phases for the rectifier. I would split the wire into 2 traces and connect that to the ends of the rectifier

Yeah, that is what I am going to do.

This is the circuit that I will be using for the Arduino, with some extra things. I just need to figure out how to power it, and that is why I am asking about the rectifier

You are confusing "phase" with the + and - halves of a sine wave which is what your AC power is. A full wave rectifier is actually a steering mechanism that inverts the - part of the sine wave and adds it to the + side. You can also easily make a DC supply with just a single diode, but need a larger filter capacitor because of the missing 1/2 of the sine wave.
In ALL cases you need to form a COMPLETE circuit for any current to flow and your initial explanation does not seem to do that.

ok, but it seems that in that case, both the black 'common" and red "24VAC" lines will be supplying 24 volts AC (I think). is AC like DC in where you have to have a "ground/neutral" wire for the circuit to work?