Help with hooking up LM2596 Step Down to 24VRms thermostat wire.

Hi.

I was hoping to get some help on hooking up a LM2596 DC-DC Buck Converter Step Down Module to a 24VRms thermostat wire. The thermostat wire has two wires. I measure 24V Rms across them. The boiler turns on when the two wires are connected.

I was hoping to half rectify the AC on the wire to power a Arduino using the LM2596.
The arduino is reponsible for switching on and off the thermostat by connecting or disconnecting the two wires.

This is the module.
https://www.amazon.com/RioRand-LM2596-Converter-1-23V-30V-1Pcs-LM2596/dp/B008BHAOQO

What i don't understand is how I connect the AC and DC grounds in a half-wave rectifier situation. I believe I simply need a diode out front of the module as the cap is already there before the LM2596 input.

Any pointers much appreciated.

I think you will find that you can't do what you want from your description - typically, the 24vac is connected in series with the thermostat and control relay for the boiler so while you see 24vac when you measure the voltage across the thermostat when it is open, as soon as you put any load (like your buck converter) across that, two things will happen - the voltage will drop way down because of the control relay impedance AND, it will probably think the thermostat has closed and it will turn on the boiler. You can pull limited power (those transformers are pretty small usually) directly from the transformer, but at the thermostat, you are just part of a series circuit.

And when the thermostat does come on you lose your power source!

I am curious as to how these work ?

Why 24 V AC ? Safety ?

To provide power to a clever controller ?

I don't know all the facts, but 24vac is much safer than either 110 or 220 as far as people etc. are concerned (pretty hard to electrocute yourself with 24vac). It is also current limited, so there is far less danger of starting a fire etc with bad connections. The codes are much more relaxed for running that sort of "low voltage wiring" around the residential environment than they are for your mains. 24v is a nice number from the standpoint of low enough voltage to not be a danger to most things but still high enough to operate things that take some current without needing heavy wires.

I have fallen of many horses.
Plumbing was one of them.
240 V thermostats used to be the norm. There was battery radio available then.

I suspect but do not know that using 24 V ac makes the wiring regs easier to comply with for retrofit.

Where I have seen the 240V thermostats was where they were directly in-line with things like baseboard heaters (and they switched the full load of the heater, not just a control relay). My electric water heater has 240vac thermostats on it against the side of the tank (upper and lower), but it is covered to prevent busy fingers from getting near it. Anything that is remotely controlled (doorbells, furnace with remote thermostats etc.) tend to be 24vac. We had 24vac on the thermostat in the house I grew up in -- that was built in 1950, so the 24vac goes back a ways. Typically the 24v ("LV") wiring is just a twisted pair and can easily be run all sorts of places as opposed to the normal "mains" wiring that has lots of requirements for how to make junctions, how to protect the wires, what size the wires have to be etc.

Why 24 V AC ? Safety ?

Yep, it's all about meeting safety standards. They use the next available winding ratio for transformers that's below the 30Vac limit (42.4 Vac peak or 60 Vdc) for any supply without a limited current circuit.