How to convert 24v AC to power RPi?

Question

Hi all,

This is also related to my sprinkler project...

My current sprinkler controller is old and parts are not available, and I'd like to replace it with a Raspberry Pi.

The sprinkler valves are 24v AC (this is standard), and I can use the 24v adapter from my old controller.

The easiest solution is, of course, to use a separate plug-in to power the RPi with my usual RPi power adapter, and have the 24v just applied to the valve switching bits.

But to simplify things, and reduce number of outlets and power cords, it would be nice to power the RPi with the same power supply that's already coming in.

I would like to use standard parts if possible.

Maybe I'm just not looking in the right places for the right things, but I can't find anything that can turn the 24 volts into a low enough voltage to where I could use a regulator chip.

Has anyone run into this before?

Thanks!

-Nico

dougw · Accepted Answer

That is what I did with the Henrietta Project which runs off 24VAC from the furnace. Note the large capacitor after the rectifiers which ensures enough DC voltage is available for the DC-DC converters to work.

ntewinkel · Answer

Just FYI, I've ordered an adjustable step-down module that can take an input of up to 40v and bring it down to as low as 1.25v. It was only a few dollars and will save me figuring out the rest of the circuit. My plan is to rectify the 24v and then run it through that module, and connect a micro-usb end to power the Pi. From what I've read, the micro-usb is the only safe way to power the Pi (as opposed to direct GPIO connection). -Nico

ntewinkel · Answer

Oh hey, I think I found something!

24VAC to 5VDC Conversion « RAYSHOBBY.NET

Interesting, I didn't realize that 24v (RMS) actually hits 34v when rectified, which then requires a 70v diode... I would have royally screwed that one up if I had not read that part! I usually do try for a comfortable amount of leeway, but in this case I might have used the 50v 1N4001 I have a fair number of!

How to convert 24v AC to power RPi?

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