Hello, I am currently working on a project to build a smart thermostat with an Arduino Uno and Bluetooth for remote access. I have the general design prototype working good. You can check it out on YouTube at Building a Smart Thermostat Part 1 - YouTube
Currently the design is powered by a wall wart plug in power supply. I would like to get rid of that and power it off of the 24 VAC signal coming from the HVAC system. Does anybody know a power supply part number out there that can run off of 24 VAC and outputs about ~9 VDC (really any DC value that is compatible with the Uno's onboard regulator). Thanks in advanced.
Finding a 120V to 5V source is much easier.
Any cell phone wall charger with a USB output can be used to power the Arduino via the USB connector.
Pick one for $5-6.
I did this exact thing. I used the 24VAC that comes from my AC unit and rectified it to give me around 37VDC which I filtered and then sent to a 5V regulator to power the thermostat. I'll never do that again, it was silly.
What I would do now is get a variable buck power supply off ebay that can take at least 40VAC input and regulate it down to what I need, then put a 100uf cap and a couple of small value caps on the output to filter the trash that these little supplies always have and use that.
I have had incredible luck with the one I built a few years ago, but it was expensive and it runs hotter than I like. I describe the project on my blog if you want to see how I did it. But, like I said, I'd do it differently if I was doing it now.
Thanks for the input and personal experience. I think in my mind I was thinking that I need a regulated DCV but the Arduino Uno has a regulator on it so I can just rectify the AC and smooth it out a bit with caps and feed a raw DCV into the regulator. I will report back on the results and what I end up using.
Keep in mind the voltage limits of the Arduino. If you rectify and filter the 24 volts from a heater or AC unit you'll get around 37 volts. If you apply that to an arduino, you'll get some black smoke and a few popping sounds as the board burns up.
Yes, noted. I am either going to add another transformer stage to the VAC to step it down more or a series resistor. If the design has a fairly constant average current I can choose a resistor value that will deliver a voltage in the range of the Arduino regulator. The resistor approach is not the most energy efficient, but may be better than a bulky transformer.