24vac to 5v for Arduino

I have a pretty basic question. I'd like advice for the most economic way to convert 24vac to power an Arduino.

My project concept: I want to run an arduino at my HVAC thermostat to monitor the "on" time. It's a very simple coding/wiring project. Ideally, I'll power the breadboard right at the thermostat (HVAC systems use 24vac) so I'd like a small, breadboard mounted transformer/regulator. The only thing I'm powering off the arduino is an 7x5 LED array. It's not a matter of understanding what I need, I'm just looking for suggestions of the most compact, practical way to do this.

BTW - using a Diecimila right now but will probably switch to a nano mounted to a 3x4" breadboard - I don't want a "finished product" but would be nice if it's a compact design.

Thanks!

“7x5 LED array” = 5 digit 7-segment display?

Bridge rectifier, capacitor and a little DCDC buck converter will do fine. Without the leds you could even go lineair.

Have you determined there is enough power available in your 24 vac source to operate another device?

Paul

24vac-to-12Vdc

SWITCHING REGULATOR available in different voltage versions

VENDOR

septillion--

5x7 red LED dot matrix. SunLED XMMR18C, specifically. Probably only using 20-50mA to power it? So just use a rectifier and a small buck converter, no transformer. Is that correct?

Paul--
It's a small transformer (stepping down from household ac) on the HVAC circuit, but I can't imagine it's not capable of what's needed to power an Arduino/Nano with basically nothing running off the board. I can double check the transformer.

5x7 red LED dot matrix. SunLED XMMR18C, specifically. Probably only using 20-50mA to power it?[/quote]For each LED. :wink:

So just use a rectifier and a small buck converter, no transformer. Is that correct?

Correct. A step-down transformer may seem logical but "good luck" finding a transformer with a 24V primary, and if you find one it will probably be expensive. And, you'd still need to rectify, filter, and regulate.

Note that when you rectify & filter 24VAC (RMS), the capacitor charges-up the AC peak (about 33V) so if you use a buck regulator or DC-DC converter, make sure it can handle that (with some safety margin).

"good luck" finding a transformer with a 24V primary

24V rectifier

With all the other constructive help, may I remind you there is no "GROUND" connection with your 24 VAC transformer.

Paul

With all the other constructive help, may I remind you there is no "GROUND" connection with your 24 VAC transformer

It doesn't matter because the 24Vac is tapped off the HVAC power and the rectifier has a ground.

I want to run an arduino at my HVAC thermostat to monitor the "on" time. It's a very simple coding/wiring project. Ideally, I'll power the breadboard right at the thermostat (HVAC systems use 24vac) so I'd like a small, breadboard mounted transformer/regulator.

raschemmel:
It doesn't matter because the 24Vac is tapped off the HVAC power and the rectifier has a ground.

True, if only a local ground for his project works.

Paul

gtsdrs:
5x7 red LED dot matrix. SunLED XMMR18C, specifically. Probably only using 20-50mA to power it?

Depends on how you drive it. Quick look at datasheet says 30mA (per led) max. So if you drive it super bright, whole column on, that would mean 7 x 30mA = 210mA. But I think <5mA per led is already fine :slight_smile: