On a semi-related topic..

I'm making my own internet-connected thermostat. Pretty simple stuff, except for how to power it.

Here in Canada (and the USA), thermostats get 24VAC from the heating system. So my puzzle is how best to convert that into regulated 5VDC or less, to power the micro-controller I'm using (atmega328p). No worries about lots of voltage/current for relay coils, as I'm using a solid-state relay (aka. an opto-isolated TRIAC circuit).

I googled the subject, and discoved lots of people with the exact same question, and the "best" answers are always the same: rectify the 24VAC with a diode bridge, filter it with a huge capacitor, and then feed it into a DC-DC converter circuit (a form of switching supply) to get whatever regulated DC voltage I want.

Seems like a lot of fuss though.

Then I looked at the teensy little 100-250VAC adapters that litter my work area. These all put out regulated 5VDC (+/-0.4V), in the form of a USB-A jack.

Mmm.. they're all just switching supplies internally, with some kind of voltage feedback to allow them to adapt to inputs of 100-250VAC, I wonder if they'll also adapt to lower inputs?

Tried three no-name "China chargers", and they all work just fine on the 32VAC (output) transformer at my work bench. Not quite as low as 24VAC, but that's all I have handy. The 9VDC switching adapter I have here also puts out 9VDC just fine when powered by only 32VAC. Encouraging.

Then I tried a Blackberry charger.. it outputs only 4VDC, with fluctuations. No good.

Tried the Samsung charger from my Galaxy Nexus kit: 0VDC output. Same thing with the ASUS charger for the Nexus 7 tablet: 0VDC.

Obviously everyone isn't using quite the same circuitry.
Any of you electronic types have any opinions or suggestions on this?

Right now, I'm planning to just go with one of the "China chargers", since they work. I'll have to retest against true 24VAC first though.. mmmm.. perhaps the doorbell transformer.. mm...