I just took a 24v AC wall wart power supply and shoved it in the same terminals as the hot and neutral control on the smart thermostat. The A in AC makes this a

I just took a 24v AC wall wart power supply and shoved it in the same terminals as the hot and neutral control on the smart thermostat. The A in AC makes this arrangement work just fine to allow the battery of the thermostat to charge without zapping anything.