Quote Originally Posted by AndyUK View Post
I've never seen 24VAC used for control circuitry before, but clearly it's the default.
Andy, you're showing your age (or lack of it) 24v AC dates back to when control systems were relay based (eg the 1960's Marconi BD272 250kW radio transmitter). AC is better for 'wetting' relay contacts and obviously does away with the expense of a rectifier and smoothing.

Kit (aka Old Fart)