I have recently installed a whole home humidifier that runs on 24 VAC. I currently am running it off a power brick plugged into a standard 120VAC outlet. I would like to set up the humidifier such that it only runs when the blower is running. I have put a meter across the C and G terminals where the thermostat is wired in and noticed that when the blower is on there is 24VAC across the terminals. When the blower is not running there is 0 volts across the terminals. Can I wire my humidifier directly across these terminals? I'm just curious if those terminals are meant to drive anything. It appears that the humidifer draws about 200 milliamps.

What is a power brick? Does your furnace have a PC board or fan control board?
Just hook up the primary of a 24V transformer to your heat speed terminal and nuetral. When ever the heat calls for heat/blower it sends 120v to the blower and powers the transformer, pretty simple huh?