How should I connect my humidifier to HUM terminals?
The HUM terminal on most furnaces is a 120VAC terminal. If you want to use the HUM terminal to control the humidifier, you're going to have to use a step down transformer, to drop that down to 24VAC. 

To measure the voltage on the HUM terminal:


Set your volt meter to measure AC volts.
Place one probe on one HUM terminal.
Place the other probe on the other HUM terminal.


When the furnace is running in heat mode, you should measure ~120 volts about 1 second after the blower motor starts.


  "HUM relay closes on any heating call (HP/Gas)
  approximately 1 second after the blower motor
  starts"  


- From manufacturer's literature

According to the manufacturer, the HUM terminal is rated for 1 ampere at 120 volts.



You may be able to simply connect the wire from the humidifier to the W terminal in the furnace, instead of directly to the transformer.  That way, the humidifier will only get power when the thermostat is calling for heat. However, without knowing more details about your system, it's impossible to say for sure if that will work.  Please provide the make and model of the humidifier, and a wiring diagram/photos, if you want a more accurate answer.