A humidity controller is a much like a humidistat.  Only bigger and sits outside the dehumidifying/humidifying product.

A humidity controller is a device that is used to control humidifying or dehumidifying products such as dehumidifiers or humidifiers fans, etc.  It is like a power box that turns dehumidifying products ON or OFF, to maintain the desired humidity level.  In a humidity controller, you plug the power cord of your humidifying or dehumidifying products into the humidity controller, which in turn is plugged into a wall outlet.  You leave your humidifying or dehumidifying product on the ON position.  You then set a %RH on the humidity controller.  The humidity controller will then power On or OFF your humidifying or dehumidifying products to reach the desired humidity level.  Once the desired humidity level is reached (± 5%RH)  the controller will turn off power to your humidifying or dehumidifying products.  When the humidity level increase again above the desired level (usually 5%RH - 10%RH above the desired level)  the humidity controller will power back up the humidifying or dehumidifying products.

Note: If your humidifying or dehumidifying products has a built in humidistat leave the unit on the ON or MAX ON position.  That way your humidifying or dehumidifying product doesn't turn ON or OFF before the humidity controller does.

A humidity controller is a great accessory for controlling devices such as ventillation fans or where greater control is desired in humidifying or dehumidifying products.