There are various types of accessories connected to a greenhouse controller. This is a summary of the basic connection types for different accessories.
1. Fans, AC powered louvers, and other AC powered accessories.
Most greenhouse controllers do not switch AC power. The switching of power is handled by a separate box referred to as an “interface box” or a “contactor box”. The greenhouse controller is programmed to send a signal to the interface box when it wants a certain accessory to be activated. The contactor box responds by switching the power on to the appropriate unit, such as a fan or AC controlled louver. For a more detailed explanation see our paper entitled “Understanding Interface Boxes”, as well as in the product manual.
2. Low voltage DC accessories
Rollup curtains and roof vents are typically operated by 24 V DC motors, or “linear actuators”, which are essentially DC motors that drive a piston/shaft forward and backward to push open a vent. These 24 V accessories can be operated directly from most controllers. They require two terminal connections, since the polarity of the DC voltage must be reversed to cause the drives to operate in the forward and reverse (open and close for vents) directions. There are also interface boxes with special capability to drive DC motors for expanded capability. These are also explained in our article “Understanding interface Boxes”, as well as in the product manual.
3. Heaters
Heaters are not typically controlled by switching AC power, which is not a capability of the controller in any case. Instead, heaters typically have a thermostat line. The heater is turned on by completing a connection between the two thermostat wires. Therefore, the controller has “dry contacts” to accomplish this. A set of dry contacts is simply a pair of terminals that are connected through a relay contact. When the relay is operated, contact is made between the two terminals to which the thermostat line is connected, turning on the heater.