Solid State Relays are designed for controlling, silently and proportionally, a large amount of power in response to a pulsed signal (suitable for DDC, Direct Digital Control). They incorporate a dual SCR (Silicon Controlled Rectifier) to provide full-wave capability and Zero Crossing Drive Circuitry. The opto-coupler provides the interface to the logic level input signal and electrical input-to-output isolation. Note: SSRs are position sensitive (i.e. the fins of the heatsink must be in a vertical position to ensure proper cooling). It is therefore important to specify in which position the heater is to be installed.