With no moving parts, solid-state relays avoid the mechanical failure modes of traditional relays.  Shown on this slide are the common solid-state switching solutions. These solutions pass a signal current through an LED on the input side. The output side can be a Photo-Transistor to switch a DC load or a Photo-Triac to switch an AC load. A MOSFET driver uses a photo-sensitive element that converts the light into a voltage signal which is passed over a control circuit to drive a MOSFET located externally on the board. A PhotoMOS relay is essentially an MOSFET driver that includes an internal MOSFET for the output. In fact, there are actually two MOSFETs per each channel which allows Panasonic’s PhotoMOS to switch both AC and DC loads.