The optically isolated SSR provided the ability to switch high voltage and large currents using lower DC voltages to control them. Sometimes referred to as the ‘hockey-puck’ relay, because of its shape, it provided some of the very first PC controlled high voltage/current systems.

The SSR can be described as a switch in simplistic terms but without it, modern control systems would be hard pressed to start heavy current devices such as pumps, heaters, etc. The standard hockey-puck shaped relay soon transformed into other useful variants that enable mounting on circuit boards, relay boards and the like.