Solid state relays have been around for over 30 years, they were invented by Crydom engineers in 1972. Over time there have been changes in control systems, improvements in current and voltage capability, additional options made available and so on. However, the basic element of a solid state relay is the output switch, sometimes a triac but more often back to back SCRs. This output switch is the key part of a solid state relay, it is the component which does the work. This note will describe the various methods of controlling solid state relays, the options available, some application considerations and then will describe in detail the trade offs, options, strengths and weaknesses of the output silicon devices and how they are thermally managed.