A solid-state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. The tradeoff comes from the fact that every transistor has a small voltage drop across it, typically 0.7 vdc. If the SSR has a load of 10 amps, there is 7 watts of heat to dissipate. A hundred amp load would require fans to dissipate 70 watts of heat.
A Silicon or Semiconductor Controlled Rectifiers (SCR) is a 4-layer solid state device that controls current flow. SCR's are made with voltage ratings of up to 2500 volts, and with current ratings up to 3000 amperes per device. SCR's are used in power switching, phase control, chopper, battery chargers, and inverter circuits. Industrially they are applied to produce variable DC voltages for motors (from a few to several thousand HP) from AC line voltage. They can also be used in some electric vehicles to modulate the working voltage in a Jacobson circuit. Another common application is phase control circuits used with inductive loads. SCR's can also be found in welding power supplies where they are used to maintain a constant output current or voltage. Large silicon-controlled rectifier assemblies with many individual devices connected in series are used in high-voltage DC converter stations.