Relays have enabled low-voltage signals to control high-voltage systems since before the invention of the transistor. With very low on-resistance, the venerable electromechanical relay (EMR) still finds use today, but the newest solid-state relays (SSRs) offer considerable advantages with respect to lifetime reliability, switching speed, power consumption, and tolerance for shock and vibration.
SSRs also offer advantages over photorelays (or optical relays), which face achievable-power-transfer limits and are subject to internal LED degradation. SSRs find use in a variety of automotive and industrial applications, including inrush current limiting and insulation-resistance monitoring.