The Solid State Relays (SSR) are electronic devices used for switching resistive or inductive AC loads with many advantages over the conventional relays. A control voltage applied to the device INPUT turns the SSR ON, energizing the load. The conduction effectively occurs at the next zero crossing of the mains voltage. When the INPUT signal is removed, the SSR turns OFF when the load reaches a current equal to zero. This means that the load switching may be delayed by ½ of mains period (or 8.3 ms for
the 60 Hz mains). Turning the output ON and OFF only on the mains voltage zero crossing brings important performance advantages to the system: practically no EMI is generated during the load switching and the SSR is submitted to less severe switching conditions. On the other hand, the SSR is suitable to AC loads only (it cannot be used to switch DC loads).