Solid-state relays (SSRs) control load currents through solid-state switches such as triacs, SCRs, or power transistors. These elements are controlled by input signals coupled to the switching devices through isolation mechanisms such as transformers, reed relays, or optoisolators. Some solid-state relays also incorporate snubber circuits or zero-crossing detectors to reduce spikes and transients generated by load-current interruptions. Since semiconductor switches can dissipate significant amounts of power, solid-state relays must generally be heat sinked to minimize operating temperature.
Applications are where rapid on/off cycling would quickly wear out conventional electromechanical relays. General-purpose SSRs have on/off cycle lifetimes as high as 100,000 actuations. SSRs that can be actuated with conventional CMOS and TTL logic-level voltages are available.
The chief failure mechanism of an SSR is mechanical fatigue in the power semiconductor structure caused by thermal cycling. However, thermal-cycling effects can be controlled by matching the required load-cycling characteristics to the relay. Heat sinks for most conditions are available or are a part of the SSR. (More information on SSRs can be found in the Relay and Switch section.)
