Solid-state relays are much smaller than corresponding single-pole relays. They are also faster, dissipate less power, and withstand a larger number of operations. And unlike electromechanical relays, solid-state switches exhibit no bounce on closing.

Solid-state devices are generally preferred where switch life must be independent of the number of switching cycles, where switching times must be less than 2 msec, and for bounce-free or zero-current switching. They are also generally chosen for applications subject to severe shock or vibration.

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 switched 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 interrupting load current. 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.

SSR failure modes are primarily determined by the triac or SSR switching characteristics. Most failures take the form of SSR false turn on with no turn-on signal. For example, turn on may occur if operating temperatures exceed the thyristor rating. Also, transients from the switched load or from an ac line can momentarily exceed the thyristor breakover voltage, or steeply rising load voltages can couple into the thyristor input through stray capacitances in the thyristor and cause turn on. This latter effect, called dv/dt turn on, occurs in highly inductive circuits immediately after the circuit attempts to turn off. To combat dv/dt turn on, some SSRs use back-to-back SSRs with reverse bias in place of triacs.

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 qualities to relay characteristics. Proper heat sinks for most conditions are available or are an integral part of the SSR.

SSRs generate heat because of the voltage drop present in all semiconductor devices. A 40-A relay, for example, typically drops 1.2 V during conduction and, thus, dissipates 50 W of heat. However, SSR heat generation generally does not require special system design. These devices usually mount on circuit boards or control panels containing other semiconductor devices. Cooling and heat-sinking methods used for these devices are likely to be adequate for the SSR.

Some SSRs designed for controlling ac loads incorporate a zero-voltage turn-on circuit that switches the load on or off only when the power-line sine wave passes through zero. Highly capacitive loads such as lamps and heaters which produce high inrush currents at turn on generate little electromagnetic interference if actuated when line voltage is zero. However, inductive loads such as motors and transformers can saturate during the first half cycle after turn on and produce maximum interference when switched on as line voltage passes through zero. Zero-voltage switches should not be used.

Many SSRs have built-in transient suppressors connected in parallel with the semiconductor switch output. Common suppressors include RC networks (sometimes called snubbers), Zener or clipper diodes, varistors (voltage-dependent resistors), and RC/diode dump circuits.

I/O modules: Input/output (I/O) plastic encapsulated modules are designed to allow microprocessors or hard-wired logic circuits to control or sense industrial loads.

Ac output modules allow logic-level voltages to control switches, usually triacs, that turn ac loads on and off. Many incorporate the same types of transient protection and zero-voltage switching available in conventional solid-state relays.

Dc output modules allow logic-level voltages to control a solid-state switch, usually a power transistor, that turns dc loads on and off. These modules are also similar to dc solid-state relays.

Ac input load-sensing modules generate a logic-level voltage (typically TTL or CMOS) that corresponds to the presence or absence of an ac load voltage. As in output modules, load-sensing modules have transient protection and are generally optoisolated. Dc input load-sensing modules perform a similar function for dc loads.

Transducer-sensing input modules accept low-level signals from a specific transducer type, such as thermocouples or strain gages. The module conditions these signals through operations such as linearization and amplification. Module output can then be sent to an a/d converter for conversion. Some transducer-sensing modules also provide frequency or current outputs for signal conditioning.