Relays are utilized for a variety of applications. In electrical and electronic circuits, electrically actuated switches or relays are commonly used for performing switching functions, i.e. opening and closing a circuit, in response to a control signal. Mechanical relays are used to control the flow of current across wires in a circuit. Electromechanical relays are often used so that a relatively low power control signal can be used to control a higher power signal. An electromagnetic coil is energized by the control signal to cause an armature to change position. The armature may carry one or more movable contacts. A frame carries one or more corresponding fixed contacts which serve as switches to control the higher power signal based upon movement of the armature relative to the frame. Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. Motor control and protection devices for electric motors generally include electro-mechanical or solid state electronic devices for activating and deactivating the electric motor based on various operating conditions and the loading on the electric motor. In most appliances, several different types of relays (switches) are necessary to insure that the variety of switching circuitry in the appliance operates properly. There are applications in which the operation of a relay is of vital importance in performing a control function and a failure thereof may result in equipment failure of serious consequence.

Advances in semiconductor technology have allowed the substitution of electromechanical relays with solid state relays. A solid state relay provides isolation between a control circuit and a switched circuit and may replace an electromechanical device such as a reed relay. Solid state relays have all solid state components, and do not require any moving parts. A solid state relay has all its components made from solid state devices and involves no mechanical movement. They are compatible with digital circuitry and have a wide variety of uses with such circuits. Switching is carried out using a power semiconductor device capable of handling high voltages and large currents. A solid-state relay may include a driver circuit and a field-effect transistor (FET) providing the output signal and having a control terminal or gate connected to the driver circuit. The driver circuit receives the control signal and operates or drives the output FET based upon the control signal. A typical solid state relay consists of a light-emitting diode (LED) optically coupled across an electrically isolating gap to a photovoltaic array. The photodiode array is electrically connected to an output device such as a field effect transistor (FET). Light from the LED creates a voltage across the photovoltaic array and activates the output FET. FET is the preferred switching element in a solid-state relay because it presents comparatively less electric resistance when it is in a conductive state than a triac in the same state and therefore generates less heat. As a result, an FET requires smaller heat dissipating fins and can reduce the overall size of the solid-state relay.

Solid state relays have many advantages over electromechanical relays, such as increased lifetime, clean, bounceless operation, decreased electrical noise, compatibility with digital circuitry, and resistance to corrosion. Electromechanical realys, which rely on a pair of contacts, a mercury switch, or similar metallic connector, have near-zero-ohm impedance when closed. Consequently, these devices can be used to control quite high currents without difficulty. The electromechanical relay exhibits a high degree of immunity to false switching due to voltage transients and other electrical noise which is often present in an industrial environment. However, because there is a physical closure of contacts required, arcing usually occurs when the relay is actuated. This produces switching noise, at a minimum, and will also produce pitting and erosion of the contacts. Another major drawback of the electromechanical relay is an inherently short life relative to that of solid state devices. Electromechanical switching devices, such as relays, provide a very low voltage drop at the switch closure thereby afording low power dissipation. As used in A/C power control applications, mechanical contacts cannot conveniently be timed to apply power to the load as the source voltage crosses zero voltage, or to interrupt the application of power as the load current passes through zero so as to minimize transient disturbances. Mechanical wear, electrical arcing and slow response also limit the application of electromechanical devices to applications requiring low closing cycle rates.

The solid state relay is known to overcome the problem of applying a source voltage to a load as the source voltage crosses zero voltage. Solid state relays are also able to interrupt load current as the load current crosses zero. Solid-state relays, which do not have mechanical moving parts and are therefore likely to offer improved reliability, are candidates to replace mechanical relays if they can be designed to meet the performance requirements. In general, solid state relays have several advantages including an increased lifetime, particularly at a high rate of switching; an elimination of contact bounce; decreased electrical noise; compatibility with digital circuitry; the ability to be used in explosive environments since there are no contacts across which arcs can form; and low voltage turn-on that reduces both the electromagnetic interference and stress on the attached load. The lack of physical contacts and moving elements also provides increased resistance to corrosion and the elimination of mechanical noise. The reliability, speed, quietness, and efficiency of solid-state relays make them an attractive alternative to mechanical (electromechanically operated) relays. As a result, solid-state relays have significantly displaced mechanical relays in low and medium power applications. More particularly, both normally-open (form A) and normally-closed (form B) single pole relays have found wide application.

Typically, the functions desired of a solid state relay are directly analogous to the functions of electromechanical relays. The required functions include those referred to as normally open (NO), normally closed (NC), and latched. In normally open operation, the solid state relay is to be OFF or to present an open circuit between the source and the load unless and until a control signal is applied to the power switch to close it and complete the circuit between the source and the load. In the normally open mode, the power switch will remain closed only so long as such a control signal is applied to it. In normally closed operation, the converse is to occur with the switch ON and the load circuit closed except when a control signal of a certain type is present that causes and maintains interruption of the load circuit. A solid state relay is usually configured so that an input circuit and an output circuit are electrically insulated from each other by a photocoupler, and a main switching element interposed in the output circuit is operated in accordance with an electric signal applied to the input circuit, thereby closing or opening a load connected to the output circuit. Most solid-state relays are the normally-open, form A, (normally-off) type where the relay is non-conducting until actuated. This type of relay implemented either in monolithic form in hybrid form where the individual components are fixed to a non-conducting substrate, such as a ceramic or polyimid plate.

Solid-state relays have been widely utilized in place of mechanical relays in view of many advantages including miniaturized configuration, low-energy consumption, and high-speed switching performance. Solid-state relays have significantly displaced mechanical relays in low and medium power applications. Generally, solid state relay circuits have been used for switching an AC power supply applied to a load such as a motor, a signal lamp, an electromagnetic valve (solenoid valve), and the like, which requires a high frequency operation. Solid state switching devices are now available for switching both AC and DC circuits at relatively large currents. The solid-state relay can be successfully incorporated in a small device, such as an automatic test equipment for testing LSI chips. Most solid-state relays are optically isolated and contain zero crossing firing circuits which cut down on radio frequency interference (RFI). Optically-coupled solid-state relays (SSR) are applied to perform switching in telecommunication, battery powered devices, programmable controllers, electronic instruments, and industrial controls, such as microprocessor control of solenoids, lights, motors, etc. In general, In a relay-controlled circuit which is used for signaling or control functions, relatively small currents are switched. In many such applications the relay takes the form of a semiconductor or solid state switch. In other applications where relatively large currents are switched or because of the environment, electromechanical relays are commonly used.

However, there are disadvantages. Solid state relays introduce a substantial voltage drop at the point of circuit closure, thereby providing relatively high power dissipation in comparison to electromechanical switching devices. For this reason solid state relays find their greatest application in controlling electrical service to loads requiring relatively low load currents. The solid state relay is inherently susceptible to electrical noise such that voltage transients with relatively low level rates of voltage change may produce unwanted switching of the relay. It is not suitable, without special associated circuitry, for use in industrial control circuits where false switching cannot be tolerated. One of the critical limitations of solid state relays is the speed of the switching action. This is the result of a capacitance inherent in the output FET. This problem is magnified when high-power circuits must be switched since larger FETs must be used. Accordingly, a larger capacitance must be charged and discharged.