electrical system Electricity is the set of physics, physical Phenomenon, phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagne ...

s, a static relay is a type of

relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...

, an electrically operated switch, that has no moving parts. Static relays are contrasted with

electromechanical relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated swit ...

s, which use moving parts to create a switching action. Both types of relay control electrical circuits through a switch that is open or closed depending upon an electrical input. Static relays have been designed to perform similar functions with the use of electronic circuit control as an electromechanical relay performs with the use of moving parts or elements. For example, in an induction type electromechanical relay, the time delay for the switching action can be adjusted by adjusting the distance traveled by the disc, whereas in a static relay the delay can be set by adjusting the value of the resistance in an R-C time delay circuit. Static relays may be based on analog solid state circuits, digital logic circuits, or microprocessor-based designs. Some authors use the term "static relay" to refer only to

solid state relay Solid state contactor PCB mount solid-state DIL relay A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. They serve the same function ...

Structure

A static relay consists of: * An input circuit that measures the value of desired property * A

comparator In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals V_+ and V_- and one binary digital output V_\text. The output is ideally : ...

circuit that compares the measured value to a preset threshold * An optional time delay circuit that controls the timing of the switch action after the input has reached the threshold * A power supply for the static relay circuits For example, an overcurrent protective relay may have an

AC to DC power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a r ...

for the input circuit, a level detector circuit and an RC time delay circuit. While early comparators used discrete

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...

circuits, modern designs use

operational amplifier An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...

References

Solid state switches {{electric-stub