Relays are a common device found in a wide variety of assemblies and systems, coming in the form of electrically operated switches that utilize electromagnets or solid-state technology for their functionality. With a relay, one can control a circuit with a low-power signal or manage several circuits with a single signal. In either application, relays guarantee electrical isolation between controlling and controlled circuits. To help you better understand how relays may be implemented and used correctly, we will provide a brief overview of them below.
While there are various types of relays that one may use, most consist of two main circuits that are known as the primary and secondary side. The primary side or circuit is what provides the control signal for relay optional, and its functions are generally managed by a switch, thermostat, or sensor. Additionally, the primary circuit will have connection with a low voltage DC supply, and it will contain the electromagnetic coil that produces a magnetic field as current is passed through. Meanwhile, the secondary circuit features a circuit that handles the load that is to be controlled and switched.
As electrical current is induced to the electromagnetic coil, a magnetic field will begin to be produced with its strength being determined based on the amount of windings that are made as the wire is coiled. By governing the current that flows through the coil, the magnetic field itself may also be managed. Toward the end of this coil is an element known as the armature, that of which is an adjustable piece that pivots. As the magnetic field is created and grows stronger, the electromagnet will draw the armature toward it. Meanwhile, ceasing the flow of current will cause the magnetic field to drop as the armature returns to its original position. To assist in the overall movement of the armature, a small spring will often be used to create tension in a certain direction. The amrature itself will be attached to a contactor, and this allows for the relays to open and close the secondary side circuit as the armature adjusts.
There are two major forms of electromechanical relays that exist on the market, those of which come in the form of normally-open and normally-closed types. With a normally-open relay, no electricity will flow into the secondary circuit while the magnetic field is down, and inducing current will cause armature to adjust, close the circuit with the contactor, and establish a complete circuit. With a normally-closed type, the secondary circuit will remain completed when there is no current, meaning that the load is on. Once a load is induced in the primary circuit, the generated electromagnetic field will cause the armature to break the circuit, disconnecting the supply of electricity to the wider load.
While the information provided above describes how most relays work, things are different with solid state relays. That is because they are devoid of any moving parts, meaning they lack armatures and other adjusting elements. Instead, they rely on the electrical and optical properties of solid-state semiconductors to both conduct switching and enact input and output isolation. In the primary side of a solid state relay, an LED light takes the place of the electromagnet, and it will shine a beam of light into the secondary side when induced with current. When this occurs, a phototransistor on the secondary side will receive the light and cause a change, making or breaking the circuit. As such, solid state relays are fairly similar in their overall functionality when compared to more mechanical options, yet they can exhibit longer service lives and other benefits as a result of lacking any moving parts.
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