Molly Ayala
A relay is an electrically operated switch that is used in almost every electrical and I&C installation. They are used to control high-power or high-voltage circuits with a low-power circuit. Relays are either electromechanical or solid-state relays (SSR). Electromechanical relays use a magnetic force to open or close contacts, while SSRs use non-moving electronic devices. Relays are represented by symbols in electrical schematics, which consist of two parts: the coil and the contacts. The coil creates an electromagnetic field, while the contacts are the conducting part of the switch that opens or closes the circuit. There are many different types of relays, including ISO relays, vacuum relays, and latching relays, each with unique characteristics and applications.
Characteristics and Values of an FL Relay Electrical Schematic
| Characteristics | Values |
|---|---|
| Definition | An FL relay is an electrically operated switch. |
| Function | Used to control one electrical circuit by opening and closing contacts in another circuit. |
| Coil | An important part of the relay; it converts electrical current into an electromagnetic flux, which operates the relay's contacts. |
| Contacts | The conducting part of the switch that makes (closes) or breaks (opens) a circuit. |
| Types | Electromechanical, Solid-state (SSRs), Static, Timing, Vacuum, Reed, Thermal, Time Delay, Latching, Single-pole, Double-pole, ISO, ISO 280, etc. |
| Applications | Used in electrical and I&C installations, automotive industry, sewage pumping stations, telephone exchanges, telegraph lines, etc. |
| Benefits | Reduced cost and weight, efficient power transfer, control of multiple circuits, protection against electrical abnormalities, etc. |
| Diagrams | Wiring diagrams and instrumentation diagrams may differ; schematics depend on factors like the designer, environment, standards, etc. |
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Relay coil terminals
A relay is an electrically operated switch that uses an electromagnet (coil) to operate its internal mechanical switching mechanism (contacts). When a relay contact is open, it switches the power on for a circuit when the coil is activated.
The relay coil is designed to be energised with alternating current (AC) or direct current (DC), depending on the type of coil. When the coil is energised, the generated electromagnetic field pulls the nearby spring-loaded pole metal, implementing the switching of the contacts.
The number and type of contacts depends on the model of the relay. Single-pole relays have one contact field, usually switched between NC (normally closed) and NO (normally open). In a two-pole relay, applying voltage to the relay coil will trigger switching in two fields of contacts at once.
In summary, relay coil terminals are an essential part of the relay, as they are the points where voltage is applied to power the coils and ultimately control the flow of electricity in a circuit.
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Relay symbols
A relay is an electrically operated switch that uses an electromagnet (coil) to operate its internal mechanical switching mechanism (contacts). The relay symbol consists of two parts: the coil and the contacts. The coil in a relay is only one, while the contacts in a electrical relay schematic may be many. The number and type of contacts depend on the model of the relay.
In a simple electromagnetic relay, the coil is a wire wrapped around a soft iron core (a solenoid), with an iron yoke that provides a low reluctance path for magnetic flux. The relay also has a movable iron armature, hinged to the yoke and mechanically linked to one or more sets of moving contacts. The armature is held in place by a spring, creating an air gap in the magnetic circuit when the relay is de-energized.
The coil of the relay is characterised by its supply voltage, which can be either DC or AC power depending on the type of coil. The contacts are characterised by their state (open or closed) when the coil is at rest and when it is energised. In most cases, three types of contacts are used: normally open (NO), normally closed (NC), and changeover (CO) contacts. Single-pole relays have one contact field, usually switched between NC and NO. In a two-pole relay, applying voltage to the relay coil will trigger switching in two fields of contacts at once.
Relay coil terminals have no polarity unless the relay coil is protected by a diode, in which case the coil terminal connected to the diode's anode must be connected to negative. ISO relays are designed for use in the automotive industry, with the newer ISO 280 relays featuring smaller pin terminals for use in compact power distribution units and holders.
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Relay types
A relay is an electrically operated switch that uses an electromagnet (coil) to operate its internal mechanical switching mechanism (contacts). There are several types of relays, each with unique characteristics and applications. Here are some of the commonly used relay types:
Electromagnetic Relays
Electromagnetic relays are the most common type of relays. They consist of a coil of wire wrapped around a soft iron core (a solenoid), an iron yoke, a movable iron armature, and one or more sets of contacts. The armature is hinged to the yoke and mechanically linked to the contacts. When the coil is energised, it creates a magnetic field, attracting the armature and changing the state of the contacts.
Solid-State Relays (SSRs)
SSRs operate at high speeds and consume less power than electromagnetic relays. They have a longer lifespan due to the absence of physical contacts that could wear out. However, they may experience a voltage drop across the semiconductor, resulting in power loss through heat dissipation.
Machine Tool Relays
Machine tool relays are standardised for industrial control of machine tools, transfer machines, and sequential control. They are characterised by a large number of contacts, easily convertible from normally open to normally closed status, replaceable coils, and a compact form factor.
ISO Relays
ISO relays are designed for the automotive industry, adhering to a standard pattern for their electrical terminals. The newer ISO 280 relays use smaller 2.8mm pin terminals, making them suitable for compact power distribution units and holders.
Static Relays
Static relays consist of electronic circuitry that emulates the functions of moving parts in an electromagnetic relay. They can be designed with intentional delays in operating their contacts, using mechanisms like copper disks or dashpots to control the timing.
Vacuum Relays
Vacuum relays are sensitive relays with their contacts mounted in an evacuated glass housing. This design allows them to handle high radio-frequency voltages without flashover between contacts, even with minimal contact spacing.
These are just a few examples of relay types, and there are many more specialised relays designed for specific applications. Each type of relay has its own unique characteristics, advantages, and limitations, making them suitable for different electrical and electronic systems.
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Relay functions
A relay is an electrically operated switch that is a basic component in almost every electrical and I&C installation. It allows circuits to be switched by electrical equipment, controlling a high-power or high-voltage circuit with a low-power circuit. Relays commonly use an electromagnet (a coil) to operate their internal mechanical switching mechanism (contacts).
The relay symbol consists of two parts: the coil and the contacts. The coil in a relay is always single, while the contacts in a relay may vary in number and type depending on the model of the relay. The coil is characterised by its supply voltage, which can be DC or AC. The contacts are characterised by their state (open or closed) when the coil is at rest and when it is energised. In most cases, three types of contacts are used: normally open (NO), normally closed (NC), and changeover (CO).
In a single-pole relay, there is one contact field, which is usually switched between NC and NO. A two-pole relay, on the other hand, will trigger switching in two fields of contacts simultaneously when voltage is applied to the relay coil. A double throw relay has two switch electrical specifications: one for the normally open terminal and the other for the normally closed terminal.
A static relay consists of electronic circuitry that emulates the characteristics achieved by moving parts in an electromagnetic relay. A vacuum relay is a sensitive relay with its contacts mounted in an evacuated glass housing, allowing it to handle radio-frequency voltages as high as 20,000 volts. A mercury relay uses mercury as the switching element and is used where contact erosion would be a problem for conventional relay contacts.
Relays offer several benefits, especially in automotive applications. They reduce cost and weight by eliminating the need for a high-current rated switch or cable in the switching circuit. They also provide flexibility in positioning, allowing efficient power transfer to various electrical accessories in a vehicle, such as headlights, electric motors, and heaters.
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Relay applications
A relay is an electrically operated switch that uses an electromagnet (coil) to operate its internal mechanical switching mechanism (contacts). They are used in electrical and electronic control or switching circuits and are basic devices in almost every electrical and I&C installation. Relays are used to control a high-power or high-voltage circuit with a low-power circuit.
Relays are used in a variety of applications, including:
- Automotive Industry: ISO relays are designed for use in the automotive industry, controlling circuits for headlights, electric motors, heaters, etc.
- Telecommunications: Relays were historically used in Strowger and crossbar telephone exchanges, as well as in the first public relay-based telephone exchange in the UK, installed in 1922.
- Sewage Pumping Stations: Electromagnetic relays are used in sewage pumping stations to control the flow of sewage.
- Industrial Automation: Relays are used in industrial automation systems, such as PLC, HMI, and control systems.
- Vacuum Relays: Vacuum relays are used to handle radio-frequency voltages as high as 20,000 volts, allowing for galvanic isolation in high-voltage circuits.
- Timing Relays: These relays are designed for intentional delays in operating their contacts, with varying time periods achieved through different mechanisms.
- Solid-State Relays (SSRs): SSRs use semiconductor transistors, thyristors, or triacs as their switching devices, offering an alternative to electromechanical relays.
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Frequently asked questions
A relay is an electrically operated switch. It uses an electromagnet (coil) to operate its internal mechanical switching mechanism (contacts).
There are two main types of relays: electromechanical relays and solid-state relays (SSRs). Electromechanical relays are electro-magnetic devices that convert a magnetic flux into a pulling mechanical force. SSRs, on the other hand, have no physical contacts and switching is totally electronic.
Relays allow a low current circuit to control one or more higher current circuits. They provide benefits such as reduced cost and weight, efficient power transfer, and the ability to control high-power or high-voltage circuits with low power circuits.
A relay electrical schematic consists of two parts: the coil and the contacts. The coil is the energizing side, while the contacts are on the contact side. The schematic may differ depending on various factors such as the designer, environment, and standards followed. Understanding how relays work and their features is essential to interpreting the schematic.
