Molly Ayala
Electrical relays are electrically operated switches that open and close circuits by receiving electrical signals from outside sources. They are used to control a circuit by an independent low-power signal and to control several circuits with a single signal. Relays are made up of a coil, which receives an electric signal and converts it to a mechanical action, and contacts that open and close the electric circuit. The two main types of relays are solid-state relays (SSR) and electromechanical relays (EMR).
| Characteristics | Values |
|---|---|
| Definition | Electrically operated switch |
| Function | Open and close circuits |
| Control | Controlled by electrical signals from outside sources |
| Types | Electromechanical relays, solid-state relays, MOS FET relays, latching relays, magnetic latching relays, high-voltage relays, timer and time-delay relays |
| Uses | Control electrical circuits, protect electrical circuits, control power to devices, control complex switching systems, control multiple circuits with one signal, switch starting coils, heating elements, pilot lights, audible alarms |
| Construction | Coil, core, electromagnet, switch, mechanical parts |
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What You'll Learn
How relays work
Relays are electrically operated switches that open and close circuits by receiving electrical signals from outside sources. They were first used in long-distance telegraph circuits as signal repeaters. They are used to control a circuit by an independent low-power signal and to control several circuits with one signal.
A relay consists of a coil, which receives an electric signal and converts it to a mechanical action, and contacts that open and close the electric circuit. When the coil is energised, a current flows through it, creating a magnetic field. This magnetic field attracts a ferrous plate, which is part of the armature, and the armature opens and closes the contacts. An attached spring returns the armature to its original position.
There are two main types of relays: electromechanical relays (EMR) and solid-state relays (SSRs). EMRs use a magnetic force to open and close contacts, while SSRs switch the current using non-moving electronic devices and have no contacts. SSRs are faster, smaller, more reliable, and longer-lasting than EMRs, but they are typically more expensive and less efficient.
Relays can be used in various applications, such as in cars to control multiple circuits for headlights, electric motors, and heaters. They can also be used to protect electrical circuits from overload or faults and to prevent equipment damage by detecting electrical abnormalities.
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Types of relays
Relays are electrically operated switches that open and close circuits by receiving electrical signals from outside sources. They are used to control a circuit by an independent low-power signal and to control several circuits by using one signal. Relays can be broadly categorized into movable contacts (mechanical relays) and no movable contacts (MOS FET relay, solid-state relay).
Electromechanical Relays (EMR)
The traditional electromechanical relay uses an electromagnet to open or close the contacts. They are the most basic type of relay and function using the standard electromagnetic coil that can manipulate the moveable contact. However, the physical motion can take longer and lead to internal arcing, which can degrade the relay over time.
Solid-State Relays (SSR)
Solid-state relays use semiconductor properties for control without relying on moving parts. They switch the current using non-moving electronic devices such as silicon-controlled rectifiers. SSRs are faster, more compact, and quieter than EMRs, but they cannot be repaired if defective and must be replaced entirely.
Reed Relays
Reed relays are a type of EMR that use an iron reed that switches between two contacts in the presence of a magnetic field.
Latching Relays
Latching relays require only a single pulse of control power to operate the switch persistently. They function as storage circuits and help reduce power consumption in the application circuit, as the coil does not need to be charged continuously.
Temperature or Thermal Relays
Temperature relays are dedicated electrical relays that input excitation in the form of heat. They are suitable for direct temperature control of different units.
Protective Relays
Protective relays are a type of temperature relay that input excitation in the form of current. They are used as protection against current overload and are suitable for various electrical users. They are critical components of the electrical power grid, preventing damage to electrical equipment by detecting electrical abnormalities.
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History of relays
The history of electrical relays dates back to the 1800s, playing a pivotal role in the evolution of telegraphy and modern communications. In 1809, Samuel Thomas von Sömmerring designed an electrolytic relay as an alarm for an electrochemical telegraph. The relay was further developed in the 1830s by inventors such as Joseph Henry, who sought to improve the electrical telegraph. Henry's experiments, though not published, are credited with combining natural philosophy, mechanical aptitude, and a keen interest in telegraphy.
Edward Davy also made significant contributions to relay design in 1837, proposing a practical electromagnetic relay design, the 'make-and-break' type, which improved upon earlier mercury-based designs. However, an official patent was granted to Samuel Morse in 1840 for his telegraph, now recognised as a relay. As telegraph networks expanded in the 1840s and 1850s, relays evolved and found their way into various electrical systems.
Relays became integral in the telephone system, serving as the heart of long-distance telephony by enabling point-to-point connections and amplifying feeble signals. They were extensively used in telephone exchanges, contributing to the evolution of the switch. In the 20th century, relays continued to find applications in computing. Konrad Zuse built the first relay-based computer in 1941, utilising relays before they were replaced by faster vacuum tubes and transistors.
Today, relays are still commonly used in automation to control motors and other high-power elements, such as in elevators, water pumps, and lighting systems. Additionally, protective relays or safety relays are employed in modern electric power systems to safeguard circuits from overload or faults, ensuring reliability and safety.
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Relays vs. circuit breakers
Relays and circuit breakers are both used for circuit protection and are electrically operated switches. However, they are not interchangeable. Relays are smaller and generally used to switch smaller currents in a control circuit. They are also used to control a circuit by an independent low-power signal and to control several circuits by one signal. Relays are either electromechanical relays (EMR) or solid-state relays (SSR). EMRs use electromagnets to open or close contacts, while SSRs have no contacts and switching is totally electronic. SSRs are faster and require less voltage to operate, but EMRs provide a cleaner ON or OFF condition.
Circuit breakers, on the other hand, are typically very large switchable devices that are designed to prevent and overcome any damage from short circuits and overloads. They are automated switches that open and close contacts within the system within a short duration of time. They are typically intended for overcurrent protection, while relays can provide both overcurrent and overvoltage protection. Circuit breakers can be found in most residential settings, where they are used to sense and prevent excessive current draw by devices in the home.
In terms of appearance, relays consist of coils that act as electromagnetic fields. The solenoid, or moving part, is referred to as an armature and opens and closes fixed and moving contacts when the coils are energised. Relays generally come in two varieties: normally open (NO) and normally closed (NC). NC relays are typical and provide a closed circuit as long as there is no fault in the system.
Circuit breakers can contain relays that control when the circuit breaker opens or closes. The operation of both switches is dependent on each other. The relay recognises the fault in the circuit and transfers the information to the circuit breaker, which then decides whether to interrupt the flow of current.
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How to build a relay
A relay is an electrically operated switch that can be used to control a circuit. Relays can be used to control several circuits by using one signal. There are two main types of relays: solid-state relays (SSR) and electromechanical relays (EMR).
Solid-state relays use semiconductors and have no moving parts. They are faster, more durable, and more compact than electromechanical relays. On the other hand, electromechanical relays use mechanical components that move and are controlled by a coil.
To build a basic electromechanical relay, you will need an iron nail and some insulated wire. Here are the steps to follow:
- Wind the insulated wire into a coil around the iron nail. Ensure that you have enough windings to prevent overheating.
- Connect one end of the wire to a power source and the other end to a switch.
- Complete the circuit by connecting the other end of the switch back to the power source.
- When power is applied to the coil, it creates a magnetic field, and the nail becomes a temporary electromagnet.
- You can now attach other components to the relay to control their power.
It is important to note that building your own relay may not provide the same level of precision and safety as a manufactured relay. Additionally, relays are typically used for low-power applications, and attempting to control high-power devices may require additional circuitry.
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Frequently asked questions
An electrical relay is an electrically operated switch that opens and closes circuits by receiving electrical signals from outside sources.
An electrical relay consists of a coil, an electromagnet, and a mechanical switch. The coil is a wire wound around a metal core. The coil of wire causes an electromagnetic field.
There are two main types of electrical relays: solid-state relays (SSRs) and electromechanical relays (EMRs). SSRs switch the current using non-moving electronic devices and have no moving parts. EMRs use mechanical components that move and are controlled by a magnetic force.
Relays receive an electrical signal and send the signal to other equipment by turning the switch on and off. Relays control one electrical circuit by opening and closing contacts in another circuit.
Relays are used in many applications to control different amounts of currents and numbers of circuits. For example, relays are used to switch on and off high-power devices like lamps or garage door motors with just a small DC voltage signal. Relays are also used to switch starting coils, heating elements, pilot lights, and audible alarms.
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