Relays Definition

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Electronics Relays Definition

A relay is a type of switch that’s operated electronically to turn current on or off in an electric circuit. Relays work similarly to a regular light switch, where power is switched on and off by moving the switch lever. The main difference is that a relay is controlled by an electrical circuit instead of being pressed manually. Relays have many uses and are often used when isolation is needed or when a circuit needs to be controlled with a low-power signal.

How Do Relays Work

Whether they are electromechanical or electronic relays, or designed to switch signals or high-power loads, all relays have the same basic parts and principles. On the primary side, relays turn an electrical input into an intermediate, non-electric physical signal. This non-electric signal is then used to control a switching element (secondary side), like contacts, which switch and carry the electrical current (the output or load current). Relays use this non-electric signal to create the necessary galvanic isolation between the input and output circuits. Since high-current capacity switches are more expensive, relays help save money by allowing one output to activate several circuits and functions.

Relays can also perform logic functions, like latching an output on or off from a short input. When a switch can’t handle high current or is controlled by an electronic circuit, switch circuits can be used to control the relay. One important feature is that relays help limit inrush current, which can be very high, by addressing arcing and contact bouncing. If you’re dealing with high inrush currents, you’ll need a high inrush relay. During the design and testing phases of any application, care must be taken to ensure the peak inrush current doesn’t exceed the relay’s specs.

Types of Relays

There are also specialized relays used for very high-power applications, known as contactors.

Electromechanical Relay (EMR)

An electromechanical relay (EMR) uses a physical moving arm that’s triggered by voltage applied to the relay coil. This arm connects contacts in the output part of the relay, completing the circuit and allowing current to flow to the connected equipment. The movement of the arm often makes a clicking sound, which can be used as an audible indicator of operation.

Solid State Relay (SSR)

Solid state relays work similarly to EMRs but without moving parts, which increases their long-term reliability. SSRs use semiconductor switching elements, like thyristors, triacs, and diodes, to transmit and energize the output signal. When triggered, the input signal acts like a switch, allowing a high-voltage signal to pass through the SSR’s output components.

Signal Relays

A signal relay has a secondary molded coil that provides excellent insulation between the coil and the contact. Since signal relays are used for switching current, they often feature c-contact construction and can be used with both reflow and surface mount devices. With gold-plated contacts and bifurcated crossbar designs, signal relays are ideal for switching loads under 2A, ensuring reliable contact even with small signal loads. They’re commonly found in machine tools, molding machines, welding machines, security devices, gaming machines, and testing and measurement equipment.

Automotive Relays

Automotive relays are built to handle DC voltages in passenger comfort and entertainment systems, as well as high-power levels in extreme conditions. Nearly all of a vehicle’s harnesses and box-modules—covering the passenger and engine areas, body control, powertrain, and modules for seats, roof, doors, and fans—use automotive relays. They are essential in cars, trucks, buses, and even construction and off-road vehicles for vital switching functions. Examples include PCB relays, automotive contactors, plug-in relays, high-voltage relays, and high-current relays.

DC Power Relays

Power relays use an electromagnetic coil to open or close a circuit, allowing continuous power flow. These affordable relays typically consist of an armature, a spring, and one or more contacts. For a normally open (NO) relay, when electricity is applied, the magnet pulls the armature toward the coil until it touches a contact, closing the circuit. In a normally closed (NC) relay, the electromagnetic coil pushes the armature away from the contact, opening the circuit. Most power relays are monostable (non-latching), meaning they have one stable position and stay in a non-energized state unless power is applied.

MOS FET Relays

MOSFET relays are semiconductor relays with MOSFET outputs. These non-contact relays are used in a wide range of applications, including semiconductor testing equipment, measurement tools, and security devices, and are mainly used for switching and connecting signals.

Guide on Buying Electrical Relays

Top 10 Electric Relays Manufactures

The top 10 electronics relay manufacturers in the market include Sensata Technologies - Crydom, Omron Automation & Safety Services, Finder Relays, Inc., TE Connectivity, Panasonic, Phoenix Contact, Carlo Gavazzi, Omron Electronics Components, IXYS Integrated Circuits Division/Littelfuse, and Weidmuller.

Frequently Asked Question

What is relay used for ?

A relay is mainly used to protect electrical systems from excessive voltage or current, helping ensure that any connected equipment operates safely. Relays are found in all sorts of places, from commercial and industrial settings to home and consumer electronics.

Why use a relay instead of a switch?

Whether you choose a relay or a switch depends on the application. Switches work well for everyday household devices, but relays are better suited for sensitive remote control systems. Picking the right component is key for safety and durability.

What is the main advantage of a relay?

The biggest advantage of a relay is that it can control a high-power circuit with a low-power signal. Relays can handle both AC and DC currents and are ideal for switching high voltages and large currents, often over 5 amps.