In the 18th century, scientists considered electricity and magnetism to be two completely unrelated physical phenomena. After Danish physicist Johann Gerhard Oersted discovered the magnetic effect of electric current in 1820, British physicist Michael Faraday discovered electromagnetic induction in 1831. These discoveries confirmed that electrical energy and magnetic energy could be interconverted, laying the foundation for the later development of electric motors and generators; humanity thus entered the electrical age thanks to these inventions. In the 1830s, American physicist Joseph Henry invented the relay using electromagnetic induction while studying circuit control. The earliest relay was the electromagnetic relay, which used the phenomenon of the generation and disappearance of magnetic force in an electromagnet when energized and de-energized to control the opening and closing of another high-voltage, high-current circuit. Its advent enabled remote control and protection of circuits. The relay is a great invention in the history of human science and technology; it is not only the foundation of electrical engineering but also an important foundation for electronic and microelectronic technologies.
Relays are automatic switching elements with isolation functions, widely used in remote control, telemetry, communication, automatic control, mechatronics and power electronic equipment, and are one of the most important control elements.
Relays generally have a sensing mechanism (input section) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); an actuator (output section) that can control the controlled circuit to "on" and "off"; and an intermediate mechanism (drive section) between the input and output sections of the relay to couple and isolate the input quantity, process the function, and drive the output section.
The commonly used relays include the following types:
(a) Intermediate Relay
An intermediate relay is an electrical component that uses a small current to control a large current. Its principle is basically the same as a contactor; both control the connection between the moving and stationary armatures after the coil is energized, thus starting and stopping electrical equipment. Its load capacity is generally no greater than 5A. The electromagnetic coil of an intermediate relay can be powered by either DC or AC. In relay protection and automatic control systems, it is used to expand the number of control contacts and increase their capacity. In control circuits, it is used to transmit signals (simultaneously transmitting signals to several control elements) and to control multiple lines simultaneously. Specifically, intermediate relays have the following applications:
1. Increase contact capacity and quantity
2. Commonly found on electronic circuit boards when used in small contactors.
3. The function of a switch
4. Low voltage control of high voltage
(ii) Time relay
Its main function is to delay power-on or power-off, commonly found in star-delta reduced voltage starting and autotransformer reduced voltage starting, as well as some forward/reverse and trolley reciprocating control circuits, etc. Although the principle is relatively simple, it is essential basic knowledge for electrical engineers.
(III) Thermal relay
Primarily used as an overload protection element for electric motors, it causes a change in the internal bimetallic strip when the current is too high, pushing a linkage to disconnect the control circuit. Its simple structure, small size, and low price have led to its widespread application in daily life.
(iv) Solid State Relay
The new type of contactless switch composed of solid-state electronic components has the purpose of connecting and disconnecting circuits without sparks. It has a wide range of applications, and has both amplification and isolation functions. It also has a longer service life, faster speed, and higher reliability.
The above four types are commonly used relays in electrical work. Their wiring methods are relatively simple and will not be elaborated further.
