What is an Electromagnetic Relay?

Catalog

Ⅰ Development History

In the 18th century, scientists believed that electricity and magnetism were two physical phenomena that were not related to each other. After the Danish physicist Oersted discovered the magnetic effect of electric current is 1820, the British physicist Faraday discovered electromagnetic induction in 1831. These discoveries confirmed that electrical and magnetic energy can be transformed into each other, which laid the foundation for the birth of later electric motors and generators. Human beings have entered the electrical age because of these inventions. In the 1830s, American physicist Joseph Henry used electromagnetic induction to invent the relay when he was studying circuit control. The earliest relay is an electromagnetic relay, which uses the phenomenon of the generation and disappearance of the magnetic force of the electromagnet when the power is turned on and off, to control the opening and closing of another circuit with high voltage and high current. Its appearance makes the circuit remote control and protection work. 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 of electronic technology and microelectronics technology.

Ⅱ Main effect

Relay is an automatic switching element with isolation function, which is widely used in remote control, telemetry, communication, automatic control, mechatronics and power electronic equipment, and is one of the most important control elements.

electromagnetic relay

Relays generally have an induction mechanism (input part) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.). It has an actuator (output part) that can realize the "on" and "off" control of the controlled circuit. Between the input part and output part of the relay, there is also an intermediate mechanism (drive part) for coupling and isolating the input, functional processing, and driving the output part.

As a control element, in summary, the relay has the following functions:

1) Expand the control range: For example, when the control signal of a multi-contact relay reaches a certain value, you can switch, break, and connect multiple circuits at the same time according to different forms of contact groups.

2) Amplification: For example, sensitive relays, intermediate relays, etc., can control a large power circuit with a very small control quantity.

3) Integrated signal: For example, when multiple control signals are input to a multi-winding relay in a prescribed form, they will be relatively integrated to achieve the predetermined control effect.

4) Automatic, remote control, and monitoring: For example, the relay on the automatic device and other electrical appliances can form a program control circuit to realize automatic operation.

Ⅲ Principle and characteristics

An electromagnetic relay is a switch that uses an electromagnet to control the on and off of the working circuit.

electromagnetic relay structure

(1) Structure: The main components of the electromagnetic relay are electromagnet A, armature B, spring C, moving contact D, static contact E. (as shown in the figure)

(2) The working circuit can be divided into two parts: low-voltage control circuit and high-voltage working circuit. The low-voltage control circuit includes electromagnetic relay coil (electromagnet A), low-voltage power supply E1, switch S; high-voltage working circuit includes high-voltage power supply E2, motor M, electromagnetic The contact D and E parts of the relay.

(3) Working principle-close the switch S in the low-voltage control circuit, the current passes through the coil of the electromagnet A to generate a magnetic field, thereby generating a gravitational force on the armature B, making the moving and static contacts D and E contact, the working circuit is closed, and the motor works ; When the low-voltage switch S is turned off, the current in the coil disappears, the armature B under the action of the spring C disconnects the moving and static contacts D and E, the working circuit is disconnected, and the motor stops working.

As long as a certain voltage is applied to both ends of the coil, a certain current will flow in the coil, which will produce electromagnetic effects. Under the action of the electromagnetic force, the armature will overcome the pull force of the return spring and attract to the core, thereby driving the armature. Then the moving contact and the static contact (normally open contact) are pulled together. When the coil is powered off, the electromagnetic attraction will also disappear, and the armature will return to its original position under the reaction force of the spring, releasing the moving contact and the original static contact (normally closed contact) so as to achieve the purpose of conducting and cutting off in the circuit. The "normally open and normally closed" contacts of the relay can be distinguished as follows: the static contact that is in the off state when the relay coil is not energized is called "normally open contact"; the static contact that is in the on-state is called "normally closed contact".

Ⅳ Technical Parameters

Rated working voltage

It refers to the voltage required by the coil when the relay works normally. Depending on the model of the relay, it can be AC voltage or DC voltage.

DC Resistance

It refers to the DC resistance of the coil in the relay, which can be measured by a multimeter.

Pull-in current

It refers to the minimum current that the relay can produce the pull-in action. In normal use, the given current must be slightly larger than the pull-in current, so that the relay can work stably. As for the working voltage applied to the coil, generally do not exceed 1.5 times the rated working voltage, otherwise it will generate a larger current and burn the coil.

Release current

It refers to the maximum current that the relay generates to release the action. When the current in the pull-in state of the relay is reduced to a certain extent, the relay will return to the unpowered release state. The current at this time is much smaller than the pull-in current.

Contact switch voltage and current

It refers to the voltage and current allowed by the relay. It determines the magnitude of the voltage and current that the relay can control, and this value cannot be exceeded during use, otherwise, it is easy to damage the contacts of the relay.

Symbol contact

The relay coil is represented by a rectangular box symbol in the circuit. If the relay has two coils, draw two parallel rectangular boxes. At the same time, mark the text symbol "J" of the relay in or beside the long box. There are two ways to represent the contacts of the relay: one is to draw them directly on the side of the long box, which is more intuitive. The other is to draw each contact into its own control circuit according to the needs of circuit connection. Usually the same text symbol is marked next to the contact and coil of the same relay, and the contact group is numbered to show the difference. There are three basic forms of relay contacts:

1. The two contacts of the moving-on type (H-type) coil are disconnected when the coil is not powered, and the two contacts are closed after power on.

2. The two contacts of the movable-break (D-type) coil are closed when the coil is not powered, and the two contacts are disconnected after power on.

3. Conversion type (Z type), a contact group type. This kind of contact group has three contacts in total, that is, the middle is a moving contact, and the upper and lower are static contacts. When the coil is not powered, the movable contact and one of the static contacts are disconnected and the other is closed. After the coil is powered, the movable contact moves to make the original open into closed, and the original closed into the open state. Such contact groups are called changeover contacts.

Ⅴ Application 

Electromagnetic relay is an important component in electric bells, telephones, and automatic control circuit devices. Its essence is a switch controlled by an electromagnet. It plays a role similar to a switch in the circuit: (1) use low voltage and weak current to control high voltage, Strong current; (2) Realize remote control and automatic control. Electromagnetic relays are widely used in automatic control (such as refrigerators, cars, elevators, machine tools In the control circuit) and the communication field.

Other types of relays

Thermal reed relay

The thermal reed relay is a new type of thermal switch that uses thermal-magnetic materials to detect and control temperature. It is composed of a temperature-sensitive magnetic ring, a constant magnetic ring, a dry reed tube, a thermally conductive mounting sheet, a plastic substrate, and some other accessories. The thermal reed relay does not use coil excitation, but the magnetic force generated by the constant magnetic ring drives the switch. Whether the constant magnetic ring can provide magnetic force to the reed tube is determined by the temperature control characteristics of the temperature-sensitive magnetic ring.

Solid State Relay (SSR)

A solid-state relay is a four-terminal device with two terminals as input and the other two terminals as output. An isolation device is used in the middle to achieve electrical isolation between input and output.

Solid-state relay

Solid-state relays (SSR) can be divided into AC type and DC type according to the type of load power supply. According to the switch type, it can be divided into normally open type and normally closed type. According to the isolation type, it can be divided into hybrid type, transformer isolation type, and photoelectric isolation type, with the photoelectric isolation type being the most.

The solid-state relay is a branch of the relay product. It is made by using semiconductor devices instead of traditional coils. It does not need to rely on the movement of mechanical parts to control the opening and closing of the switch, which effectively improves the reliability and service life. Since the coil is no longer used, it will not encounter problems such as electromagnetic interference and contact "sparks", and the safety has also been greatly improved. Solid-state relays have lower power and are more suitable for high-end products, meeting the national energy conservation and environmental protection requirements.