How relays work and what they do?

In many household appliances (e.g. automatic washing machines, electric ovens for heating, etc.), relays are installed on top of the appliances to prevent damage to the appliances due to excessive current. A relay is an electronic control device, in fact, a smaller current to control a larger current is "automatic switch". So how do relays work? What are their functions? Here's a look.

1. Basic concept

Relay is an electrical control device, is when the input quantity (excitation) changes to meet the specified requirements, in the electrical output circuit to make the controlled amount of a predetermined step change in an electrical appliance. It has an interactive relationship between the control system (also called the input circuit) and the controlled system (also called the output circuit). Usually used in automatic control circuits, it is actually an "automatic switch" that uses a small current to control the operation of a large current. Therefore, it plays the role of automatic regulation, safety protection, and conversion circuit in the circuit.

In the 1830s, the American physicist Joseph Henry invented the relay by using electromagnetic induction in his research on circuit control. The earliest relays were electromagnetic relays, which used the phenomenon of magnetic force generated and lost by an electromagnet under energization and de-energization to control the opening and closing of another circuit with high voltage and high current, and its emergence enabled the smooth remote control and protection of circuits, etc. Relay is a great invention in the history of human science and technology, it is not only the basis of electrical engineering, but also an important foundation for electronics and microelectronics.

Refer to this article to learn about relay symbols

2. Main functions

(1) Amplification: For example, sensitive relays, intermediate relays, etc., with a very small amount of control, can control very high-power circuits.

(2) Integrated signals: For example, when multiple control signals are fed into a multi-winding relay in a prescribed form, they are compared and integrated to achieve a predetermined control effect.

(3) Automatic, remote control, monitoring: For example, relays on automatic devices, together with other appliances, can form programmed control lines, thus enabling automated operation.

(4) Expanding the control range: For example, when a multi-contact relay control signal reaches a certain value, it can switch, open and turn on multiple circuits at the same time according to the different forms of contact groups.

3.Basic structure

The relay is composed of four parts, which are coil, magnetic circuit, reaction spring and contacts.

The purpose of the coil is to generate electromagnetic suction when energized, which drives the armature of the magnetic circuit to close, and makes the contacts produce displacement action.

The magnetic circuit consists of the core, choke and armature, and its task is to establish a magnetic circuit channel for the magnetic flux generated by the coil.

In the magnetic circuit, the most important is the magnetic circuit air gap, which is a gap between the armature and the core. When the coil is not energized air gap is the maximum value, the contact is the initial state; after the coil is energized, the air gap is zero, the contact is dislocated to the action state.

The role of the reaction spring is to provide the armature with the opposite direction of the action of the repulsive force, when the coil is powered off it can help the armature and contact reset.

The contact is used to perform external control output, which consists of normally closed contacts and normally open contacts. When the coil is energized, the normally closed contact opens and the normally open contact closes, and when the coil is de-energized and released, the normally closed contact and the normally open contact are reset to the initial state.

4.Working Principle

The changeover contact of the relay is a moving contact and two static contacts of the relay. The moving contact and static contact 1 in the closed state, known as normally closed contact, moving contact and static contact 2 in the open state, known as normally open contact.

When the coil is powered, its moving contact and static contact 1 immediately disconnected and closed with static contact 2, cut off the static contact 1 control line, contact through the static contact 2 control line.

When the coil is de-energized, the moving contact is reset, that is, the moving contact is reset with the static contact 2 and closed with the static contact 1, cutting off the control line of the static contact 2 and connecting the control line of the static contact 1.

Press the button SB, the line is connected, the relay K coil is powered, normally closed contact K-1 disconnected, cut off the power supply of light bulb EL1, light bulb EL1 off; at the same time, normally open contact K-2 closed, the power supply of light bulb EL2 on, light bulb EL2 lit.

Release the button SB, the line is broken, the relay K coil de-energized, normally closed contact K-1 reset closed, the power supply to the lamp EL1, lamp EL1 light; at the same time, normally open contact K-2 reset open, cut off the power supply to the lamp EL2, lamp EL2 off.

The above is the concept of relays, circuit symbols, the main role, the basic structure and working principle is introduced. Relay after more than 100 years of development, has formed a variety of forms, such as time relays, temperature relays, reed relays, thermal relays, differential relays, light relays, sound relays, Hall relays, etc., and now there are solid-state relays, from mechanical to electronic, various forms are available.