What is an Electromagnetic Relay?

An electromagnetic relay is an electronic control device. It has a control system (also called an input circuit) and a controlled system (also called an output circuit). It is usually used in automatic control circuits. It actually uses a smaller current and a lower current. A kind of "automatic switch" that controls the larger current and higher voltage. Therefore, it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit.

Chinese name: Electromagnetic relay
Foreign name: electromagnetic relay

Nature: Electronic control device
Make up: Control system and controlled system
Application: Automatic control circuit

Principle and characteristics of electromagnetic relay

The electromagnetic relay is a switch that uses an electromagnet to control the working circuit.

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

(2) The working circuit can be divided into two parts: a low-voltage control circuit and a high-voltage working circuit. The low-voltage control circuit includes an electromagnetic relay coil (electromagnet A), a low-voltage power source E ₁ , and a switch S. The high-voltage working circuit includes a high-voltage power source E ₂ and a motor M. , Contact D, E part of the electromagnetic relay.

(3) Working principle-close the switch S in the low-voltage control circuit, and the current generates a magnetic field through the coil of the electromagnet A, thereby generating an attractive force on the armature B, making the dynamic and static contacts D and E contact, the working circuit closed, and the motor working ; When the low-voltage switch S is turned off, the current in the coil disappears, and the armature B disconnects the dynamic and static contacts D and E under the action of the spring C, the working circuit is disconnected, and the motor stops working. ^[1]

As long as a certain voltage is applied across the coil, a certain current will flow in the coil, which will generate an electromagnetic effect. The armature will be attracted to the core by the force of the return spring against the force of the electromagnetic force, thereby driving the armature. The moving contact and the static contact (normally open contact) are attracted. When the coil is de-energized, the electromagnetic suction force also disappears, and the armature will return to the original position under the reaction force of the spring, so that the movable contact and the original static contact (normally closed contact) are released. This attracts and releases, so as to achieve the purpose of conducting and cutting in the circuit. The "normally open, normally closed" contacts of a relay can be distinguished as follows: static contacts that are in the open state when the relay coil is not energized are called "normally open contacts"; static contacts that are in the on state are called It is "normally closed contact".

Technical parameters of electromagnetic relay

Rated working voltage of electromagnetic relay

Refers to the voltage required by the coil during normal operation of the relay. Depending on the type of relay, you can

Electromagnetic relay It is AC voltage or DC voltage.

DC resistance of electromagnetic relay

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

Electromagnetic relay pull-in current

It refers to the minimum current that the relay can generate. 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 a larger current will be generated and the coil will be burned.

Electromagnetic relay release current

It refers to the maximum current that the relay will release. When the current in the relay pull-in state is reduced to a certain level, the relay will return to the release state without power. The current at this time is much smaller than the pull-in current.

Electromagnetic relay contacts switch voltage and current

Refers to the voltage and current allowed by the relay. It determines the size of the voltage and current that the relay can control. Do not exceed this value during use, otherwise the contacts of the relay will be easily damaged.

Electromagnetic relay test

Contact resistance of electromagnetic relay

Use a universal meter resistance file to measure the resistance of normally closed contacts and moving points. The resistance value should be 0;

Electromagnetic relay The resistance of points and moving points is infinite. This makes it possible to distinguish which is a normally closed contact and which is a normally open contact.

Electromagnetic relays measure coil resistance

The resistance value of the relay coil can be measured with the universal meter R × 10, so as to determine whether the coil has an open circuit phenomenon.

Electromagnetic relay measures pull-in voltage and pull-in current

Find an adjustable regulated power supply and ammeter, input a set of voltage to the relay, and connect an ammeter in the power supply circuit for monitoring. Slowly increase the power supply voltage. When you hear the relay closing sound, write down the closing voltage and closing current. In order to be accurate, you can try several times to find the average.

Electromagnetic relay measures release voltage and release current

It is also connected as above. After the relay is pulled in, the power supply voltage is gradually reduced. When you hear the release sound of the relay again, note the voltage and current at this time. You can also try a few more times to obtain an average release Voltage and release current. Generally, the release voltage of the relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it cannot be used normally, which will threaten the stability of the circuit , Work is not reliable.

Electromagnetic relay symbol contact

The relay coil is represented by a long box symbol in the circuit. If the relay has two coils, draw two long boxes side by side. At the same time, mark the relay's text symbol "J" in or near the long box. Relay contacts are represented in two ways

Electromagnetic relay Method: One is to draw them directly on the side of the long box. This representation is more intuitive. The other is to draw each contact into its own control circuit according to the needs of the circuit connection. Usually, the same text symbols are marked next to the contacts and coils of the same relay, and the contact group is numbered To show the difference. Relay contacts come in three basic forms:

1. The movable contact (H type) coil is open when the current is not applied. After the current is applied, the two contacts are closed. It is represented by the "H" pinyin prefix.

2. When the dynamic break type (D) coil is not energized, the two contacts are closed, and the two contacts are opened after energization. It is indicated by a hyphenated pinyin prefix "D".

3. Conversion type (Z type) This is a contact group type. This kind of contact group has three contacts in total, namely the moving contact in the middle, and one static contact on the top and bottom. When the coil is not energized, the moving contact and one of the static contacts are opened and the other is closed. After the coil is energized, the moving contact is moved, so that the originally opened is closed, and the originally closed is opened. purpose. Such contact groups are called switching contacts. It is indicated by the pinyin prefix "z" of "Zhu".

Selection of electromagnetic relay

The electromagnetic relay understands the necessary conditions

The power supply voltage of the control circuit, the maximum current that can be provided;

the voltage and current in the controlled circuit;

The controlled circuit needs several sets of contacts. When selecting a relay, the power supply voltage of the general control circuit can be used as a basis for selection. The control circuit should be able to provide the relay with sufficient working current, otherwise the relay pull-in is unstable.

Information on electromagnetic relays

After consulting the relevant information to determine the conditions of use, you can find the relevant information to find the model and specification number of the required relay. If you already have a relay at hand, you can check whether it is available based on the information. Finally, consider whether the size is appropriate.

Electromagnetic relay attention to the volume of the appliance

If it is used for general electrical appliances, in addition to considering the volume of the chassis, the small relay mainly considers the circuit board installation layout. For small appliances, such as toys and remote control devices, subminiature relay products should be selected.

Application of electromagnetic relay

Electromagnetic relays are important components in electric bells, telephones and automatic control circuit devices. Their essence is switches controlled by electromagnets, which play a role similar to switches in circuits: (1) control high voltage with low voltage, weak current, Strong current; (2) Long-distance operation and automatic control. ^[2]

The use of electromagnetic relays can use low-voltage and weak-current control circuits to control high-voltage and strong-current working circuits, and can achieve remote control and production automation. Electromagnetic relays are widely used in automatic control (such as refrigerators, cars, elevators, machine tools) Control circuits) and communications. ^[1]

Other types of electromagnetic relays

(Thermal reed relay) Electromagnetic relay thermal reed relay

Thermal reed relay is a new type of thermal switch that uses thermal magnetic material to detect and control temperature. It consists of a temperature-sensitive magnetic ring, a constant magnetic ring, a reed switch, 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 action. Whether the constant magnetic ring can provide magnetic force to the reed switch is determined by the temperature control characteristics of the temperature-sensitive magnetic ring.

(SSR:Solid-state relay) Electromagnetic relay solid state relay (SSR: Solid-state relay)

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

solid state relay

The solid state relay can be divided into AC type and DC type according to the load power type. 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 photoelectric isolation type being the most.

Solid state relay is a branch of relay products. It is made of 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 reliability and service life. Because the coil is no longer used, it will not encounter problems such as electromagnetic interference and contact "spark", and it has also greatly improved safety. The solid-state relay has a lower power and is more suitable for high-end products, which meets the requirements of national energy conservation and environmental protection.