A relay is an electrical control device. It has an interactive relationship between the control system and the controlled system. Usually used in automated 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 adjustment, safety protection, and conversion circuit in the circuit.

A relay is an electrical appliance that turns on or off a circuit according to changes in physical quantities such as voltage, current, temperature, speed, or time. The main function of relays is protection and control. Although there are many types of relays, they are usually divided into two categories: protection relays and control relays. Common relays include thermal relays, electromagnetic relays, time relays, temperature relays, and speed relays. This article will introduce different types of relays.

I. Classified by working principle

1. Electromagnetic relay

An electromagnetic relay is a switch that uses an electromagnet to control the on and off of the operating circuit. It uses low voltage and weak current to control high voltage and high current circuits, and can also end remote operation and production automation. It plays an increasingly important effect in modern days.

Working principle: When the electromagnet is energized, the armature is sucked down to close the operating circuit. When the electromagnet is de-energized, it loses its magnetism, and the tension spring pulls the armature up to block the operating circuit.

Function: Use low voltage to control high voltage; remote control; active control.

Symbols of electromagnetic relay: (a) General symbol of coil; (b) Current relay coil; (c) Voltage relay coil; (d) Contact

2. Thermal relay

Thermal relays are generally composed of heating elements, control contacts, and action systems, reset mechanisms, current setting devices and temperature compensation elements. It generates heat from the current flowing into the thermal element, which causes the bimetallic strips with different expansion coefficients to deform. When the deformation reaches a certain point, it pushes the connecting rod to disconnect the control circuit, so that the contactor loses power and the main circuit disconnects, in order to protect the motor from overloading.

There are some types of thermal relays:

1) Temperature relay: A relay that acts when the outside temperature reaches the specified temperature.

2) Electrothermal relay: A relay that uses the electrical energy in the control circuit to convert it into heat, and when it fulfills the specified requirements, it will act as a relay.

3. Photoelectric relay

A relay that uses the photoelectric effect to operate. It is composed of light-emitting elements (such as small incandescent lamps, gallium phosphide diodes, etc.) and photosensitive devices. When the signal added to the light-emitting element reaches a certain value, the effect of light causes the resistance of the photosensitive device to change sharply, thereby closing or breaking the circuit.

4. Polarized relay

A polarized relay refers to a relay that is actuated by the combined action of the polarized magnetic field and the magnetic field generated by the control current through the control coil. The polarized magnetic field is generally generated by magnetic steel or a polarized coil with a direct current; the suction direction of the relay armature depends on the direction of the current flowing in the control winding. It can be used as components of pulse generation, DC and AC conversion, summation, differentiation, and signal amplification in automatic devices, remote control, and telemetry devices, and communication equipment. It has the outstanding advantages of high sensitivity and fast action.

II. Classified by the nature of the input signal

1. Current Relay

The coil of the current relay is connected in series in the circuit and acts according to the magnitude of the coil current. The coil wire of this kind of relay has few thick turns and low coil impedance.

Diagram of the current relay

The function of the current relay: It works according to the current signal, and also determines the contact action according to the current of the coil. The coil needs to be connected in series with the load circuit when installing. According to the coil current, it can be divided into AC and DC. According to the action current, it can be divided into overcurrent and undercurrent.

2. Voltage relay

The voltage relay has many coils turns and thin wires. It is connected in parallel in the loop when working, and the circuit is turned on or off according to the voltage across the coil.

Diagram of the voltage relay

The function of the voltage relay: It works according to the voltage signal, and determines the action of the contact according to the magnitude of the coil voltage. The coil needs to be connected in parallel with the load during installation. The voltage relay can be divided into AC and DC according to the coil voltage and can be divided into overvoltage and undervoltage according to the operating voltage.

3. Auxiliary relay

The power supply used for the electromagnetic coil of the intermediate relay is DC and AC. Commonly used intermediate relays have two series, JZ7 and JZ8.

Diagram of the auxiliary relay

The role of the intermediate relay: play a role in converting and transmitting control signals. Its input signal is the power-on and power-off signal of the coil, while the output signal is the contact action of the intermediate relay. In essence, it is a kind of voltage relay, which has the characteristics of a large number of contacts.

4. Time relay

When the input signal is added or removed, the output part needs a delay or time limit to close or disconnect the relay of the controlled circuit until a specified time.

Symbols of electromagnetic relay: (a) The general symbol of the coil; (b) The power-on delay coil; (c) The power-off delay coil; (d) Delayed closing dynamic closing contact; (e) Delayed opening dynamic breaking contact; (f) Time-delayed opening of moving-on contacts; (g) Time-delayed closing of moving-breaking contacts; (h) Momentary moving-on contacts; (i) Momentary moving-breaking contacts

The function of the time relay: It belongs to a kind of input signal (that is, when the coil is energized or cut off), and the signal will be output after a preset time delay (contact closing or opening action) Relay.

5. Speed relay

The speed relay is a relay that acts when the speed reaches a specified value. The speed relay is usually connected with the shaft of the motor when in use. The function of the speed relay is to coordinate the signal with the contactor depending on the speed to realize the reverse braking of the motor. Therefore, the speed relay is also called the reverse brake relay.

The shaft of the speed relay is connected with the shaft of the motor. There is a cylindrical permanent magnet on the shaft. The outer ring of the permanent magnet is embedded with a squirrel-cage winding and can be rotated to a certain angle.

A speed relay

III. Classified by relay contact load

1. Micro-power relay: When the contact open circuit voltage is DC 27V, the contact rated load current (resistive) is 0.1A, 0.2A.

2. Weak power relay: When the contact open circuit voltage is DC 27V, the contact rated load current (resistive) is 0.5 ampere, 1 ampere relay

3. Medium power relay: When the contact open circuit voltage is 27 volts DC, the contact rated load current (resistive) is 2 amperes and 5 amperes.

4. High-power relay: When the contact open circuit voltage is 27 volts DC, the contact rated load current (resistive) is 10 amperes, 15 amperes, 20 amperes, 25 amperes, 40 amperes.

Note: As an example, only one type of DC resistive load value is given here, and other loads are determined by the product technical conditions according to the corresponding conversion relationship.

IV. Classified by the dimensions of the relay

1. Miniature relays: Relays with the longest side dimension not greater than 10 mm.

2. Subminiature relays: Relays whose longest side dimension is greater than 10 mm but not greater than 25 mm.

3. Small relays: Relays whose longest side dimension is greater than 25 mm but not greater than 50 mm.

Note: For sealed or enclosed relays, the external dimensions are the maximum dimensions of the relay body in three mutually perpendicular directions, excluding the dimensions of mounting parts, terminals, crimping, crimping, flagging, and sealing solder joints.

V. Classified by the protection characteristics of the relay

1. Hermetically sealed relays: Use welding or other methods to seal the contacts and coils in the 2. enclosure, isolate them from the surrounding medium, and have a low leakage rate.

2. Enclosed relay: A relay in which the contacts and coils are enclosed (unsealed) and protected in a casing

3. Open type relay: A relay that does not require a protective cover to protect the contacts and coils, etc.