A relay is an electrical control device used to convert a certain amount of electricity, such as voltage, current, or non-electricity, such as temperature, pressure, time, speed, etc., into a switch value to realize the automatic control function of the circuit. It is an automatic switching element with isolation function, which is widely used in automatic control, mechatronics and power electronic equipment, and is one of the most important control elements.

Relays are classified according to the input quantity, which can be divided into voltage relays, current relays, time relays, speed relays, pressure relays, etc.; according to the working principle, they can be divided into electromagnetic, induction, electric, electronic, etc.; according to their purpose, they can be divided into control Relays, protection relays, etc.

Generally speaking, our commonly used relays mainly include intermediate relays, voltage relays, current relays, time relays and thermal relays; most of the relays used in our control circuits are electromagnetic relays, so electromagnetic relays are widely used with low voltage In the control system, let's introduce several commonly used relays respectively.

Intermediate relay is one of the most commonly used relays. Its structure is basically the same as that of contactors. Intermediate relays can replace small contactors when the load capacity is relatively small. It can also be used to expand the capacity and number of contacts, and it can also be used in control. The intermediate signal is transmitted in the circuit, and the function of the intermediate relay is to control a large current with a small current, or to control a strong current with a weak current. Intermediate relays have many contacts. There are 8-pin, 11-pin, and 14-pin relays. Different numbers of pins are actually divided into two types: two open and two closed, three open and three closed, and four open and four closed types.

The time relay is a very important component in the electrical control system. In many control systems, it is necessary to use the time relay to realize the delay control and play the role of delaying closing or opening in the circuit. The time relay is an automatic control electrical appliance that uses the principle of electromagnetic or mechanical action to delay the closing or breaking of the contact. Its characteristic is that there is a delay from the time the attracting coil receives the signal to the action of the contact.

The input of the current relay is current, which is a relay that acts according to the magnitude of the input current. The coil of the current relay is serially connected to the circuit to reflect the change of the circuit current. The coil has a small number of turns, a thick wire and a small impedance.

The input of the voltage relay is the voltage of the circuit, which acts according to the input voltage. Similar to current relays, voltage relays are also divided into undervoltage relays and overvoltage relays. The voltage relay is connected in parallel in the circuit when it is working, so the coil has many turns, the wire is thin, and the impedance is large. It reflects the voltage change in the circuit and is used for the voltage protection of the circuit.
Voltage relays are commonly used in power system relay protection, but are rarely used in low-voltage control circuits.