1. The working principle and characteristics of electromagnetic relays
Electromagnetic relays are generally composed of an iron core, coil, armature, contact spring, etc. As long as voltage is applied to both ends of the coil, a current will flow through the coil, generating electromagnetic effects. The armature will overcome the pulling force of the return spring and attract the iron core under the attraction of electromagnetic force, thereby driving the moving contact of the armature to engage with the static contact (normally open contact).

1. Rated working voltage
It refers to the voltage required by the coil of a relay during normal operation, which is the control voltage of the control circuit. Depending on the model of the relay, it can be AC voltage or DC voltage.

The relay coil is represented by a rectangular symbol in a circuit. If the relay has two coils, draw two parallel rectangular boxes. At the same time, mark the text symbol "J" for the relay inside or next to the rectangular box. There are two ways to represent the contacts of a relay: one is to draw them directly on one side of a rectangular box, which is a more intuitive representation. Another method is to draw each contact point into their respective control circuit according to the needs of circuit connection. Usually, the same text symbol is marked next to the contact point and coil of the same relay, and the contact group is numbered to indicate differentiation.