24vdc relay is an electrical control device, which is an electrical appliance that causes the controlled quantity to undergo a predetermined step change in the electrical output circuit when the change of the input quantity meets the specified requirements. The 24vdc relay has an interactive relationship between the control system (also called the input loop) and the controlled system (also called the output loop).

There are three basic line forms of 24vdc relay:
　　1. The two contacts of the moving-on (normally open) (H-type) coil are disconnected when the coil is not energized, and the two contacts are closed when the coil is energized. It is represented by the pinyin prefix "H" of the ligature.

　　 2. The two contacts are closed when the dynamic-break (normally closed) (D-type) coil is not energized, and the two contacts are disconnected after energization. It is indicated by the pinyin prefix "D" of hyphenation.

　　 3. Conversion type (Z type) This is a contact group type 24vdc relay. This kind of contact group has three contacts in total, that is, the middle is a moving contact, and the upper and the lower are a static contact. When the coil is not energized, the movable contact and one of the static contacts are disconnected and the other is closed. After the coil is energized, the movable contact moves to make the original open to be closed, and the original closed to the open state to reach the converted state. Purpose. Such a contact group is called a changeover contact. It is indicated by the pinyin prefix "z" of the word "zhuan".