Relays are typically composed of two main components: the coil and the contact set. In circuit diagrams, the graphical representation of a relay also consists of these two parts. A long rectangle symbolizes the coil, while a set of contact symbols represents the contact group. When the circuit is relatively simple, the contacts are often drawn directly next to the coil symbol. This method of representation is known as a centralized layout.
Electrical symbols and contact types:
The relay coil is represented by a long rectangular symbol in the diagram. If a relay has two coils, two separate rectangles should be drawn. Alongside or near each rectangle, the text symbol "J" is usually marked to identify the relay. There are two common ways to represent the contacts of a relay. One approach is to draw the contacts directly adjacent to the coil symbol, making it more intuitive. The second method involves placing the contacts in their respective control circuits based on the needs of the overall system. Typically, the same text symbol is used for both the coil and its corresponding contacts, and each contact group is labeled with a number to distinguish them clearly.
There are three basic types of relay contacts:
1. Normally open (H type): When the coil is not energized, the contacts remain open. Once the coil is activated, the contacts close. This type is denoted by the first letter "H" of the Chinese pinyin for "open."
2. Normally closed (D type): When the coil is de-energized, the contacts are closed. Upon energization, the contacts open. This is represented by the first letter "D" from the Chinese pinyin for "closed."
3. Changeover (Z type): This contact configuration includes three terminals—two fixed contacts and one movable contact. When the coil is not powered, the movable contact is connected to one of the fixed contacts while the other is open. When the coil is energized, the movable contact switches, closing the previously open contact and opening the one that was closed. This allows for switching between two different circuits, hence the name "changeover contact." It is represented by the first letter "Z" of the Chinese pinyin for "turn."
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