Understanding NO and NC Contacts in Programmable Logic Controllers (PLCs)

Programmable Logic Controllers (PLCs) are the backbone of industrial control systems. They are used to control the mechanical operations of various machinery and systems. A key aspect of PLC programming involves understanding the concepts of NO (Normally Open) and NC (Normally Closed) contacts. This article will delve into these terms, their applications, and how they are used in both relay and ladder logic diagrams.

Introduction to NO and NC in PLCs

In the realm of PLCs, Normal Open (NO) and Normal Closed (NC) refer to the states of contacts used in relay or ladder logic diagrams. These contacts control the flow of electricity through a circuit and are fundamental in creating complex control logic.

NO (Normally Open) Contacts in PLCs

No contact is open when the controlling device (such as a switch or relay) is not energized. When the controlling device is energized, the NO contact closes, allowing the current to flow through the circuit.

NO contacts are typically used to create a condition that must be true for the circuit to operate. For example, in a simple house light control system, the NO contact ensures that the light will turn on only when the switch is pressed (energized).

NC (Normally Closed) Contacts in PLCs

NC contacts are closed when the controlling device is not energized, allowing current to flow through the circuit. When the controlling device is activated, the NC contact opens, interrupting the current flow.

NC contacts are often used for safety interlocks or to ensure that a circuit should operate only when a certain condition is met. An example could be a safety door in a factory. The NC contact ensures that the machinery will stop if the door is opened.

NO and NC in Relays and Ladder Logic

The terms NO and NC are not only physical contacts but are also used in PLC programming to represent the state of relays in ladder logic diagrams. For a NO contact in ladder logic:

It means that the output will be 1 (true) when the relay is activated The output will be 0 (false) if the relay is deactivated

Similarly, for an NC contact in ladder logic:

It means the output will be 0 (false) when the relay is activated The output will be 1 (true) if the relay is deactivated

Normally Open (NO) and Normally Closed (NC) Switches

Normally Open (NO) Switches have contacts that are open when the switch is off. This means that the electrical supply/connection is broken until the switch is toggled to the ON position, at which point the electrical connection is active.

Example: A light switch is a typical NO switch. The light is off when the switch is off, and it turns on when the switch is turned on.

Normally Closed (NC) Switches

Normally Closed (NC) Switches have contacts that are closed when the switch is off. This means that the electrical supply/connection is active until the switch is toggled to the OFF position, at which point the electrical connection is broken.

Example: A safety door switch is a typical NC switch. The machinery is deactivated when the door is closed, and it activates when the door is opened (thus breaking the electrical connection).

Conclusion

Understanding the behavior of NO and NC contacts is essential for effective PLC programming. These contacts can be used in a variety of scenarios, from simple light controls to complex safety interlocks. Whether you are designing a new control system or troubleshooting an existing one, a deep understanding of NO and NC contacts will significantly enhance your ability to manipulate the system's behavior through ladder logic diagrams.

By mastering NO and NC contacts, you can create more reliable and efficient control systems, ensuring that your industrial processes run smoothly and safely.