Normally closed (NC) and normally open (NO) contacts are terms used in the context of electrical switches and relays, but what’s the difference between a normally closed vs normally open switch? These terms describe the default state of a contact in a switch or relay when no external force or input is applied.
Electrical devices work by using electrical circuits, which include a few basic components. You need a power source, such as a battery, and you also need a device called a load that consumes the electricity, like a lightbulb. A circuit can then include a switch to connect and disconnect the source and load.
Not all switches work the same. Sometimes you need a switch to close the circuit when you actuate it, while other times you need one to open the circuit. This is where normally open and normally closed contacts come in.
Normally Closed vs Normally Open Switch Contacts
Normally Open Switch
In a circuit with normally open contacts, electricity doesn’t flow through the circuit until you actuate the switch. So the circuit is open (not complete) when the switch is in its normal, resting state. When there’s no external force or input applied, the contacts remain apart, and the current cannot flow through the circuit.
When the switch is actuated or activated, the contacts close, allowing current to flow through the circuit. An example of a device using normally open contacts is a doorbell. A doorbell doesn’t ring on its own, you have to push the button. Pushing the button closes the circuit, making the doorbell ring.
Normally Closed Switch
In a circuit with normally closed contacts, electricity flows through the circuit until you actuate the switch. The circuit is closed (complete) when the switch is in its normal, resting state. When there’s no external force or input applied, the contacts are in physical contact with each other, allowing current to flow through the circuit.
When the switch is actuated or activated, the contacts open, breaking the circuit and interrupting the flow of current. An example of a device using normally closed contacts is an emergency stop button on a machine. The machine will keep running until you press the emergency stop button, thus opening the circuit and stopping the machine.
Another example of a normally closed contact is a refrigerator light switch. When the refrigerator door is closed, the circuit is complete and the light is off. Opening the door breaks the circuit, turning on the light.
In summary, the terms “normally open” and “normally closed” refer to the default states of contacts in a switch or relay. They describe whether the contacts are open or closed when there is no external influence. These concepts are fundamental in understanding the behavior of electrical circuits and are widely used in various applications.
Normally Open vs. Normally Closed Notes
Video Explainer from E-Switch
Questions and Answers
What is the difference between normally open and normally closed switches?
A normally open (NO) switch has contacts that are open in their normal, unactuated state and close when the switch is activated. A normally closed (NC) switch has contacts that are closed in their normal state and open when the switch is activated.
How do I know if I need a normally open or normally closed switch?
Consider what you want the circuit to do when the switch is in its normal, unactuated state. A normally open switch is appropriate when the circuit should remain open until the switch is activated, while a normally closed switch is appropriate when the circuit should remain closed until the switch is activated.
What is the difference between NO and NC contacts?
NO stands for normally open, meaning the contacts are open when the switch is in its normal state. NC stands for normally closed, meaning the contacts are closed when the switch is in its normal state. When the switch is actuated, the contacts change to the opposite state.
Can a switch have both normally open and normally closed contacts?
Yes. Some switches have both normally open and normally closed contacts, allowing one switch to control different parts of a circuit or provide different circuit responses when actuated. The available contact configuration depends on the switch design.
What are normally open switches commonly used for?
Normally open switches are commonly used in applications where a circuit should remain open until a user or device activates the switch. Examples include pushbuttons, doorbells, control inputs and other circuits that need to initiate an action when the switch is activated.
What are normally closed switches commonly used for?
Normally closed switches are commonly used in applications where a circuit should remain closed during normal operation and open when the switch is activated. Examples include emergency-stop circuits, safety controls and other applications where activating the switch needs to interrupt the circuit.
