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 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.