Learn the Difference of NO and NC Relay Switch

NO NC Relay Switch – Relay is a switch operated by electrical signals.  A relay consists of a set of input terminals to control a set of operating contact switch terminals. The switch may have any number of applications such as contact, break contact, and combination of those two. Relay is mainly used to control high power circuits using separated low power signals.

The design of a relay is quite simple. It consists of:

• Yoke
• Coil 
• Armature
• Spring
• Contacts

Yoke is used to protect all of the relay’s components. Yoke is made from iron with low reluctance for magnetic flux.

Armature is made from iron and able to move. Armature is hinged to the yoke and connected to the moving contacts. The armature is also attached to a spring to make it move to its original position when the coil is not energized anymore and makes an air gap between the magnetic circuit.

The coil is the part we use to control the switching of the contacts. It is made from an iron core wrapped by wire conductor, like a solenoid. When the coil is energized (voltage is applied to it), together with the iron core it produces magnetic flux to move the armature.

When an electric current is applied to the coil, it energizes the coil to produce magnetic flux and moves the armature. The movement of armature results in the change of contact state from NO (Normally Open) to NC (Normally Closed) or vice versa.

What is NO Switch Contact?

Like its name implies, a NO (Normally Open) switch contact or “a contact” is a switch or relay whose initial state is in open-circuit. The switch will stay in open-circuit state or simply open state if the condition is not met. We need to press the actuator to change to switch into close state. When the switch contact is closed then the current starts flowing.

On the other hand, an electrical switch needs a trigger to change the state. Just like a relay, we need to energize the coil to make the NO contact changes to NC contact. A push button is also considered as a NO contact when not pressed and a NC contact when pressed.

What is NC Switch Contact

A NC (Normally Closed) switch contact or “b contact” is the opposite of the NO contact. A NC contact has its initial state is closed-circuit. The switch contact will stay closed until the actuator is pressed or a condition is met. At the initial state, the current is already flowing. When it is pressed, then switch contact becomes open-circuit and current stopped flowing. Just like a relay, we need to energize the coil to make the NC contact changes to NO contact.

How Relay Switch Works

Its working principle is the electromagnetic circuit to generate magnetic force to open and close the electric switch inside it. The difference between relay and contactor is the contactor has a higher maximum current rating.

The difference of NO and NC relay switch:

• NO (Normally Open) Switch = Still stays in open state unless the condition is met.
• NC (Normally Closed) Switch = Still stays in closed state unless the condition is met.

After the electric current is no longer applied to the coil, the magnetic flux disappears and the spring pulls the armature back to its original position. Hence the state of the contacts return to their states before electric current was applied.

The simple relay above works in two basic operations:

• Normally Closed (NC) where its initial state stays closed until the coil is energized.
• Normally Open (NO) where its initial state stays open until the coil is energized.

The explanation structure above can be read below:

1. An iron core is wrapped by a wire coil to produce a magnetic field to pull or release the armature.
2. The coil is where we give low power to energize the magnetic field on the iron core.
3. The armature is connected to the spring and switch contacts. It can make the relay changes from NO to NC or vice versa.
4. Switch contacts are connected to the high power circuit and its state is either NO or NC depending on how we operate.

Pole and Throw

As mentioned before, relay is basically a switch so it also has the same terminology as switches. The type of switch’s connections are NO (Normally Open) and NC (Normally Closed) switch contacts.

Before learning about pole and throw combination, we need to learn what is pole and throw first. We can assume that:

1. Pole is the moving part. The moving part means the pole is attached to the armature which is moving according to the coil state.
2. Throw is the steady part. The steady part means the throw connects to the rest of the circuit on the load side.

If the pole is connected to the throw then the switch is NC. Otherwise, it is NO.

We will meet NO and NC a lot when interacting with relay and other switches. There are several types of relay contact forms used in industries and below are the most common example we will encounter:

• SPST-NO (Single Pole Single Throw – Normally Open). This has a single contact form, only connects and disconnects the circuit. Including the two terminals for the contact, it also has two terminals for the coil. This type has four terminals in total.

• SPDT (Single Pole Double Throw). This has double contact forms, when the coil is energized, it disconnects from contact B and connects to contact A or vice versa. Including the two terminals for the coil, it also has three terminals for the contacts. This type has five terminals in total.

• DPST (Double Pole Single Throw). This has a pair of SPST switches controlled by a single coil. Along with two terminals for the coil, it also has four terminals for a pair of SPST contacts. This type has six terminals in total.

• DPDT (Double Pole Double Throw). This has a pair of SPDT switches controlled by a single coil. Along with two terminals for the coil, it also has six terminals for a pair of SPDT contacts. This type has eight terminals in total.

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Application of Relay

The application of control systems and automation circuits need the logic state and logic gates. The logic gates are represented by open and closed state of switches or relays. Later we will use the terms of NO (Normally Open) and NC (Normally closed) when talking about relays and switches when the coil is generated.

Below shows the applications of relay in electrical system:

1. Relay is used to operate Logic Gate/ Logic Operation/ Logic Function.
2. Relay is used to provide Time Delay Function.
3. Relay is used to control high power circuits using low power signals.
4. Relay is used as a protective device of motors and other applications from short circuits or very high voltage.