About Relays

What Is a Relay?

This section explains relay circuits — the fundamental basis of sequence control. Understanding relay circuits makes it significantly easier to create PLC programs (ladder diagrams). Let's first look at what a relay is.

The photo above shows a typical relay. It is attached to a terminal block so wiring can be done with screws. Normally, the transparent relay body and the black terminal block are purchased separately. What role does this relay play?

The photo above is a side view of a relay. The blue-taped part inside is the coil. When voltage is applied to this coil, it becomes an electromagnet and attracts the contacts on the right side. The attracted contacts create a switching action. Coils come in AC and DC types, so always check the voltage and type before use.

Relay Contacts: NO Contact (a-contact) and NC Contact (b-contact)

Because the photos are hard to follow, let's use illustrations. The illustration below shows a relay in its normal (de-energized) state.

In this state, contacts C and B are connected (conducting), and C and A are separated. Next, when voltage is applied to the relay coil, the coil becomes an electromagnet and attracts the contacts.

After the coil is energized, C and A become connected (conducting), and C and B separate.

• NO Contact / a-contact (C–A): Normally open — no continuity normally, turns ON when the coil is energized
• NC Contact / b-contact (C–B): Normally closed — continuity normally, turns OFF when the coil is energized
• COM (Common): The common terminal (C) shared by both the NO and NC contacts

Relay Socket

Relays are housed in a transparent cover, and those with 2 or 4 contacts are commonly used for control purposes. The relay terminals protrude downward and are typically inserted into a terminal block (socket) for use. Let's confirm the terminal arrangement when inserted into a socket.

The figure above shows a typical arrangement. Terminal numbers are marked on both the top of the relay and on the socket side. Generally, the inner terminals form the NO (a-contact). Always verify this when using a relay for the first time.

Operating a Relay

Let's now connect and operate the relay. First, define the electrical symbols as shown in the diagram below.

These are simplified symbols. Symbols may vary by manufacturer, but they are adequate for understanding circuits. Let's look at a simple circuit.

In the diagram above, pressing the push-button switch sends current to the relay coil, the contacts operate, and the lamp lights up. Releasing the switch turns the lamp off. This circuit itself is no different from directly switching a lamp with a push button, but the next circuit shows why a relay is useful.

In this circuit, the lamp has been replaced with an LED. Since an LED operates on DC power, it cannot be directly connected to an AC push-button switch. The relay contact is used to switch the DC LED circuit. By using a relay, circuits operating at different voltages or from different power sources can be safely switched.

About Timer Relays

A timer relay has the same basic structure as a regular relay, but differs in that the timing of contact operation can be set to any desired delay.

In the circuit above, if the timer is set to 1 second: pressing the push-button switch sends current to the timer coil. However, the contacts do not operate immediately — instead, the lamp lights up 1 second later. In this way, the timing of operation can be shifted using a timer. Timer relays are an extremely important component in sequence and relay control.

Now that you have a solid understanding of relay basics, the next section explains how to input signals from sensors into relays. A control circuit is meaningless if it cannot receive input signals, so let's confirm the signal input method before moving on to relay control.

Frequently Asked Questions (FAQ)