Summary： This article focuses on relays definition, working and operation, configurations, types, parts, advantages, and applications. You will understand what energized and de-energized relays are at the end.
In power electronics, power flow control is one of the critical areas if you need to have a successful system performance. The system's success is determined by the stability of the power flowing through it and how it responds when it becomes unstable. Either overvoltage Undervoltage or short circuits can cause instability. A relay plays a critical role in power supply into electronic systems. Most high-end industrial systems use relays to maximize their performance. Relays can be defined as electrical switches that can be operated mechanically or electrically. The use of an electromagnet achieves the switching mechanism in the relays. There exists another mechanism employed in attaining this switching. Relays differ based on where they find helpful. In this article, we will focus on fundamental theories of the relays. Let us walk together!
Relays are significant when low power is required to control an electric circuit. It is also relevant when a single signal contains multiple courses. The use of relays dates back to the invention of telephones. They were vital when it came to switching calls during the telephone exchange. Relays were also critical electric components for telegraphy of long distances. The purpose was to aid in switching signals originating from one source to the expected destination. With the invention of modern computers, relays have been utilized in performing Boolean operations. High-end relays called contactors control the power flow that contains the motor speeds.
A relay comprises only four main parts: electromagnet, switch point contacts, movable armature, and spring. It has an electromagnetic relay consisting of a wire coil surrounded by a well-placed iron core. A movable armature and switching point contacts are provided with a low reluctance magnetic flux. The connection of the movable armature is done to the yoke. The yoke is connected to the system switching point contacts. The parts are then held together by a spring. The purpose of the spring is to produce an air gap within the connecting circuit any time the relay is de-energized.
Let’s use the diagram below to explain how a relay works.
Figure 1: Relay Inner Section Representation Courtesy of Simon Mugo
The above is the representation of the inner section of the relay. As you can see, the iron core has a control coil wired around it. Input power is injected into the electromagnet iron core and coil through an electric switch and contacts to the connected load. When electric current starts to flow through the relay control coil, the electromagnetic wave is energized, intensifying the relay magnetic field. This makes the upper contact pad start being attracted to the fixed lower pad, thus closing the contact and leading to a short circuit of the power flowing toward the load. The opposite would happen if the contacts were closed when the system was de-energized. The contacts will move in opposite to open the circuit. Remember, we have typically completed and normally open types of relays. As soon as the current flowing in the coil goes off, the movable armature is returned by a mechanical force back to its original position. The point is approximately half the magnetic one, determined by the spring and the gravitational force.
While designing relays, they are aimed at two critical basic operations, that is, low voltage applications and high voltage applications. For low-voltage applications, the focus is on circuit noise reduction, while for high-voltage applications, the focus is on reducing arcing in power systems.
Figure 2: Operation of the Relay Image Courtesy of Simon Mugo
All the relays are built around similar basics. In the discussion, our focus will be based on a four-pin relay, as shown in the figure above. We shall use two color codes to dismantle the theory behind a relay where red represents the load circuit while green represents the control part. There is a connected control coil at the control circuit, and at the load circuit, there is a switch connection. The purpose of the ring is to control the switch. Two different steps occur during the control of the relay: energization and de-energization.
Energized Relay
This means the relay is ON. At this point, the current flow through the electric coil is represented by pins labeled 1 and 3, creating electromagnetism. The electromagnetism triggers the closure of pins labeled 2 and 4, allowing an electric current to flow through the circuit across the switch represented by pins 2 and 4. This controls the load connected to the relay through pin 4. The relay will appear as shown below when energized.
Figure 3:Energized Relay
De-Energized Relay
Here, the relay is OFF. Immediately, the current flow through the relay’s pins 1 and 3 is stopped, and the switch pins 1 and 4 open, thus preventing the current flowing through the load circuit. This action turns the relay into de-energized mode, which is said to be OFF.
Figure 4: De-energized Relay
Relays operate in the same way as the electric switches. A relay uses the mechanism of pole and throw and is said to be switching one or more poles. There exist three ways in which the contacts of the bars can be thrown, and we can discuss them as follows.
Normally Closed Contact (NC)
This one is also known as break contact. It is the opposite of ordinarily open, where the circuit is disconnected when you activate the relay, and when you deactivate it, the course is started.
Normally Open Contact (NO)
This one is also known as making contact. Activation of the relay closes the circuit, and deactivation disconnects the circuit.
Change-over Contacts (CO)
This one is also known as Double-Throw (DT) Contacts. It finds uses in controlling circuits of two types. They maintain both NC contact and NO contact by using a common terminal.
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