Technical Guides

What is Relay?

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Relay is a very common device in industry in general and automation in particular. This device has the function of opening and closing contacts by electrical signals. We can use low current to control the opening and closing of a circuit with higher current. In this article, we will learn briefly about this type of relay

Classification relay

There are 2 main factors for us to choose Relay as follows:

+ Control voltage level: AC or DC. The common message levels are 5VDC, 12VDC, 24VDC, 220VAC, …

+ Number of contact pairs: normally the relay will have 1 pair of contacts, 2 pairs of contacts, 4 pairs of contacts, it is often standardized as 5-pin relay, 8-pin, 14-pin, …

How does relay work?

Relay is composed of two main components: coil and contact. When current flows in the coil, it produces a magnetic field that attracts the contacts. This in turn reverses the current state of the point. When the coil loses power, the contacts will no longer be attracted and return to its normal state.

A complete relay device will include two main components as follows:

+ Relay body: including the main parts of a relay to operate, including the coil and contactor…

+ Base or socket: With the legs of the relay body normally works already. However, to ease the connection, we will have a component called the base of the dyke or the socket. This base is attached to the relay and contacts of the relay contacts to the ends of the wires or screwed for convenient connection.

When we choose the relay, we must keep in mind that the relay body and base must go well together. Mainly two main factors: the number of relay pins and the type of pins (flat or round) …

The operation of the circuit can be summarized as follows:

+ With the switch open, coil CR is de-energized.

The circuit to the green pilot light is completed through the normally closed contact, so this light will be on.

At the same time, the circuit to the red pilot light is opened through the normally open contact, so this light will be off.

+ With the switch closed, the coil is energized.

The normally open contact closes to switch the red pilot light on.

At the same time, the normally closed contact opens to switch the green pilot light off.

The working principle of Relay

We already know how the relay works in the previous section, but why the relay can work like that, let’s take a look at the structure inside the relay to know its principles.

The current flowing through the intermediate relay will flow through the inner coil. It creates a magnetic attraction. This magnetic field acts on an internal lever. This phenomenon causes the electrical contacts to be closed or closed. That will change the state of the relay. The number of electrical contacts changed may be one or more, depending on the design.

Relay has 2 independent working circuits. A relay control circuit: Whether the current flows through the coil or not (or means that the relay control is ON or OFF). A circuit control the current we need to control whether or not the relay relies on the ON or OFF status of the relay.

There are 3 symbols on the relay: NO, NC and COM:

+ COM (common): is the common pin, it is always connected to one of the other 2 pins. As for which pins it connects with, it depends on the relay’s operating state.

+ NC (Normally Closed): Normally, it is closed. This means that when the relay is in the OFF state, the COM pin will be connected to this pin.

+ NO (Normally Open): When the relay is ON (the current flows through the coil), the COM pin will be connected to this pin.

>>> Therefore, we will connect COM and NC when you want to control the current when the relay is OFF. And when the relay is ON, the current is cut off. Otherwise, connect COM and NO.

Advantages and disadvantages when using relay

There is a device that has the same features as a relay but is made of a semiconductor, the transistor. Relay and transistor both have the same function of switching the circuit. One used mechanical, one used semiconductor. For example, when you buy a PLC, it will also have two output options, the relay type and the transistor type. So when to use the relay, when to use the transistor? Let’s compare advantages and disadvantages of two types to find the best choice.

– Advantages of relay:

+ Relay can switch AC, DC. Transistor can only switch DC

+ Relay can switch high voltage, but the transistor is not possible

+ Relay is the best option when switching large currents (> 5A)

+ Relay can switch on several pairs of contacts at the same time.

– Disadvantages of relay:

+ Relays are more bulky than transistors when needing to switch small currents.

+ Relay cannot switch quickly. Transistors can switch many times in a second

+ The relay consumes more power due to the need for current to flow in the coil

+ The output of electronic devices may not provide enough current for relay control. Therefore, it is necessary to use an external transistor or power source to buffer the current for the relay

+ The durability of the relay is much lower than the transistor

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