How does a relay work? | Normally Open | Normally Closed | Steps towards learning Automation – 02


In this episode, we will see how relays work. It’s relatively easy for anyone with an understanding of relay type control to program PLCs with little guidance. Some of you must have already used relays. Relays are used in our day-to-day devices like refrigerators, washing machines, heaters and air-conditioning. The earliest application of relays started during long distance telegraphy wherein you pressed and released a letter to type it interrupted the current flow. The signals traveled through wire from transmitter to receiver. Here the pulses turned the electromagnet on and off pulling and releasing a lever. A pencil at the bottom of the lever recorded
the code on a moving paper. In fact, some of the first computers ever
built used relays to implement boolean gates. As we can see from the applications a relay is used mostly as a switch. But do you know that relays operate on a low
power and can be used to control a circuit of a very high power. Let’s demystify relays A relay consists of two separate and completely independent circuits. One is control circuit and another is load circuit. One circuit drives another circuit. Let’s understand the components that make a relay. Electromagnet, Armature and Spring. Let’s first understand an electromagnet. We take a metal rod and wind a conductive wire usually insulated copper around it. It is powered using a battery. We know that an electric current has the ability
to produce magnetic field in a plane perpendicular to the direction of current flow. The electromagnet uses this principle. Current is introduced only when the switch is closed. When current flows it energizes the coil and creates a magnetic field. Electromagnets are useful because you can turn the magnet on and off by completing or interrupting the circuit Lets see the armature. There is a magnetic object near electromagnet for the purpose of making it move when the coil is energized with electric current. The armature acts as a switch to complete the second circuit. When current runs through the control circuit
and energizes the coil, it creates a small magnetic field which pulls the arm of the
switch towards the coil and completes the second circuit. Thus, the light glows. When the electromagnet is not energized the spring pulls the armature away and the circuit is not complete. The light is off. These parts are safely held with the help
of a spring. The relay coil may only consume fractions of power while the load circuit may be able to conduct hundreds of times that amount of power to a load. In effect, a relay acts as a binary amplifier. Relays are often used for isolating one voltage level from another. This is why relays are used to safely control heavy load circuit. When you use relays, you have to decide
whether it should work as normally open or normally closed. Normally open or normally closed relays depend
upon the configuration of armature. In the example that we have shown, this is an example of normally open relay The normally closed relay looks this way. Here the armature is placed in such a way
that the load circuit is completed, when the control switch is turned off. When you develop a PLC program you will be using relay logic. But, instead of hardware logic with actuators, you will now have ladder logic contacts or boolean instructions. Just like the input actuators you have both normally open and normally closed contacts available in PLC programming. Often, they are referred as Examine if closed and Examine if open In the next video, we will talk about it more. Thank you for watching this video If you have liked the video give it a thumbs up and hit the bell icon to never miss an update from practical ninjas

Leave a Reply

Your email address will not be published. Required fields are marked *