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.
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