Power and Control Circuit

 

 

 

 

While designing the Relay Logic you might have noticed that every time for an output we do connect a relay coil, yet the relay coil is only one side of complete control system. The coil might be of a relay or contactor but in both cases if you recall we have a NO, and NC on the other side. Both function differently when the coil is energized. One makes the contact while the other breaks it. And as the particular output is connected to these NO and NC side the logic alone can’t define the procedure through which we have acquired this control. And we need an additional circuit that tells us how the load is connected to the relay/contactor’s NO/NC side and what work it might do.

Control Logic/Circuit

For example if we say that we are given a NO switch and want the motor connected to it to turn off whenever the switch is pressed.

The first thought that comes to our mind is that it is not possible as a NO switch makes contact when pressed so there is no way it can turn off the output as it will make the current flow through it.

But here we do miss one thing that we do energize a relay coil by a switch not the load (motor), and every relay has a NO and NC. NO which makes the contact when energized and NC which breaks the contact when energized so just we need to do is connect the motor to the NC side.(click here to check out relay working)

Now if we recall that the switch is connected to the relay coil, and the motor to NC side of the relay. Now pressing the switch energizes the relay coil which will break the contact with NC and henceforth the supply to the motor will be broken.

Power Circuit

Yet the issue still persists that the relay logic design only clarifies about the logic that controls the relay coil called the Control Circuit.

This control logic provided no clue of which contact (NO or NC) is used to connect the load. Hence a circuit diagram should be drawn to clarify the connections of load through contactor/relay, this circuit diagram usually contains 3-phase loads and draws a lot of power hence the name Power Circuit.

For further clarification check out the diagram of power and control circuit.