There are many people, even with technical training, who even today ask themselves, what is a relay for?
Since it was invented in 1835 by Joseph Henry, the relay has being going hand in hand with the world of industrial automation.
The self-regulation of everyday processes by intervening in simple but effective procedures as opening the door of our garage when we get home , activate the coil of the contactor that starts the motor of the elevator that lift us to the floor of our house or even turning on the TV screen when we press the remote control from our sofa.
Resembling the electrical circuit where the kitchen´s lamp is connected, there is a switch that when manually operated, the lamp turns on or off requiring our physical presence by pressing the switch with our finger in order to change its position.
However, it is significantly more interesting to use the relay for other purposes.
I begin to understand …
So, from now on, if someone asks us “what is a relay for?”, we can say that a relay is a switch to which we have added a “system” that replaces our finger to swap its position in such manner that the contacts either meet or separate letting or impeding the current throughout the circuit we want control.
Nevertheless, due to the technological monumental progress in electricity and electronics, it may seem that the relay is obsolete as we have been using it for more than 180 years since its invention.
Still, hard as it may seems, the use of relays is at its highest peak as a result of the necessity of the automated systems that require their inclusion in the design and are currently in great demand.
Much better with an example …
Furthermore, to clarify the previous explanations we will use as an example the garage door.
Whenever we want to open the door of our garage, we will have to operate the motor that provides the door with the mechanical power to slide along the rails that guide its wheels or, if it is folding, to turn on its hinges.
Whether the type of door is smaller or larger, you will need either a small motor or one of higher power for its movement, but in all cases, the electrical circuit to which the motor is connected will circulate several amps at a voltage that normally will be the service voltage in alternating current of 110 or 230 volts.
When we press the button “remote control” of our garage, from our car, the chain of events that occur is as follows:
Our pulsation on the remote control closes a small circuit on the electronic board of our “remote control”.
The small power circuit mentioned before usually work at voltages below 6 volts in direct current that is supplied by an internal “battery” in which the circuit is only responsible for emitting a radio-frequency encoded signal where is received by the detection antenna located in the control cabinet inside our garage.
The control voltage …
This receiving board is always “listening” to catch and decode the signal emitted by our “remote control” where the circuits of this receiving plate are normally supplied with small control voltages, between 12 and 24 volts of direct current that are provided by a small power supply connected to the network.
Throughout this plate´s circuits, only a few milliamperes circulate to the voltages mentioned, and are responsible for feeding the electronic components.
If the decoding of the signal is done correctly, a small electronic component is activated normally a transistor that gives way to the current of the circuit to which the coil of our relay is connected.
Relay operation …
At this moment the coil of the relay to be energized generates a small magnetic field around an iron core which attracts towards itself a moving part also made of iron to close the magnetic circuit.
Besides, the mobile piece of iron has incorporated a bronze sheet united by one of its ends to the conductor cable of the circuit to which the motor of the door is connected where is fastened by means of an insulating material.
Next, the sheet carries at its other end an electrical contact of a good conductive material that is ordinarily an alloy of copper with other elements.
Lastly, the movement of electromagnetic attraction on the moving piece of iron ends with the seat of the mobile contact on another similar junction that is “fixed” and subject to a conductive sheet placed in to turn on an insulating material and connected by its other end to the cable that continues towards the engine.
In this way it is achieved that the ends of the circuit that were initially separated (open contacts) come together and manage to close the circuit allowing to connect the motor and open the door due to the action of the electromagnet described above.
As we mentioned at the beginning, it is possible that the motor needed to open the door has to be of high power and its current range exceeds the normal load switching capacity of a relay, which Its usually roams in intensities of between 5 and 30 amps at a maximum voltage of 250 volts AC (for resistive loads).
Now everything is much clearer …
In these circumstances, it is necessary to interpose a “contactor” that performs a function similar to that of the relay but with a much higher working range between the relay and the motor, being able to switch circuits with connected loads that demand up to several hundred amps at voltages greater than 600 volts.
In this instance, the relay is used to “pick up” the small signal coming from the transistor of the electronic board and when turn on, close at the same time the circuit to which the contactor coil is connected.
This causes its contacts to close the powerful motor of several kilowatts.
We hope we have oriented you a little and answered your question of what is a relay for? Thanks for joining us here.