USE OF A RELAY FOR MULTIPLE CONTROL POINTS (THEORY)
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In this theoretical lesson we will see how with a single relay we can control a group of lamps from an arbitrary number of locations.
In the previous theoretical lesson, we have seen how by using a button and a relay, it is possible to control the switching on and off of a lamp or group of lamps.
It is immediate to ask whether it is possible to extend the concepts analyzed in the previous lesson to an arbitrary number of control points.
We know that the control point of this circuit is represented by the push-button. We must therefore ask ourselves if it is possible to connect more buttons to this circuit.
Let’s consider the case with three buttons.
We now need to make connections that allow each button to energize the relay coil.
To do this, it is sufficient to replicate the connections made for the first button.
It is noted that compared to the case with a single button, what changes is that all the buttons are connected in parallel.
This means that each button is connected to the hot conductor and the relay coil.
With this circuit, it is clear that more buttons can be added with little effort.
Compared to the approach based on the use of intermediate switches,
it is noted that the number of wires to be used is reduced and furthermore the addition of new buttons does not involve any modification on the existing connections.
This is not true in the case of intermediate switch, since the addition of a new switch
requires a modification to the existing connections in the electrical system.
The disadvantages of the relay-based approach are that the relay is not a cheap component and its use becomes economically convenient when we need to install three or more control points.
Furthermore, older or lower quality relays are usually quite noisy.
The relay is also a device that tends to deteriorate more rapidly than switches and is therefore more likely to be among the first components that must be replaced over the years of the electrical system’s life.
There are relay types in which the coil is powered at lower voltage. This means that the buttons are not connected directly to the hot wire, but are connected to a lower voltage power supply. In the following figure we are considering for example a voltage equal to 24 V.
In this way, the operating points (represented by the buttons) are powered with a voltage that would not be dangerous in the event of accidental contact with the conductors. The price to pay in this configuration is the use of a power supply capable of providing the lower voltage required to power the coil.
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Well, let’s go back to our previous configuration and analyze the case of a wiring in a room with three entrances.
In this room we will place a lamp, the three push-buttons and finally the relay.
Before making the connections, we need to lay the electrical conduits.
Let’s assume that the hot and neutral wires are available at the junction box where the relay is mounted.
In this case, we lay three electrical conduits that will be used to make the connections from the buttons to the relay,
and finally a fourth electrical conduit that will be used to make the connections from the relay to the lamp.
In particular, we use the fourth electrical conduit to connect first the neutral wire to the lamp, and then one of the two terminals of the relay switch to the lamp.
We can proceed by connecting the neutral wire to one of the two terminals of the relay coil.
We can proceed now with the connections involving the push-buttons and the relay.
To this aim, we connect one terminal of each push-button to the free terminal of the relay switch.
We will use a different wire color for these connections in order to easily identify the wires coming from the buttons when inspecting the junction box where the relay is mounted.
As indicated in the following figure, the three wires will pass through the three electrical conduits numbered 1, 2 and 3,
in accordance with the original design, illustrated below:
We can proceed by connecting the other terminals of the push-buttons.
In this case we have an additional black wire to be inserted in the electrical conduits numbered 1, 2 and 3.
We can see that these wires are connected together. This can be accomplished for example by using a screw terminal.
The connection just made can be completed by connecting the three wires to the free terminal of the relay coil.
Also for this connection, we are using the black wire.
In conclusion, we can see that we have used the blue wire for the neutral conductor, the brown wire for the hot conductor and for the relay switch, and finally the black wire for push-buttons connections.
In accordance with our original design, all the wires are housed in the electrical conduits, in the electrical boxes, and in the junction box.
To conclude, let’s have a final look at the connection diagram.
Well, we have come to the end of this lesson dedicated to the relay. More specifically, we have seen how to use a single relay to control a group of lamps from an arbitrary number of locations.
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