USE OF A RELAY TO CONTROL TWO GROUPS OF LAMPS (THEORY)
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In this theoretical lesson we will see how to use a single relay to control two lamps or two groups of lamps.
The relay system analyzed in the previous lesson allows us to control a single group of lamps.
In fact, the relay used in this system is equipped with a single single-pole single-throw switch and therefore the relay can assume only two states.
More specifically, we can have both bulbs off, or both bulbs on.
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If, on the other hand, we have to control individually the two lamps, we can use a four-state relay.
Controlling two lamps individually means that we want to be able to turn on a light bulb or the other light bulb, following a well-defined sequence as indicated by the 4 states represented in the following figure.
Let’s consider the case with a single command point which will then be extended later to the case with more command points.
It is observed that the four-state relay has two switches, instead of just one as in the case of the two-state relay.
The two switches are connected to the two of lamps (or groups of lamps) to be controlled.
The push-button is connected to the hot wire and, when it is pressed, it creates electrical continuity of the hot wire with the relay coil.
When this happens, the coil commands the two relay switches. In our example, the two switches are then closed.
In this way, both lamps light up and remain on, even when the button is released.
If we press the button a second time, the coil opens the first switch of the relay and consequently the corresponding lamp turns off,
and remains off even when the button is released.
If we press the button a third time, the coil closes the second switch and simultaneously opens the first switch, thus reversing the previous situation.
In particular, we have that the lamp that was off turns on, and the lamp that was on turns off.
Finally, by pressing the button a fourth time, the coil opens both switches, and consequently both lamps will be turned off,
and remain so even when the button is released.
At this point the switching sequence can begin again.
In fact, by pressing the button again, both lamps will be turned on.
It must be said that not all four-state relays have this switching sequence.
Among the various types, we mention a second example a four-state relays with a switching sequence where:
both lamps off, then the first lamp off and the second lamp on, then the first lamp on and the second lamp off, and finally both lamps on. At this point, the cycle resumes with both lamps off.
This tells us that before buying a four-state relay, we need to carefully analyze its characteristics.
For the sake of completeness, we note that the use of three-state relays is also quite widespread. A typical sequence available with these relays is shown in the figure.
Well, we have come to the end of this lesson dedicated to the to use of a single relay to control two groups of lamps.
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