INTERMEDIATE SWITCH USED AS SPST OR SPDT SWITCH (THEORY & PRACTICE)
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In this lesson we will see how an intermediate switch can be used as a single-pole single-throw or as a single-pole double-throw switch.
In the previous lesson, we have seen how an intermediate switch is able to solve circuit situations that cannot be managed by using single-pole single-throw switches or single-pole double-throw switches.
It is instructive to ask whether the opposite applies, that is, whether an intermediate switch can be downgraded to a SPST or SPDT switch.
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This can be useful, for example, in those cases where we have an intermediate switch, but we need to install an SPST or SPDT switch.
Let’s start with the single-pole single-throw switch. We have already seen in previous lessons that this switch has two possible states: open and closed.
In particular, inside a lighting system, the closed switch allows you to turn on a light bulb while the open switch allows you to turn it off.
Let’s now verify if with an intermediate switch we can replicate the same functionality.
The intermediate switch also has two possible states, as shown in the following figure.
However, we have seen that it has four terminals, while the SPST switch has only two.
In particular, in the figure we indicate with red crosses the two terminals that must not be used.
In this case, we are using the top terminals of the intermediate switch,
but it is evident that we can use equivalently the other pair of terminals.
On the contrary, it is not allowed to use the right or left pair of terminals.
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Well, let’s go back to our initial setup
and connect a light bulb to the intermediate switch. We can see that the two states of the intermediate switch allow us to turn the bulb on and off exactly as in the case of the SPST switch.
We can therefore infer that it is possible to use an intermediate switch as an SPST switch.
Let’s do a similar analysis, but this time with an SPDT switch. As we know, this switch also has two possible states, indicated in the figure with “up” and “down”.
We also observe that the SPDT switch has three terminals.
So let’s check if it is possible to replicate its functionality with an intermediate switch.
In this case, it is easy to verify that it is sufficient to use only three of the intermediate switch terminals. In the following figure we indicate with a red cross the terminal that we will not use in our tests.
If we now connect the bulbs to the intermediate switch, we notice that they assume the same configuration obtained with the SPDT switch.
We can therefore conclude that an intermediate switch can be used as an SPDT switch.
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Let’s put the concepts into practice analyzing some simple circuits. Let’s start with the bulb-switch circuit.
In this circuit, we can replace the SPST switch with an intermediate switch, with an intermediate switch,
We note, in particular, that the functioning of the circuit is not altered.
Let us now consider another circuit configuration. In this case, let’s analyze the circuit that allows us to control a light bulb from two different control points.
Specifically, let’s try to replace one of the two single-pole double-throw switch with an intermediate switch.
We have seen that we must leave one of its four terminals unused.
By carrying out the tests of this circuit, we therefore notice that it continues to function correctly,
In summary, the tests carried out in this lesson allow us to state that the intermediate switch can always be used instead of the SPST and SPDT switches.
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We can also apply these concepts to a real circuit. In particular, let’s consider the circuit created at the beginning of this course, in which we used a single-pole single-throw switch
to control a light bulb.
To do this, we open the electrical box in which the switch is mounted.
We can see the presence of the two terminals of the SPST switch.
We know we can replace this switch with an intermediate switch using just two terminals, as illustrated in the following picture.
We can therefore proceed by closing the electrical box.
You will observe that by flipping the intermediate switch several times, it is possible to turn the light on and off just like in the case of a single-pole single-throw switch.
Well, we have come to the end of this lesson. We have analyzed the possibility to convert an intermediate switch into a single-pole single-throw switch or into a single-pole double-throw switch.
We suggest you immediately consolidate the concepts you have just learned by carefully reviewing this lesson and above all by going through all videos of our course, by accessing our playlists.
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