SPDT SWITCH USED AS SPST SWITCH (THEORY & PRACTICE)

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In this lesson we will see how a single-pole double-throw switch can be used as a single-pole single-throw switch.

In the previous lesson, we have seen how a single-pole double-throw switch is able to solve circuit situations that cannot be managed by using single-pole single-throw switches.

It is instructive to ask whether the opposite applies, that is, whether an SPDT switch can be downgraded to a simple SPST switch.

This can be useful, for example, in those cases where we have an SPDT switch, but we need to install an SPST switch.

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In particular, starting from the circuit that controls a lamp from two points,

we want to remove a control point.

If we do that, we will have a situation where it is not clear how to connect the two traveler wires to the single wire coming from the lamp.

It is clear that if we connect both wires to the lamp, it will be always on!

In fact, regardless of the state of the switch, there is always an electrical connection between the phase cable and the bulb:

In this case, the solution is pretty simple.

Instead of connecting both wires to the light bulb, we connect only one, leaving the other traveler terminal disconnected.

With this change made to the circuit, we are now able to achieve the desired functionality.

In fact, if we now operate the switch, we are able to turn the light bulb on and off.

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We can see that when we flip the SPDT switch, the position of the lever, inside it, changes. In this case we establish a connection of the hot conductor with the light bulb, which consequently turns on.

By flipping the SPDT a second time, the lever changes position again, thus interrupting the connection just created and consequently the light bulb will be turned off.

This is precisely the operation illustrated in the case of a single-pole single-throw switch.

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With this simple analysis, we can answer to our question.

In particular, we can state that the two schemes under analysis are completely equivalent.

In conclusion, we have seen that a single-pole double-throw switch can be converted into a single-pole single-throw switch by leaving a contact unused.

Of course, it is important to note that the contact to be left unused must not be the one permanently connected to the lever.

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We can also apply the concepts we have just learned 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.

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We can see the presence of the two terminals of the SPST switch.

We know we can replace this switch with a single-pole double-throw switch using just two terminals, as illustrated in the picture.

We can therefore proceed by closing the electrical box and the installation is completed!

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Well, we have come to the end of this lesson. We have analyzed the possibility to convert a single-pole double-throw switch into a single-pole single-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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Thanks for your attention and … see you next time!

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