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Switches

Momentary Switches

Another common term used to describe switches is momentary, and the associated NO, NC acronyms. This means as it says, it is only momentarily activated and returns to its original state when the user lets go of the switch. The common push button is of this type. The acronyms refer to the 'normal' state of the switch, with no user actuation. It is either Normally Open, meaning off; or Normally Closed, meaning on.

These momentary switches are very often SPST switches, but of course it is possible to have the other types as well. Common uses would be devices that you do not want to operate on their own - for example an electric saw, or a car horn. They only run when you are actively pulling the trigger or pushing the button (for obvious reasons); and are of the NO (normally open) type.

Ratings

Perhaps the easiest part to understand; the ratings specify how much electricity the switch is able to handle without overheating or damaging the contacts. You will typically find two such ratings, where are amperage and voltage. For more detail on what amperage and voltage mean, see my related article on basic electricity.

The amperage tells us how large of a load the switch can safely control; and is normally specified in Amps. This is primarily determined by the size and hardness of the contacts, and the interconnecting wiring. It is common to allow some safety margin when selecting a switch, however higher current ratings do quickly increase the cost of a switch.

The voltage tells us how much voltage the contacts can withstand in the off position; and is primarily determined by the space between the contacts in the off position - which needs to be a sufficient distance to avoid arcing. It also is and indication of the minimum insulation properties to the outside of the switch. For low voltage use, you will commonly find switches rated much higher than needed (often 120v ratings for 12v circuits). This is simply because its easier to build a single switch to cover many applications; and the higher rating simply provides a large safety margin while adding little if any to the cost or complexity.

In the next section, I'll show howthe various switches are indicated on a wiring diagram (schematic); and show a few common real-world circuits.

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