Electronics Principles V11

Clive W. Humphris


Switch contacts when open provide an interruption of the current flow within a circuit and when closed completes the conducting path. Shown is one of the simplest of schematic diagrams that consists of just three components, indicated by appropriate symbols. Clearly shown are the component connections and the effect of what happens when the switch button is pushed. One of the simplest types of switch has to be the push-to-make, i.e. for a doorbell, here is a push-to-break.

Within the pages of a components catalogue you can find dozens of different combinations of switch types. When selecting a switch there are two main considerations, current rating and the maximum working voltage. Using a switch that is under-rated can be unreliable and dangerous because of arcing of the contacts or physically expose the user to an electric shock because of a voltage breakdown of the insulation.

In this diagram the battery can represent any number of cells connected in series which increase the supply voltage (potential difference) as each cell is added. Battery cells are usually in multiples of 1.5V, and those of the rechargeable type are lower at 1.2V.

To calculate the current I flowing in this simple circuit we can use Ohm's Law by applying the formula shown. Try changing the battery supply voltage and note the changing current. In a practical circuit the more current that flows the brighter the lamp would glow. Increasing the voltage and thereby the current, above that permitted by the bulb and the filament acts like a fuse.

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