Tuesday, June 24, 2008


For electricity to flow, there needs to be a path that connects all the elements together. In the diagram below, you can see how electricity can travel from the cell around in a loop through the lamp and back to the cell again provided all the wires are in their proper places.

It should be noted that we show electricity travelling from the positive (+) side of the cell around the circuit to the negative side. This is called conventional current. The slightly odd thing about this is that the electrons that constitute electrical current are negatively charged and actually travel in the opposite direction. The fact that we depict current travelling from the positive to the negative is an historical accident. Fortunately unless you're doing something esoteric like semiconductor physics, this extra layer of complexity need never worry you.

Lamp and Cell Circuit

Circuit diagrams provide a very efficient way to describe an electronic circuit. They use a small set of symbols and conventions that need to be learned but the benefits of their form over a more pictorial style are so definitive that they are used universally.

The circuit above can be diagrammed more efficiently. In order to illustrate this, here are some symbols used to depict cells and lamps.

Cell. This is often called a battery, but technically a battery is multiple cells. This is what C, D, AA and AAA cells we can get at the corner store all are. This kind of cells is rated at 1.5V.
Lamp. Like you might find in a flashlight. Lamps have voltage ratings like many things. This rating indicates the voltage that the lamp is designed to run at. It will be the highest voltage the lamp can withstand without getting too hot and burning out. Lamps may also state their wattage - the power they consume. From this and the a re-arrangement of the equation for power (I = P / V) the likely current consumed can be calculated.

With these symbols we can now construct a circuit diagram with a cell and a lamp.

Simple Lamp and Cell Circuit Diagram

This circuit diagram expresses only the essential features of the circuit. The abstract symbols hide all the myriad details of appearance that are actually irrelevant to the circuit. For example, the circuit is not influenced by the colors of the wires so they're not indicated, curves in the wire are irrelevant so the lines in the diagram are all straight, the actual physical design of the lightbulb is from an electrical point of view, immaterial, etc.

The lines on the diagram need not even be wires. They can be traces of copper on a printed circuit board (PCB) or pretty much anything that conducts. In a car, for example, wires that go from parts of the motor, headlights and other devices back to the negative terminal of the battery are not needed. The metal of the car's chassis and body are the return path to the battery and complete the circuit.

Another very important abstraction is the connection of wires. People are often confused by this. The diagram below shows the same circuit as before except for the addition of another lamp. The problems arise when people try to figure out exactly how to connect the things together. How literally does the diagram need to be followed?

Dual Lamp Circuit

Take the top section of the circuit. It depicts a long section of conductor with one lamp getting its power from it about half way along and another getting its power at the far end. As illustrated below, this shouldn't be taken literally. Since the wires in this circuit are pretty much perfect conductors, it really doesn't matter. As long as all the things that are shown to be connected together are connected, the circuit will work.

Dual Lamp Circuit Wiring Alternatives

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