DIY – Lighting – A Short Primer on Electricity

Want to install a dimmer switch, replace incandescent bulbs with CFLs (Compact Fluorescent Lights) or LEDs, or perform many other home lighting projects? If so, it’s helpful to know a little bit about what drives them all: electricity.

Two of the most basic concepts needed are ‘voltage’ and ‘current’. Current moves down a wire, pushed by a certain voltage. Current is the quantity of electricity flowing, measured in amperes or amps (A). Voltage is the force with which it’s pushed down the wire, measured in volts (V).

The analogy to water moving down a hose is often used at this point and it’s a good one. But replace the water with marbles, since electricity comes in packets, called electrons.

Actually, it’s the electric wave moving down the OUTSIDE of a wire, not the electrons moving within it, that powers a lamp. One way to prove it is to have a very long wire with a bulb at the end. Turn on the switch and the light gets lit long before the slow-moving electrons can get there. The electric wave moves at nearly the speed of light. But that’s far more technical information than we need here.

But what about the most common term, ‘watts’? Watts is a measure of electrical power. It is nothing but the product of amps x volts, W = V x A.

And, just what makes that bulb glow, the current? Not exactly. Not by itself, anyway. Something else is needed: resistance (R). As current flows through the wire, driven by a certain voltage, it encounters the tungsten filament in the bulb. It vibrates the atoms in the wire. But just as with most things, they resist that push. That resistance is measured in ohms. The term comes from the name of a physicist who studied the subject.

Mathematically, voltage = current x resistance. But the resistance of the wire is determined by the material it’s made of and its shape and length, not how much current and voltage are present in circuit. So, a standard incandescent on a circuit with an on-off switch has a fixed resistance.

When you change an ‘ordinary’ bulb from 100-watts to, say, 60-watts you’re effectively altering the amount of resistance in the circuit. The voltage and current aren’t in your direct control.

When you install a dimmer switch, you’re altering the amount of power indirectly by changing the resistance. A dimmer changes the resistance from nearly zero (at the switch) to almost 100%, in the case of incandescent. For some types of CFL, the change may only be about 20%-80%. At a certain level, a fluorescent bulb can’t stay lit.

Next time you start to wire something, or design a lighting scheme for your home, keep in mind these basic concepts. They’ll help you interpret the diagrams and instructions you use.