# Protect LEDs with Resistors

To limit the current that flows from a 9-volt battery through your LED, you insert a resistor in your circuit. *Resistors* slow down current, like a kink in a hose slows the flow of water.

The figure shows you a variety of resistors. Every resistor has two leads, and it doesn’t matter which way you insert a resistor into a circuit. Current flows either way through a resistor. (Resistors are not semiconductors, so they are not picky.)

Resistors don’t require a minimum voltage like LEDs do (not picky!). Current flows through a resistor even with a tiny voltage applied. The higher the voltage you apply to a resistor, the higher the current that flows through the resistor — up to a point. Too much current can melt a resistor.

## Understanding resistance

Every resistor has a value known as its *resistance* (what a surprise). The higher the resistance, the more the resistor restricts current. Resistance is measured in *ohms* (pronounced “omes”), and the symbol for ohms is the Greek letter omega (which looks like an upside-down horseshoe).

Some resistances are measured in *kilohms* (pronounced “kill omes”), which means thousands of ohms. Other resistances are so large they are measured in *megohms* (pronounced “meg omes”), which means millions of ohms. (You may be familiar with the prefixes, *kilo*, which means thousands, and *mega*, which means millions, from your math classes. And you’ve probably heard of measurements such as *kilometer,* as in the *k* in a 5k race, and *megabyte,* as in “My laptop has 4 *megabytes* of RAM.”)

For your LED flashlight, you need a resistor with a value of 470 ohms. But resistors don’t have their values stamped on their cases, so you need to know how to identify a 470 ohm resistor. You can tell what the resistance of a specific resistor is by looking at the colored bands on its case. Think of the colored bands as a code. The color and position of the bands tell you the value of the resistance.

A resistor that has a stripe pattern of yellow-violet-brown and then a fourth stripe of any color is a 470 ohm resistor.

## Resistor power ratings

Resistors also come in different thicknesses. Generally, the thicker the resistor, the more electrical energy it can handle before having a meltdown. When resistors slow down current, a lot of heat is generated, and if too much heat is generated, the resistor will melt.

All resistors have *power ratings* measured in units of *watts* (abbreviated *W*). Ordinary resistors can handle up to 1/4 watt of energy, but you can also find 1/2-watt, 1-watt, and larger resistors for circuits that need to handle more power.

You won’t see any marks on a resistor to indicate the power rating. When you purchase a resistor, the product information will indicate the power rating.