By Cathleen Shamieh

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.)

Resistor
Resistors

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.