Resistors

A "Resistor" is an electronic component that resists the flow of electricity.

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What is a Resistor, and why should I care?

A "resistor" is an electronic component that resists the flow of electricity. This can be used to limit the amount of current that flows through a circuit and/or dispose of excess voltage. 

 

Resistor Characteristics

 

Resistance Value

Resistors have a resistance value, indicating how strongly the component resists the electrical flow.

Resistance is measured in "ohms", often indicated in technical literature by the uppercase Greek letter Omega (Ω).

Resistors with large values are often written with a suffix of "k" to mean "thousand" or "m" to indicate "million". In the United States, it is common to put the "k" or "m" at the end of the value as a suffix. In some other countries, the letter is placed in the middle of the value in place of a decimal point. Examples:

Resistors are commonly stocked in certain specific ("preferred") values. (Would you want to a run a store that carries a billion different resistor values, when values that are off by a few percent will work just as well?) Substituting a close value is quite common. 

 

Wattage

Resistors have wattage specification that indicates how much power the resistor can dissipate (as heat) before it burns up. 

 

Tolerance

It is more difficult (and more expensive) to make resistors with exact values. As a result, resistors are manufactured with a certain "tolerance", the maximum expected variation from the nominal marked resistance.

Common tolerances are: 

 

Useful Formulas

 

Ohm's Law

Ohm's Law states the relationship between the value of the resistor, the voltage applied across a resistor and the current that will vlow through it.

Simple algebra allows this equation to be written in three important ways:

Voltage = Resistance x Current

Resistance = Voltage / Current

Current = Voltage / Resistance

Where: 

 

Power

Simple algebra allows this equation to be written in three important ways:
Power = Current x Voltage

Current = Power / Voltage

Voltage = Power / Current

Where: 

 

How Resistors Are Marked

Physically large resistors often have their resistance value and wattage printed right on them.

Countries in the former Soviet nations also labeled small resistors this way.

The United States and most western European nations usually mark small resistors with a color code. 

 

Color Code

The "resistor color code" uses small colored bands on the body of a resistor to indicate its resistance value, tolerance, and sometimes other things.
Color 1st band 2nd band 3rd band (multiplier)
Black 0 0 ×100
Brown 1 1 ×101
Red 2 2 ×102
Orange 3 3 ×103
Yellow 4 4 ×104
Green 5 5 ×105
Blue 6 6 ×106
Violet 7 7 ×107
Gray 8 8 ×108
White 9 9 ×109
Gold     ×0.1
Silver     ×0.01

Resistors are often marked with a band for their tolerance: 

 

Choosing a Resistor for a project

When looking for a Resistor for use in a project, there are several things to consider: 
 

Where can I buy them?

Resistors are electronic construction parts. Don't expect to find then at Sears or ordinary stereo shops.

Look in stores that cater to electronic experimenters, such as:

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