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Reasons for Resistor Variance

There are two main reasons for resistor variance: it is cheaper to manufacture resistors if the values vary a bit, and it usually does not matter if the value of a resistor is not exact. There are companies that specialize in making resisters with exact values, but the price is more than most manufacturers of electronic devices --- who buy millions of resistors --- want to pay.

The Color Code
- Resistors are the easiest electronic component to recognize. They have colored bands around them to indicate their values. The first two bands code for integers and the third band is an exponent. If all three bands are orange --- the color code for 3 --- the value is 33,000 ohms. If there is no fourth band, the tolerance for the resistor is 20 percent. This means the manufacturer is guaranteeing that the actual value will be between 25,400 ohms and 39,600 ohms. This variance is acceptable for many applications. It does not matter if a lamp burns a little brighter or dimmer. If a circuit is critical, it often has a control knob. If the base sound is 20 percent low, you can just turn it up a little.
Tolerance
- When the value is critical the resistor can be manufactured to a higher tolerance --- for a price. These more expensive resistors have a fourth colored band. If the fourth band is silver, the tolerance is 10 percent, and if the fourth band is gold the tolerance is 5 percent. If the variance is even more critical, there are companies that will sell you resisters guaranteed to be within 1 percent, but it will cost you.
The E Series
- Not every value of resistor is manufactured by a company. The most common values are the E6 series with six equally spaced values in each "decade." That means there are six values between 100 and 1,000, six values between 1,000 and 10,000 and so on. Other series include E12,E24, E48, E96 and E192. The higher the E series, the higher the price. The largest manufacturers make only the E6 series and sometimes the E12 series. Other companies specialize in the higher series exclusively.
Other Factors
- Other factors can cause a variance in the value of a resistor. If it has ever been exposed to extreme hear or cold, or is being operated in extreme heat or cold, the value will vary. The resistors in antique radios --- the "dog-bone" resistors --- can vary by over 200 percent due to age alone. At the time they were manufactured, the tolerance was 20 percent --- the standard tolerance before the 1950s.

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