# Resistance

Resistance is the opposition to the flow of current; it an inherent property of most electronic components. It is one of the fundamental quantities of electronic circuits.

## Resistor color coding

Resistors oppose the flow of electricity, and they all have a *nominal resistance value*. If you look at most resistors, however, you’ll see no numbers on their bodies. Resistors were being manufactured before the technology existed to print text small enough to be seen clearly on their sides; to denote the nominal resistances, the **resistance color code** was born. This allows you to very quickly read the nominal resistance (and **tolerance**) of a resistor. (The tolerance describes how accurate the resistor’s value is. If a 100Ω resistor’s tolerance is 5%, then its *measured resistance* is guaranteed to be between 95Ω and 105Ω.)

In the resistance color code, each number from 0 to 9 is assigned a color. Non-precision resistors generally have 4 color bands, and each band represents a specific part of the resistor’s resistance. Take a look at the chart below:

Resistor Color Code Chart

Let’s use an example resistor to understand the chart. Our example resistor looks like this:

To locate the first band, it is easiest to *first* find the 4th, because it will almost always be either gold or silver. Gold is 5%: e.g. if you have a 100Ω resistor, its allowable range is between 95Ω and 105Ω. Silver is 10%. This resistor is gold, so we know its tolerance is 5%.

Now that you have located the fourth band, start at the opposite end. The first band represents the first digit; in our case, it is brown, which is “1”. The second band is the second digit; “0”. (So far, we have “10”.) The third band is the multiplier, or how many zeroes to add to the end of the first two digits; red is “2”, so we finally have:

1000Ω ± 5%

## Resistors in series and parallel

You will begin to encounter more complicated circuits in your electronic escapades, such as the following two:

The first is an example of resistors in **series**, i.e. one after another. The second is an example of resistors in **parallel**, i.e. two paths at the same time.

The resistances of resistors in series will just add linearly, so:

In the first example above, this works out to:

RT= R1+R2

RT= 100Ω+100Ω

RT= 200Ω

The resistance of resistors in *parallel*, however, combine in a much different fashion:

RT = (1R1+1R2)-1

In the second example above, this works out to:

RT = (1100Ω+1100Ω)-1

RT = (2100Ω)-1

RT = (100Ω2)

RT = 50Ω
If you think conceptually about what is happening here, these should both make sense. Including more resistors one after another should increase linearly. Adding *another path for the current to flow through* by using another resistor in parallel should *increase the total current*, which means a decrease in the total resistance!