Basic Electrical Units
Resistance and Resistors
Resistance is the restriction of the flow of electrons. Resistance is the opposite of current. As an analogy, think of a river flowing fast with no obstructions (current through a wire). When the water comes to a point where there are huge rocks and trees (resistance), the water loses its speed and energy.
If the resistance in a circuit increases, the current will decrease.
Resistance is designated with R and its unit is the ohm (Ω).
A resistor is a device designed to produce resistance. Resistors can be used to limit current, divide voltage, or generate heat.
There are two main types of resistors: fixed and variable.
Fixed resistors have a certain amount of resistance and cannot be changed. They are available in a wide range of varying resistances. Different types of fixed resistors include carbon-composite, metal film chip, chip array, resistor network, and radial-lead for PC board. The most commonly used resistor is the carbon-composite resistor, pictured below.
Each fixed resistor has its own unique value, as well as a tolerance value of either 5% or 10%. Depending on the type of resistor you have, you can check its value by the coding on the outer shell.
The colour-coded carbon-composite is the most popular. There are four colour bands.
- Start with the band closest to one of the terminals of the resistor. The first colour is the first digit of the value.
- The second colour represents the second digit of the value.
- The third band represents how many zeros follow the second digit (in other words, the multiplier).
- The fourth band represents tolerance in percentage. This is usually gold or silver. If a fourth band does not exist, then the tolerance would be 20%.
On a resistor using a four-band code, the first two bands are the numbers used, the third band is the multiplier, and the fourth band is the tolerance.
On a resistor using a five-band code, the first three bands are the numbers used, the fourth band is the multiplier, and the fifth band is the tolerance.
As an example, the four-band resistor below would have a value of 54 Ω ±2%
Variable resistors are used when it is necessary to change the values of a resistor easily. The two main uses of a variable resistor are as a potentiometer and a rheostat.
A potentiometer is a device with three terminals used to divide voltage in a circuit. Two of the terminals have a fixed resistance between them, while the third is connected to a wiper, or moving contact. Pictured below is a cutaway of a potentiometer.
A potentiometer is often used to tap off different voltages for measurement. They are sometimes used for control on the variable speed drives of DC machines or for adjustments in electronics.
A rheostat is similar in that it has a varying resistance; however, a rheostat is generally beefier and is used to control current in a circuit.