Basic Guide To Resistors

A resistor is an electronic component that is used to resist the flow of electrical current. The resistance restricts the flow of electrons in a circuit. Resistors are known as passive components because they do not require energy to perform, they only use energy.

• 1) Units

• 2) Resistor Colour Codes

• 3) Ohms Law

• 4) Power

• 5) Types of Resistors

1) Units

Electrical resistance is measured in Ohms which uses the Greek symbol omega (Ω). The ability of the material to resist the current flow. Resistance is measured in SI units and being able to understand the size you require is very important.  Resistors can be < 1 Ω or in turn can be Millions of Ω in size. Additional letter prefixes are used to denote values below.

 1 Ohm - 1R

1000 Ohms - 1K

1,000,000 Ohms - 1M

2) Resistor Colour Codes

Resistors are coded with coloured stripes which are used to denote both the resistance value and the tolerance. They can have 4, 5 or 6 bands, 5 bands are the most common which we will explain below.

The first 4 bands are used to calculator the resistors value and the last band represents the tolerance of the resistor. It is important to hold the resistor the correct way round when calculating the resistance, the tolerance band is usually separate to the other 4 bands which are grouped together. 

Example: Yellow, Violet, Black, Brown, Brown is a 4.7K (4700 Ohm) resistor with a 1% tolerance.

It is very handy to become familiar with how the colour code system works - however, if you're feeling lazy then you can use an online resistor colour code calculator. 

3) Ohms Law

Ohms law is an important and fundamental rule to remember when working with resistors and electronics in general. It defines the relationship between Voltage - Resistance and Current.  

"The potential difference (voltage) across an ideal conductor is proportional to the current through it" 

4) Power

 When a resistor is used the process creates HEAT. Generally, the amount of heat produced correlates with the voltage and current. Every resistor has a power rating measured in Watts which is the maximum power the resistor can handle without being damaged. The power can be calculated from the voltage across the resistor or the current through it: 

Whereby; P = Power (Watts), I = Current (Amps), V = Voltage (Volts), R = Resistance (Ohms)

Tip: it is always advisable to choose a larger power rating resistor than required.

5) Types of Resistors

There are a large variety of variable and fixed type resistors with various construction styles for each. The two main types of resistors are;

• Fixed Resistors - Most common type and have a constant resistance value that does not change.
• Variable Resistors - Have an adjustable resistance value that is normally mechanically adjusted. When used as a voltage divider they are called potentiometers.

We have included below the most common types of fixed resistors. 

Carbon Film Resistors

A film of carbon is deposited onto an insulated substrate which is then cut into a spiral body. The resistance is controlled by the varying proportion of carbon to insulation. More carbon gives less resistance and more insulation gives more resistance. Carbon resistors have largely been replaced by metal film resistors, the main reason to choose a carbon resistor is because of their lower cost.

Metal Film Resistors

A film of metal is sprayed onto a ceramic substrate and is then cut into a spiral body. Factors such as the length, thickness and width of the metal spiral determines the resistance value. Metal film resistors are low noise and are more stable when exposed to thermal changes and applied voltages compared to Carbon Resistors.

Metal Oxide Resistors

Uses a metal oxide film deposited onto a ceramic rod and protected using an epoxy coating. The resistance can be adjusted by cutting grooves in the film. Metal Oxide have a higher maximum operating temperature than standard metal film resistors.

Wirewound Resistors

Primarily used for high power applications, they are made from winding wire onto an insulating core. They come in a variety of different styles, the most common are aluminium clad and ceramic. They have a high level of reliability with a low level of temperature coefficient. These are the oldest type of resistor however they still have have high power ratings and good properties for values of low resistance.

Surface Mount Resistors

These resistors are constructed from a ceramic body and feature internal metal electrodes added on each end to make the contacts to the resistive element. The resistors come in a wide range of package sizes, the most common are 0603, 0805 and 1206. They are often found on PCBs and are popular option because of the high packing density. Surface mount resistors usually come on tape packaging (as pictured below) and feature a 3 or 4 digit code representing the value.