Resistors

Current-limiting components.

Overview

Resistors are possibly one of the most common components that you will use. A resistor has three properties that should be considered when working with an electrical circuit:

  • Resistance
  • Tolerance
  • Power rating

For a more in-depth discussion about resistors and resistance, check out Part 4 of the Electronics Tutorial.

Resistance

Resistance is measured in Ohms symbol (Ω). This may also be prefixed with a letter to indicate the scale of the value. Common scale factors relevant to resistors are:

Letter Prefix Magnitude Value
M Mega 106 10,000,000
k / K Kilo 103 10,000
m milli 10-3 0.001

Therefore, a 1kΩ; resistor has a resistance of 1,000Ω;.

The prefix letter may also be used to indicate the location of a decimal point. For example, a 4K7 resistor has a resistance of 4.7kΩ.

Tolerance

The tolerance is expressed as a percentage. Common tolerances are 1%, 5% and 10%. Lower tolerance resistors are more expensive due to the higher requirements of the manufacturing process.

A 4.7kΩ resistor with a 5% tolerance could have a value between 4,465Ω and 4,935Ω.

Power Rating

Resistance to the passage of electrical current generates heat. The amount of heat generated can be calculated using the following formula:

P = I x V

The power is stated in Watts (W) and the power rating of the resistor should always be greater than the product of the current (I) flowing through the resistor and the voltage (V) drop across the resistor.

Schematic Symbol

There are two possible symbols used in circuit diagrams (schematics):

  • Zig-zag line (IEEE)
  • Rectangle (IEC)

The use zig-zag line is more commonly used in America than the rectangle.

IEEE Symbol

Resistor Circuit Symbol