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

Resistance

The 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 Multiple
M Mega 106
k / K Kilo 103
m milli 10-3

So a 1 KΩ resistor has a resistance of 1,000 Ω.

The prefix letter may also be used to indicate the location of a decimal point. So a 4K7 resistor has a resistance of 4,700 Ω.

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 4K7 resistor with a 5% tolerance could have a value between 4,465 Ω and 4,935 Ω.

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 ⋅ V

The power is stated in Watts (symbol 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

IEC Symbol

Need IEC Symbol