Resistors
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.