ResistorsResistors

• • Resistors are used to limit Resistors are used to limit current or divide voltage, and in current or divide voltage, and in some cases, generate heatsome cases, generate heat• • Resistors come in 2 main Resistors come in 2 main categories Fixed and Variablecategories Fixed and Variable• • Common resistors are carbon-Common resistors are carbon-composition or carbon film, metal composition or carbon film, metal film, and wirewoundfilm, and wirewound

Color-code for Resistors with 3 or 4 Bands• This coding is used for 5%, 10%, and 20% This coding is used for 5%, 10%, and 20%

tolerance resistorstolerance resistors• The first three bands are used to denote The first three bands are used to denote

the rated valuethe rated value• The fourth denotes how much the device The fourth denotes how much the device

may vary from the rated valuemay vary from the rated value• 1st band is the first digit of the resistance 1st band is the first digit of the resistance

valuevalue• 2nd band is the second digit of the 2nd band is the second digit of the

resistance valueresistance value• 3rd band is the multiplier (number of zeros)3rd band is the multiplier (number of zeros)• 4th band indicates the tolerance4th band indicates the tolerance

Precision ResistorsPrecision Resistors

• Precision resistors will have a tolerance of Precision resistors will have a tolerance of 2%, 1%, 0.5%, 0.25% or 0.1%2%, 1%, 0.5%, 0.25% or 0.1%

• Some precision resistor values are Some precision resistor values are identified using the color code with 5 color identified using the color code with 5 color bands,bands,

• the first 3 bands indicate the rated the first 3 bands indicate the rated resistance valueresistance value

• the 4th band is the multiplierthe 4th band is the multiplier

• the 5th band indicates the tolerancethe 5th band indicates the tolerance

Color Coded Precision Resistors

Surface Mount Resistors

•are available as small resistor chips•They use Alphanumeric Labeling•Two or three digits, and one of the

letters R, K, or M are used to identify a resistance value

•The letter is used to indicate the multiplier, and its position is used to indicate decimal point position

Surface Mount Resistors

Variable Resistors

•Variable resistors are designed so that their resistance values can be changed

•A potentiometer is a three terminal variable resistor used to divide voltage

•A rheostat is a variable resistor used to control current

Variable Resistors

Power Resistors

•Used when the device must accommodate high currents

•Physically large surface so it can dissipate heat into the environment

•Heats the air around the device – may become very hot (heat is a byproduct of resistance)

Power Resistors

The Power Rating of Resistors

• The power rating is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat buildup

• Power rating is determined by physical composition, size, and shape of the resistor

• The power rating is not related to the ohmic value of the resistor

The Power Rating of Resistors

•Carbon film and Metal-film resistors are available in standard power ratings from 1/8 Watt to 1 Watt

•When a resistor is used in a circuit, its power rating should be at least 25% greater than the maximum power that it

will have to handle

Relative Sizes of Resistors from Relative Sizes of Resistors from 1/8 Watt to 1 Watt1/8 Watt to 1 Watt

As you can see a resistors ability to handle power is proportional to its size

The Power Rating of Resistors• When the power dissipated in a resistor is greater

than its rating, the resistor will become excessively hot

• – the device should be expected to prematurely and unexpectedly fail

• – the resistor may burn up• – its resistance value may be greatly altered• – damaged resistors may be visually detected by

charred or altered appearance of the surface,• – When a resistors power is exceeded, the suspect

resistor should be removed from the circuit and checked with an ohmmeter