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

3. Color-code for Resistors with 3 or 4 Bands This coding is used for 5%, 10%, and 20% tolerance resistors This coding is used for 5%, 10%, and 20% tolerance resistors The first three bands are used to denote the rated value The first three bands are used to denote the rated value The fourth denotes how much the device may vary from the rated value The fourth denotes how much the device may vary from the rated value 1st band is the first digit of the resistance value 1st band is the first digit of the resistance value 2nd band is the second digit of the resistance value 2nd band is the second digit of the resistance value 3rd band is the multiplier (number of zeros) 3rd band is the multiplier (number of zeros) 4th band indicates the tolerance 4th band indicates the tolerance

4. Precision Resistors Precision resistors will have a tolerance of 2%, 1%, 0.5%, 0.25% or 0.1% Precision resistors will have a tolerance of 2%, 1%, 0.5%, 0.25% or 0.1% Some precision resistor values are identified using the color code with 5 color bands, Some precision resistor values are identified using the color code with 5 color bands, the first 3 bands indicate the rated resistance value the first 3 bands indicate the rated resistance value the 4th band is the multiplier the 4th band is the multiplier the 5th band indicates the tolerance the 5th band indicates the tolerance

5. Color Coded Precision Resistors

6. 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

7. Surface Mount Resistors

8. 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

9. Variable Resistors

10. 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)

11. Power Resistors

12. 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

13. 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

14. Relative Sizes of Resistors from 1/8 Watt to 1 Watt As you can see a resistors ability to handle power is proportional to its size

15. 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