1. Resistors
Electronics 1
CVHS

2. Review
Resistance is the opposition to current flow provided by a material
The symbol for resistance is Ω
The schematic symbol for a resistor is
When measuring resistance YOU MUST ALWAYS REMOVE POWER from the circuit

3. Resistance One ohm = 1 amp @ 1 volt
A larger resistance value will allow less current to flow
A smaller resistance value will allow more current to flow

4. Resistor Uses Used to limit current flow in a circuit
Many circuits and components require different voltage and current levels
Resistors reduce these levels down to the required level for each component or area of a circuit

5. Resistor Types Fixed (resistance is preset and cannot be changed)
Variable (The internal resistance of the device can be adjusted)

6. Fixed Resistor Composition

7. Fixed resistors Carbon Film Metal film Wire wound
Wire wound ceramic coated
Symbol:
High Voltage Resistors

8. Tapped Resistors These are wire wound fixed resistors
One or more “taps” are located between the ends of the resistor to provide different resistances
Symbol:

9. Variable Resistors Used as circuit controls
Most use a wire wound resistor and a pole
Carbon coatings, and specialty materials may also be used for certain applications

10. Potentiometers Has three terminals
Shaft rotation changes the resistance between the center terminal and two end terminals
Most “Pots” are linear
One degree rotation of the shaft = the same change in resistance regardless of the shaft location

11. Potentiometers Non-linear Pots (audio taper)
One degree of shaft rotation does not = the same change in resistance at different shaft rotations.
Used in volume and tone controls
Symbol:

12. Rheostats Like Pots but only have 2 terminals
Used to adjust current in high power circuits
In low current situations, a pot may be used as a rheostat
Symbol:

13. Adjustable Used in high power applications
Used when frequent changes are not necessary
Power is usually removed from the circuit for adjustment
Symbol:

14. Thermistor Changes internal resistance with temperature
Used as temperature control and safety device.
Symbol:

15. Photocell Changes internal resistance with amount of light
Used in “electronic eye”
Symbol:

16. Resistor Power Ratings
Resistors convert electrical energy into thermal energy (heat)
As current is passed through a resistor it gets hot
The power rating of a resistor indicates how much power the resistor can dissipate without being destroyed
There is no relationship between the resistance value and the power rating of a resistor
The proper power rating for a resistor is determined by the circuit designer
Power rating is displayed by a resistor’s physical size (printed on ceramic wire resistors)

17. Review What are the two major classifications of resistors?
What does one ohm equal?
As resistance increases, current…
What is the relationship between r-value and power rating? … between power rating and size? … between r-value and size?
What is the difference between a rheostat and a pot?