Chapter 3: Resistors

Question 1

Factors affecting resistance

Answer 1

Resistivity
Length
Cross-sectional area

R = ρl/A

Question 2

American-wire gauge (AWG)

Answer 2

AWG number denotes the diameter of the wire

Larger AWG number, small diameter

Question 3

Fixed resistor

Answer 3

Carbon resistors

Wire wound resistors

Question 4

Carbon resistor

Answer 4

• Consists of a ceramic rod coated with a layer of carbon film
• To obtain a certain resistance, the carbon film is cut to produce a spiral-conducting path

Adv: inexpensive + easy to mass produce
Disadv: Varied resistance due to temperature

Used in low-power applications
Low-temperature stability
Power rating: 0.1 to 2 W
Low cost

Question 5

Wire wound resistor

Answer 5

Made by winding a thin wire such as nichrome around a cylinder that is made out of an insulating materials

Adv: high power rating
Disadv: Produces a lot of heat -> comes with cooling fins to dissipate the heat

Used in high-power applications
High-temperature stability
Power rating: 2 to 500 W
High cost

Question 6

Variable resistor

Answer 6

Used to adjust the volume control or dimmer to set the level of lighting

Types of variable resistors:

• Potentiometer
• Rheostats

Question 7

Values of resistors

Answer 7

Resistor colour code:
1st value: 1st digit
2nd value: 2nd digit
3rd value: multiplier
4th value: tolerance %

A multimeter can be used to measure the resistance of a resistor

Question 8

E24 Series

Answer 8

When choosing a suitable resistor from the E24 series, it is important to choose the value higher than the computed resistance to ensure that the current stays within the limit.

Question 9

Effective resistance in a circuit

Answer 9

Series: Rtotal = R1 + R2
Parallel: Rtotal = 1/R1 + 1/R2

Question 10

Power ratings

Answer 10

When choosing power ratings,

1. Calculate power based on V,I,R
2. Choose power rating from 0.25, 0.5, 1 or 2W that it is 2 times more than the calculated power

The size of the resistors increases with the power rating