Resistor

Color Code

Color	Digit Value	Multiplier	Multiplied Out	Tolerance
Black	0	100	1
Brown	1	101	10	±1%
Red	2	102	100	±2%
Orange	3	103	1,000
Yellow	4	104	10,000
Green	5	105	100,000	±0.5%
Blue	6	106	1,000,000	±0.25%
Violet	7	107	10,000,000	±0.1%
Gray	8	108	100,000,000	±0.05%
White	9	109	1,000,000,000
Gold		10-1	0.1	±5%
Silver		10-2	0.01	±10%

Power

Measuring power across a resistor (Ohm’s Power Law)

Ohms Law

Ohm’s law describes the fundamental behavior of resistors.

LED Current Limiting

For example, assume you have a 9V battery to power an
LED. If your LED is red, it might have a forward voltage around 1.8V. If you
want to limit the current to 10mA, use a series resistor of about 720Ω.

Kirchhoff’s Law

Kirchhoff’s law can be used to analyses networks of resistors.

Resistor networks

Voltage Dividers

Resistivity properties of materials

Material	ρ (Ω·m) at 20°C	σ (S/m) at 20°C	Temperature Coefficient (×10−3 /°C)
Silver	1.59×10−8	6.30×107	3.8
Copper	1.68×10−8	5.96×107	3.9
Gold	2.44×10−8	4.10×107	3.4
Aluminum	2.82×10−8	3.50×107	3.9
Tungsten	5.60×10−8	1.79×107	4.5
Zinc	5.90×10−8	1.69×107	3.7
Nickel	6.99×10−8	1.43×107	6.0
Lithium	9.28×10−8	1.08×107	6.0
Iron	1.00×10−7	1.00×107	5.0
Platinum	1.06×10−7	9.43×106	3.9
Tin	1.09×10−7	9.17×106	4.5
Lead	2.20×10−7	4.55×106	3.9
Manganin	4.82×10−7	2.07×106	0.002
Constantan	4.90×10−7	2.04×106	0.008
Mercury	9.80×10−7	1.02×106	0.9
Nichrome	1.10×10−6	9.09×105	0.4
Carbon (amorphous)	5×10−4 – 8×10−4	1.25×103 – 2×103	−0.5

Application of Resistors

• variable resister is used in volume control in
  the TV application,
• LED Current Limiting

Examples

Example 1

find the resistance of a copper wire of 0.75km long and having a cross sectional area of 0.01cm².
(take ρ=1.72×10^-8ohm-m).

Answers

12.9Ohm

Example 2

find the cross sectional area of an aluminium wire of 700m long and having a resistance of 0.24ohm (take ρ=2.83×10^-8ohm-m)

Answers

8.254×10^-5m².

Example 3

 if a R = 300 ohm is attached across the terminals of a V = 12 volt battery, then How much current flow throw?

Answers

(V/R)12 / 300 = 0.04 amperes flows through that.

Example 4 (Ohm’s law equations)

Consider a 1-ohm resistor in a circuit with a voltage drop from 100V till 10V across its terminals. What is the current through the resistor?

Answers

The triangle reminds us that:

Example 5 (Ohm’s law equations)

Consider a 10-ohm resistor in a circuit subject to a current of 2 Ampere and a voltage of 120V. What is the voltage drop across the resistor?

Answers

 Using the triangle shows us that:

Thus the voltage at the end terminal is 120-20
= 100 V.

Example 6 (Ohm’s Power Law)

What must be the minimal power rating of this resistor?

Answers

According to the wheel, P=I^2*R= 0.100^2*50=0.5 W. So the minimal power
rating should be at least 0.5W, but recommended is to go high above this value
for extra reliability and lifetime.

Example 7 (Ohm’s Power Law)

What is the current in the circuit?

Answers

This is a basic example of Ohm’s law. Voltage and resistance are known,
so we can calculate current with the equation:
I=V/R=6/1.2=5 A.

Example 8 (Ohm’s Power Law)

An electric heater (resistor) with a consumption of 1kW is connected in a circuit with 8A current. What is the voltage drop over the heater?

Answers

Voltage can be expressed in current and power with the formula: V=P/I= 1000/8=125 V

Video tutorial

Basic Electricity – Resistance and Ohm’s law