# Electronic Devices – Resistor Resistor is a passive two terminal electronic components. It function is to limit or reduce(resist) the flow of current or divide the voltage in a circuit. The unit of the resister is ohm(Ω). The resistance r of any material is proportional to its length and inversely proportional to its area of cross section A.

R=ρl/A

ρ=specific resistance or resistivity of the material. # Resistor Symbol # Resistor Type

• Fixed Resister
• Wire wound resistor
• Carbon film resistor
• Metal film resistor
• Carbon composition resistor
• Carbon resistor
• Film type resistor
• Chip resistor
• Variable Resister
• Rheostat
• Potentiometer
• Trimmer
• Preset

# Resistor Color Code

 Color Digit value Multiplier Multiplied Out Tolerance Black 0 100 1 Brown 1 101 10 Red 2 102 100 Orange 3 103 1,000 Yellow 4 104 10000 Green 5 105 100,000 Blue 6 106 1,000,000 Violet 7 107 10,000,000 Gray 8 108 100,000,000 White 9 109 1,000,000,000 Gold ±5% Silver ±10%

# 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.

# Resistivity properties of materials

 Material ρ (Ωm) at 20°C σ (S/m) at 20°C Temperature coefficient (1/°C) x10^-3 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 3.5×107 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 Lithium 9.28×10−8 1.08×107 6 Iron 1.0×10−7 1.00×107 5 Platinum 1.06×10−7 9.43×106 3.9 Tin 1.09×10−7 9.17×106 4.5 Lead 2.2×10−7 4.55×106 3.9 Manganin 4.82×10−7 2.07×106 0.002 Constantan 4.9×10−7 2.04×106 0.008 Mercury 9.8×10−7 1.02×106 0.9 Nichrome 1.10×10−6 9.09×105 0.4 Carbon (amorphous) 5×10−4 to 8×10−4 1.25 to 2×103 -0.5

# Resister Application

• 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).
ANS=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)
ANS=8.254×10-5m2.

## Example 3

if a R = 300 ohm resistor is attached
across the terminals of a V = 12 volt battery, then a current  of (V/R)12 / 300 = 0.04 amperes flows
through that resistor.

## 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?
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?
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? 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? 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? 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

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