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Electronic Devices – Resistor

The 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 Ω

ρ=specific resistance or resistivity of the material.

Symbol

Type

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

Color Code

ColorDigit valueMultiplierMultiplied OutTolerance
Black01001 
Brown110110 
Red2102100 
Orange31031,000 
Yellow410410000 
Green5105100,000 
Blue61061,000,000 
Violet710710,000,000 
Gray8108100,000,000 
White91091,000,000,000 
Gold   ±5%
Silver   ±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

Materialρ (Ωm) at 20°Cσ (S/m) at 20°CTemperature coefficient (1/°C) x10^-3
Silver1.59×10−86.30×1073.8
Copper1.68×10−85.96×1073.9
Gold2.44×10−84.10×1073.4
Aluminum2.82×10−83.5×1073.9
Tungsten5.60×10−81.79×1074.5
Zinc5.90×10−81.69×1073.7
Nickel6.99×10−81.43×1076
Lithium9.28×10−81.08×1076
Iron1.0×10−71.00×1075
Platinum1.06×10−79.43×1063.9
Tin1.09×10−79.17×1064.5
Lead2.2×10−74.55×1063.9
Manganin4.82×10−72.07×1060.002
Constantan4.9×10−72.04×1060.008
Mercury9.8×10−71.02×1060.9
Nichrome1.10×10−69.09×1050.4
Carbon (amorphous)5×10−4 to 8×10−41.25 to 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).

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 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.


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:


Answerer
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?


Answer:
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

Reference Website

Video tutorial

Basic Electricity – Resistance and Ohm’s law

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