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%
 

 


Resistor Power

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

 


Resistor Series and parallel connection

Series
Parallel

 

 
 

 

 
 
 

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 (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:


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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