Basic Electric Circuits Session3

Basic Electric Circuits – © 2020 Mouli Sankaran Email: mouli.sankaran@yahoo.com 2
Session 3: Focus
 A Passive element – Resistor
 Ohm’s Law
 Resistor Colour Codes
 Power Absorbed by Resistors

Passive Element - Resistor

Passive Element – Resistor (R)
 We have seen so for idealized active elements
◦ Independent/Dependent Voltage Sources
◦ Independent/Dependent Current Sources
 Now, let us se the simplest idealized passive element
◦ The linear Resistor
 German physicist, Georg Simon Ohm, who published
a pamphlet in 1827
◦ Which related voltage and current measured mathematically
for the first time
◦ We now call it as Ohm’s Law

Ohm’s Law

Ohm’s Law
 Ohm’s law states that the voltage across conducting
materials is directly proportional to the current
flowing through the material
◦ v = R * i where R is >= 0
 Where the constant of proportionality R is called the
resistance
 The unit of resistance is the ohm, which is 1 V/A and
customarily abbreviated by a capital omega Ω.

Resistor: Current-Voltage Relationship
Linear Resistor A Light Bulb

About Resistors
 Resistance is normally considered to be a positive
quantity
◦ Although negative resistances may be simulated with special
circuitry
 Again, it must be emphasized that the linear resistor is
an idealized circuit element
 Resistors can be easily manufactured and they are
relatively cheaper
 Voltage-current ratios of Resistors are reasonably
constant only within certain ranges of current, voltage,
or power,
◦ And depends also on temperature and other environmental
factors

About Resistors … contd.
 We usually refer to a linear resistor as simply a resistor
◦ Any resistor that is nonlinear will always be mentioned
specifically
 Nonlinear resistors should not necessarily be considered
undesirable elements
 For example, fuses for over-current protection are very
nonlinear in nature
◦ In fact, that property is exploited while using them in the
circuits
 Fuses offer close to zero resistance during its operating
range (current)
◦ It breaks down when the current exceeds the rated value

Resistor Colour Codes

Resistor Colour Code
Mnemonic: B.B. Roy of Great Britain Has Very Good Wine
Multiplier
Tolerance
Temperature
coefficient

The band closer to the lead is on the left
Tolerance band is larger than the rest!!!
Keep the tolerance band to your right
while reading the colour codes

Example 2: Reading Resistor Colour Codes
Resistor Value: 147 Ω1: Brown
4: Yellow
7: Violet
100 = 1: Black
Tolerance 0.5% : Green
The Actual value could be
Between:
147 ± 0.735 Ω

Resistor Colour Codes Table
Colour Digit Multiplier Tolerance
Black 0 1
Brown 1 10 ± 1%
Red 2 100 ± 2%
Orange 3 1,000
Yellow 4 10,000
Green 5 100,000 ± 0.5%
Blue 6 1,000,000 ± 0.25%
Violet 7 10,000,000 ± 0.1%
Grey 8 ± 0.05%
White 9
Gold 0.1 ± 5%
Silver 0.01 ± 10%
None ± 20%

Power Ratings of Resistors

Power Absorption by Resistor
 The Resistor symbol with current (i) passing through it
and voltage (v) across it
◦ v and i are given in passive sign convention
 The product of v and i gives power absorbed by the
Resistor
 The absorbed power appears physically as heat or
light and is always positive
 A (positive) resistor is a passive element that cannot
deliver power or store energy

Various Power Ratings of Resistors
(a) : Common resistor packages (less than 0.5 W)
(b) : A 560 Ω power resistor rated at up to 50W
(c) : 5% tolerance
10 Tera Ohm
(1012)

Power Absorption by Resistor
 Alternative expressions for the absorbed power by
resistor are
◦ p = vi = i2 R = v2 /R
 Every resistor along with its Resistance value (Ohm)
also has the maximum power that it can withstand
◦ Which is called the power rating of the resistor
 For example: Let us assume a Resistor of 100 Ω with power
rating of 2W, is connected across 230V of AC source by mistake
◦ i = v / R = 230/100 = 2.3 A
◦ Power to be absorbed by R = i2 R = (2.3)2 * 100 = 529 W
◦ This value is much above the rated power, resulting in flame and
smoke!!!
AC 230V 100 Ω
2 W

Session 3: Summary
 A Passive element – Resistor
 Ohm’s Law
 Resistor Colour Codes
 Power Absorbed by Resistors

References
Ref 1 Ref 2

Basic Electric Circuits Session3

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