00-Basic Electronics - Concepts Review (1)

Bilal Ashraf Awan - Basic Electronics 1

BASIC ELECTRONICSReview of Basic Concepts


RESISTANCE


Resistance• The property of a substance due to which it opposes the

flow of electricity (i.e. electrons) through it.

• The unit of resistance is ohm. Symbol is Ω.

• OHM• A conductor is said to have a resistance of one ohm if it permits

one ampere current to flow through it when one volt is applied across its terminals.


Laws of Resistance• The resistance R offered by a conductor depends on the

following factors:a) It varies directly as its length.

(R ∝ l)

b) It varies inversely as the cross section of the conductor. (R 1/∝ A)

c) It depends on the nature of the material (expressed through resistivity or specific-resistance ϱ)


Resistivity / Specific Resistance ϱ • The resistance between the opposite faces of a meter

cube of that material.

• Unit: ohm-meter

R = ϱ

Current

1 m3


ResistivitiesMaterial Resistivity in ohm-meter

@ 20o C x 10-8

Aluminium 2.69

Carbon 7000

Copper 1.72

Gold 2.44

Iron 9.8

Mercury 95.8

Silver 1.64

Rubber 1016


Example:• The resistance of a Cu wire 200 m long is 21 Ω. If its

thickness is 0.44mm, calculate its specific resistance.


Example:• The resistance of a Cu wire 200 m long is 21 Ω. If its

thickness is 0.44mm, calculate its specific resistance.

• SOLUTION:• l = 200m• R = 21 Ω• Thickness = d = 0.44 mm, and A =

• ANSWER:• = 1.597 x 10-8 ohm-meter


OHM’S LAW


Ohm’s Law• The ratio of potential difference V between any two points

of a conductor to the current I flowing between them is constant, provided the temperature of the conductor does not change.

• In other words, current is directly proportional to voltage and inversely proportional to resistance.


Example

• A current of 0.2 A is passed through a coil of iron wire which has a cross-sectional area of 0.01 cm2. If the resistivity of iron is 14x108 ohm-meter and p.d. across the ends of the coil is 21 volts, what is the length of the wire?

• SOLUTION:• I = 0.2 A• A = 0.01 cm2

• = 14 x 10-8 ohm-meter

• V = 21 volts• l =?

• To find R use Ohm’s law.


Example

• SOLUTION:• I = 0.2 A• A = 0.01 cm2

• = 14 x 10-8 ohm-meter

• V = 21 volts• l =?

• To find R use Ohm’s law.

• 750 m


RESISTANCE IN SERIES & PARALLEL


Resistance in Series• Joined end to end, in series.• Current is the same in all resistors.• Voltage is different.


Resistance in Parallel• Joined in parallel.• Current is different.• Voltage is same across all resistors.


Equivalent Resistance• Resistors in Series

• Resistors in Parallel


CONVENTIONAL & ELECTRON FLOW


Fluid Theory of Electricity• Franklin (1750) proposed a fluid theory of electricity

• Electricity as an invisible fluid

• Positive Charge: If a body has more than normal share of fluid• Negative Charge: If a body has less than normal share of fluid• Current flows from positive (excess) to negative (deficiency)


Conventional Current and Fluid Theory

• Conventional Current: • Current flows from positive to negative as depicted by Fluid Theory


The Electron

• Thomson (1897) discovered electron and proved that it had a negative charge

• Current Flow is because of these electrons from negative to positive in a piece of Copper wire.


Conventional or Electron Flow?• Either flow is valid for understanding electric current.


VOLTAGE SOURCES


Voltage Sources• For any electronic circuit to work, there has to be a source

of energy.

Energy Source

Voltage Source

Current Source


Voltage Source

Voltage Source

Ideal Voltage Source

Real Voltage Source


Ideal Voltage Source• A ideal voltage source produces an output that does not

depend on the value of load resistance.

• Example: a perfect battery• The current is I = V/R = 1.2 mA


Ideal Voltage Source• A ideal voltage source produces an output that does not


• Example: a perfect battery

• Changing the resistance to 30 Ω• The current is I = V/R = 1.2 mA

• Changing load resistance does not effect the voltage source


Real Voltage Source• If the load resistance is changed to zero; then …

• The current is I = V/R = 12/0 = infinity


Real Voltage Source• If the load resistance is changed to zero; then …

• The current is I = V/R = 12/0 = infinity

• No real voltage source can produce infinite current because every real voltage source has some internal resistance

• Example:• Flashlight Battery < 1 Ω• Car Battery < 0.1 Ω• Electronic Voltage source < 0.01 Ω• Ideal voltage source = 0 Ω


Real Voltage Source• The load current has to flow through the internal

resistance of the voltage source.

Ideal Voltage Source Real Voltage Source with Internal Resistance


Stiff Voltage Source

RS < 0.01 RL

• If RL is 100 times greater than RS, we ignore the internal resistance

• Any source that satisfies this condition is known as Stiff Voltage Source


Stiff Voltage Source• Example:• Suppose a voltage source has an ideal voltage of 15 V

and an internal resistance of 0.2 Ω. For what voltage of load resistance will the voltage source appear stiff?

• SOLUTION:• For a voltage source to be stiff, the following condition

applies

RS < 0.01 RL

• Multiply RL by 100, we get

• RL = 100(0.2 Ω) = 20 Ω.

• ANSWER: As long as load resistance is greater than 20 Ω, the voltage source is stiff.


CURRENT SOURCES


Current Sources• A current source produces an output current that does not


• A Current Source has very large internal resistance.•


Current Sources Symbols

• Ideal Current Source • Real Current Source


Stiff Current Source• For a current source to be stiff, the following condition

applies:• RS > 100 RL


Stiff Current Source• Example:• In the circuit given, find out

the largest acceptable value of load resistance for the current source to be stiff.

• SOLUTION:• For a voltage source to be stiff, the following condition

applies

RS > 100 RL

• Divide RS by 100, we get

• RL (max) = (10 M Ω)/100 = 100 kΩ.


NETWORK LAWS & THEOREMS


Kirchhoff’s Laws• Kirchhoff’s Current Laws (KCL) / Point Law

• The algebraic sum of the currents meeting at a point (or junction) is zero.

• Kirchhoff’s Voltage Laws (KVL) / Mesh Law• The algebraic sum of the product of current and resistance in each

of the conductors in any closed mesh (or path) in a network plus the algebraic sum of the e.m.f.s in that path) is zero.


THEVENIN’S THEOREM


Thevenin’s Theorem• Any network, when viewed from its any two terminal points,

can be replaced by a single voltage source (Vth)in series with a single resistance (Rth).

• Thevenin’s Voltage• Voltage across the load terminals when the load resistor is open.

• VTH = VOC

• Thevenin’s Resistance• The resistance that an ohm-meter measures across the load terminals

when all sources are reduced to zero and the load resistor is open.• RTH = ROC

• To zero a voltage source, replace it by a short.


Thevenin’s Theorem• Any network, when viewed from its any

two terminal points, can be replaced by a single voltage source (Vth)in series with a single resistance (Rth).

• Thevenin’s Voltage• Voltage across the load terminals when

the load resistor is open.• VTH = VOC

• Thevenin’s Resistance• The resistance that an ohm-meter

measures across the load terminals when all sources are reduced to zero and the load resistor is open.

• RTH = ROC

• To zero a voltage source, replace it by a short.

RL

A

B

RL

A

B

RTHVTH


Example-1 (Thevenin’s Th.)

00-Basic Electronics - Concepts Review (1)

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