Electric Fitting (New)

Engineering Practice

Resistance

• Electrical resistance is the

ratio of voltage drop across a resistor to current flow through the resistor.

• Polarities are governed by

the passive sign

convention.

R + v -i

v

R

i

Current = voltage / resistance

• I = V / R V = I x R

Definitions

• Voltage = potential energy / unit

charge, units = Volts

• _{Current = charge flow rate, units =}

Amps

• Resistance = friction, units = Ohms

Example

• Voltage drop when current flows

through resistor

• V₁ - V₂ = I R

I R

V₁

V₂

Power Consumed by Resistors

• Resistors

consume power.

• _{v and i are both}

positive or both negative.

R + v -i

p v i

 

v R i

 

v

i

Creating a Circuit Model

• A circuit model is usually two or more circuit

elements that are connected.

• _{A circuit model may have active elements}

(sources) as well as passive elements (such as resistors).

• _{By the assumption that electric signal}

propagation is instantaneous in a circuit, our circuit model has lumped parameters

(A lumped system is one in which the

dependent variables of interest are a function of time alone. In general, this will mean

Schematics

• _{Symbols represent circuit elements} • Lines are wires

+

Battery

Resistor

Ground

+

V

R I

Sample circuit

Example of a Circuit

Model

1000 ft AWG 14 Copper Wire

100 W Lamp

120 V Battery

120 V

0.25  _2.57 

2.57 

Kirchhoff’s Voltage Law

• _{The sum of the voltage drops around a closed}

path is zero.

• Example: -120 + V₁ + V₂ + V₃ + V₄ = 0

120 V

0.25  _2.57 

2.57 

144 

+ V₁ - + V₂

-- V₄ + + V₃

-Kirchhoff’s Current Law

• A node is a point where two or more circuit

elements are connected together.

• The sum of currents associated with a node is

zero.

I₁

I₂ I₃

Parallel And Series Resistors

Series

• Same current flows

through all Parallel

• Same voltage across all

+

Note: these points are connected together

I V R1

R₂

Series circuit

V = R₁ I + R₂ I = R_eff I R_eff = R₁ + R₂

Parallel circuit

I = V/R₁ + V/R₂ = V/R_eff 1/R_eff = 1/R₁ + 1/R₂

+

V

R_{1 I} R2

1 I2

Resistive voltage divider

• Series resistor circuit

• Reduce input voltage to

desired level

• _Advantages:

– _{simple and accurate}

– complex circuit can use

single voltage source

• Disadvantage:

– dissipates power – easy to overload – _{need R}

load << R2

New schematic symbol external connection + V_in R₁ R₂ I I V_out Resistive divider I = V_in/R_eff = V_out/R₂

Variable voltage divider

• _{Use potentiometer (= variable resistor)}

• Most common: constant output resistance

+

V_in

R_var

R_{out I} I

V_out

Variable voltage divider

V_out = V_in (R_out / (R_var + R_out) ) New schematic symbol:

Impedance

Impedance

:

• Definition : Impedance is the current resisting and impeding characteristic of load or

conductor in an AC Circuit.

•

Symbol for Impedance: Z

Z = R + jX_l - jX_c

Where, jX_l = Z_l and, -jX_c = Z_c

Revisit Ohm’s Law

Ohms Law:

•

Mathematical Statement of the Ohm’s

Law:

V = I R for DC circuits

V = I Z for AC Circuits

“GROUND”

AC-generator

• An AC generator consists of a coil wound on a

metallic core that rotates in a magnetic field.

• The ends of the coil are connected to a pair of slip

Single phase Vs Three phase

• The horsepower rating of three phase motors and

the KVA (kilo-volt-amp) rating of three-phase

transformers is about 150% greater than that for single-phase motors or transformers with a

Single phase Vs Three phase

• The power delivered by a single-phase system

falls to zero three times during each cycle.

Wye or Star connection

• The wye or star connection is made by

connecting one end of each of the three-phase windings together.

• The common point is kept as the ground or

reference.

• _{The voltage measured across a single winding or}

phase is known as the phase voltage.

• The voltage measured between the lines is known

• In a wye connected system, the line voltage is

higher than the phase voltage by a factor of 1.732. Phase current is equal to line current.

Delta Connection

• In the delta connection, line voltage is equal to

the phase voltage.

• _{The line current of a delta connection is higher}

Mains circuits

Household electric wiring plan for a flat:

mains supply cable

mains socket

cable _{ring main}

mains socket mains socket cable cable electric company’s main fuse cable consumer unit lighting circuit 30 A 5 A 15 A 30 A

main supply cable electric company’ s main fuse kilowatt-hour meter

consumer unit (‘fuse box’)

All electrical appliances are connected

in parallel.

Why?

faults in one circuit will not

affect other circuits.

mains supply cable electric company’ s main fuse kilowatt-hour meter

consumer unit (‘fuse box’)

• _{Each circuit branch has L, N & E}

wires.

•

Earth wires are connected to an

earthing electrode buried

It has a main switch:

consumer unit (‘fuse box’)

each circuit is fitted with a

fusecircuit breaker or one can switch off whole circuit

Lighting circuit

• _{1 branch for ceiling}

lamps:

• _{Protected by 5 A fuse}

5 A

lighting circuit

lighting circuit

• Each lamp is controlled by a

The lamp can be switched on at the bottom of the staircase & then switched off at the top, & vice

versa.

Two-way switches

The lamp can be switched on at the bottom of the staircase & then switched off at the top, & vice

versa.

on