EE2010 Fundamentals of Electric
Circuits
Lecture - 6
Voltage Sources, Current Sources,
Mesh Analysis
Voltage Sources in Series and in Parallel
• Ideal voltage sources cannot be combined in
parallel
(unless they have the same voltage)
Voltage Sources in Series and in Parallel
• We can connect ideal voltage sources in series.
• Voltage sources in series can be reduced to a
single voltage source
Current Sources in Series and Parallel
• ideal current sources cannot be combined in
series (unless they have the same current)
Current Sources in Series and Parallel
• Current sources in parallel can be combined as
a single current source
Example
Example
Determine the current I1 and the voltage Vs for
the network
SOURCE CONVERSIONS
the equivalence between a current source and a voltage source
exists only at their external terminals.
Example
a) Determine the current IL.
b) Convert the voltage source to a current
source.
c)Using the resulting current source of
part (b), calculate the current through
the load resistor, and compare your
answer to the result of part (a).
CURRENT SOURCES IN PARALLEL
• Voltage sources of different terminal voltages
cannot be placed in parallel because of a
violation of Kirchhoff’s voltage law.
• Current sources of different values cannot be
placed in series due to a violation of
Kirchhoff’s current law.
• However, current sources can be placed in
parallel just as voltage sources can be placed
in series.
CURRENT SOURCES IN PARALLEL
• Two or more current sources in parallel can be
replaced by a single current source having a
magnitude determined by the difference of
the sum of the currents in one direction and
the sum in the opposite direction.
• The new parallel internal resistance is the total
resistance of the resulting parallel resistive
elements.
Example
• Reduce the parallel current sources as shown in Fig. to
a single current source.
Solution:
The net source current is
I = 10 A - 6 A = 4 A
with the direction of the larger.
The net internal resistance is the parallel combination of resistors,
R1 and R2:
Example
• Reduce the parallel current sources in Fig. to a single
current source.
Solution: The net current is
MESH ANALYSIS
• The currents to be defined are called mesh or loop
currents
• A closed“loop” around the inside of each window; these
loops are similar to the loops defined in the wire mesh
fence in Fig.
• The number of mesh currents required to analyze a network will
equal the number of “windows” of the configuration.
MESH ANALYSIS
Mesh Analysis Procedure
1. Assign a distinct current in the clockwise
direction to each independent, closed loop of
the network.
2. Indicate the polarities within each loop for each
resistor as determined by the assumed direction
of loop current for that loop.
3. Apply Kirchhoff’s voltage law around each
closed loop in the clockwise direction.
4. Solve the resulting simultaneous linear
equations for the assumed loop currents
EXAMPLE
Solution:
• Step 1: Two loop currents (I1 and I2) are assigned in the
clockwise direction in the windows of the network.
• Step 2: Polarities are drawn within each window to agree with
assumed current directions.
• Step 3: Kirchhoff’s voltage law is applied around each loop in
the clockwise direction.
EXAMPLE
loop 1: + E1 - V1 - V3 = 0
(clockwise starting at point a)
EXAMPLE
EXAMPLE
EXAMPLE
Supermesh Currents
• Start as before and assign a mesh current to
each independent loop, including the current
sources, as if they were resistors or voltage
sources.
• Then mentally (redraw the network if
necessary) remove the current sources
(replace with open-circuit equivalents), and
apply Kirchhoff’s voltage law to all the
remaining independent paths of the network
using the mesh currents just defined.
Supermesh Currents
Example
Node a is then used to relate the mesh currents and the current
source using Kirchhoff’s current law: