Circuit Analysis - Methods, Laws & Theorems

This tutorial is intended to provide detailed information about the circuit or network analysis, methods of analysis and network theorems. It is just an introductory article, in which I gather all the laws, methods and theorems. This topic is usually a part of the ECE basic course on circuits. The term solving a circuit means determining all the voltage across each element and currents through each element present in a circuit.

Key Concepts:

• What are fundamental laws for solving circuits?
• Methods used for network/circuit analysis
• Network theorems
• What is the purpose of network theorems or why do we study network theorems

Fundamental Circuit Laws:

To determine current and voltage in an electric circuit, we need to gain knowledge of fundamental laws that govern electronic circuits.

1. Ohm's law
2. Kirchhoff's Voltage Law (KVL)
3. Kirchhoff's Current Law (KCL)

Ohm's Law defines a linear relationship between voltage and current in an ideal conductor. This is one of the most important and fundamental laws of electric circuits.

Kirchhoff’s Current Law is used when analysing a parallel circuit. It is based on the idea of conservation of charge. According to KCL 'current entering the node is equal to current leaving the node’.

Kirchhoff's Voltage Law is used when analysing a series circuit. It is based on the idea of conservation of potential. According to KVL 'the sum of voltage drop or potential difference across a closed loop is zero’.

Methods of Analysis:

Analysis of a circuit is the determination of output response. These methods of analysis are based on basic circuit laws. These methods are restricted to linear circuits only. The two most common methods are

1. Mesh analysis
2. Nodal analysis

Nodal analysis is nothing but an application of KCL. In the nodal analysis, we apply KCL to each node. If we have 'n’ number of nodes, then there are (n-1) linearly independent node equations. These equations are in terms of node potential.

Mesh | loop analysis is nothing but an application of KVL. In mesh analysis, we apply KVL around each loop in a circuit. If we have 'b’ branches and 'n’ nodes in a circuit then we have b-(n-1) linearly independent equations. These equations are in terms of mesh currents.

Why do we study network theorems?

I suppose you are familiar with basic network analysis methods. Like node analysis and mesh analysis. When dealing with KCL and KVL we will have a fairly large number of independent equations. As I discussed in methods of analysis, we get (n-1) node equations and b-(n-1) mesh current equations, so total ‘b’ number of equations. Did you find them time-consuming, you have to deal with so many equations (the number of node and mesh equations are equal to the number of elements/branches present in a network). For example, you have a network/circuit having 4 passive elements, then there are 4 current or voltage equations. And if there are 5 passive elements you have to solve 5 equations and so on. Analysing more complex circuits by using these methods seem to be ridiculous. What do you think?? So instead of applying tedious mesh and nodal analysis to complex networks, we develop network theorems.

Network theorems

1. Superposition theorem
2. Reciprocating theorem
3. Thevenin’s theorem
4. Norton's theorem
5. Compensation theorem
6. Millman's theorem
7. Substitution theorem
8. Maximum power transfer theorem
9. Tellegen's theorem

Applications

• Some of these apply to linear as well as non-linear circuits
• With the help of these theorems, we can solve DC as well as AC Circuits
• Easy comprehension of complex circuits

Duality In Electric Circuits

It is interesting to know how systems relate to one another. How a mechanical system can be modelled as an electrical system and observed. The concept of duality in electrical circuits is of great importance. Two phenomena are said to be dual if they can be expressed by same form of mathematical equations. This topic is usually covered under the network topology or graph theory.

Key Questions:

• What is principle of duality in electric circuits?
• List of dual pairs and their explanation
• Formation of dual networks

Principle of Duality:

Principle of duality in context of electrical networks states that

• A dual of a relationship is one in which current and voltage are interchangeable
• Two networks are dual to each other if one has mesh equation numerically identical to others node equation

List of Dual Pairs:

For evaluating a dual network, you should follow these points

1. The number of meshes in a network is equal to number of nodes in its dual network
2. The impedance of a branch common to two meshes must be equal to admittance between two nodes in the dual network
3. Voltage source common to both loops must be replaced by a current source between two nodes
4. Open switch in a network is replaced by a closed switch in its dual network or vice versa

	Elements	Dual Elements
1	Voltage (v) v = iR	Current (i) i = vG
2	Short Circuit	Open Circuit
3	Series	Parallel
4	Norton	Thevenin
5	Resistance (R)	Conductance (G)
6	Impedance	Admittance
7	KVL	KCL
8	Capacitance	Inductance

Formation of Dual Networks:

The principle of duality is applicable to planar circuits only. Carefully read the points stated below, follow each step and draw the dual circuit

1. Place a dot within each loop, these dots will become nodes of the dual network
2. Place a dot outside of the network, this dot will be the ground/datum node of the dual network
3. Carefully draw lines between nodes such that each line cuts only one element
4. If an element exclusively present in a loop, then connect the dual element in between node and ground/datum node
5. If an element is common in between two loops, then dual element is placed in between two nodes
6. Branch containing active source, consider as a separate branch
7. Now to determine polarity of voltage source and direction of current sources, consider voltage source producing clockwise current in a loop. Its dual current source will have a current direction from ground to non-reference node

Example#01: Draw dual of the given circuit.

Graphical method of drawing dual network

Example#02: Draw dual of the given circuit 

Reference:

1. Fundamentals of Electric Circuits by Alexander
2. Circuit Theory by A.V. Bakshi and U.A. Bakshi

Network Equilibrium Equations - Electrical Network Graphs

This is my fifth article on electrical network graphs. The article covers the topic  network equilibrium equations. Network equilibrium equations completely determine the state of the network at any moment.

Key Concept

• Generalized form of network equilibrium equations for circuits having sources

Branch Current and Loop Current:

Let IB be the branch currents vector and IL be the loop currents vector. BT is the transpose matrix of the fundamental loop matrix or tie set matrix. Then IB branch current can be written as follows

IB = BT.IL

Nothing is new. Actually we have to analyse the network. In actual network if there is a current source in parallel with a passive element let's consider a resistor for simplicity. Then the total branch current will either be the sum or difference of the two currents.

• If current source and resistor current is in same direction, then branch current is the difference of two currents. IB = IS - IR
• If the current source and resistor current is in opposite direction, then branch current is the sum of two currents. IB = IS + IR

In this case the relationship between branch current and loop current can be modified as follows

IB = BT.IL + IS

Where,

Is is the column matrix of order bx1 representing current source across each branch.

Branch Voltage and Node Voltage:

VB = QTVn

Where

VB = branch voltage matrix

QT = transpose of fundamental cut set matrix

Vn = node voltage matrix

Let's consider, a voltage source is present in series branch, then total voltage across this branch will be algebraic sum of source voltage and branch voltage.

• If voltage source in series with a branch with similar polarity then total branch voltage will be the sum of two voltages. V = VS + Vb
• If voltage source in series with a branch with opposite polarity then total branch voltage will be the difference of two voltages. V = VS - Vb

So the above relation between node and branch voltages can be modified as

VB = QT Vn + VS

Where,

Vs = column matrix of order bx1 representing voltage sources in series with each branch

Generalized Form of Network Equilibrium Equations:

I am not going to formulate the network equilibrium equations, I am providing reference books, from where you can get proof of these equations.

Node Equation

YVn = A[YbVs - Is]

The above equation represents (n-1) node equations

Where

Y = AYbAT  is the nodal admittance matrix. This is (n-1) X (n-1) matrix.

A = Reduced incidence matrix

Yb = branch impedance matrix, order (bxb)

Loop Equation

ZIL = B[ZbIs - VS]

The above equation represents (b+n-1) loop equations

E = Z IL

Where,

E = B(VS - ZBIS)

Z = BZbBT is the loop impedance matrix of order (b-n+1)X(b-n+1) matrix

B = fundamental loop or tie set matrix

Zb = is the branch impedance matrix, order (bxb)

E = column matrix of order (b+n-1)x1 representing loop emf

Cut-Set Equation

YCVt = QC[YbVS - IS ]

The above equation represents (n-1) cut-set equations.

J = YC Vt

Where

J = QC(IS - YbVS)

YC = QCYbQCT , is Cut-Set impedance matrix of order (n-1) X (n-1)

QC = fundamental cut-set matrix

Vt = tree voltages

References:

1. Electrical Network Analysis and Synthesis
   By U.A.Bakshi, A.V.Bakshi

Network Analysis And Synthesis
By J.S.Chitode Dr.R.M.Jalnekar

Fundamental Cut Set Matrix - Electrical Network Graphs

This is my fourth article on an electrical network graph. The article is about the cut-set matrix and how it is useful in determining branch voltages and node voltages. As the name implies cut set is a set of branches that when removed from the graph results in an unconnected graph.

Key Questions:

• What is a cut set and a fundamental cut set?
• Procedure for obtaining cut sets of a graph and writing matrix elements
• Fundamental cut set matrix and KVL
• Fundamental cut set and KCL

Cut Set and Fundamental Cut Set:

Cut set: It is a set of branches that when cut or removed from a graph, separates the graph into two groups of nodes or the graph splits into two isolated graphs. It is a minimum set of branches of a connected graph, such that if this set of branches is removed, it will reduce the rank of the graph. And also results in an unconnected graph.

• Cut set defines the minimum set of branches C, if this set of branches removed from the graph results in an unconnected graph.

• If anyone branch from a set of branches C is removed, we still get a connected graph.

Fundamental cut set: It contains only one twig and one or more links.

Procedure For Obtaining Fundamental Cut Set:

As discussed above fundamental cut set contains only one twig and one or more links. To obtain a fundamental cut set follow these steps.

• Select an oriented graph
• Select a tree and mark the links as well with the dotted line
• The number of twigs is equal to the number of cut sets
• Remove one twig and necessary links to get the fundamental cut set
• Repeat the above procedure for all twigs

Fig 1 graph and its tree

Fig 2 Fundamental Cut Sets

Have a closer look at fig 2.

• F-cut set C1 is obtained by removing a twig 5 and a link 3. The direction or orientation of C1 is in the same direction of twig 5
• F-cut set C2 is obtained by removing twig 6 and links 4 and 3. The orientation of C2 is the same as the direction of twig 6
• F-cut set 3 C3 is obtained by removing twig 1 and links 2 & 4. The orientation of C3 is the same as the direction of twig 1

Fundamental Cut Set Matrix (Q):

It describes the branches contained in the cut set and their orientation.

In the cut-set matrix, each row represents a cut set and columns represent branch voltages.

Order of Fundamental Cut Set Matrix:

The number of fundamental cut sets is equal to the number of twigs that is N-1.

B is the total number of branches.

Order of fundamental cut matrix becomes (N-1)xB

Elements of Fundamental Cut Set:

Suppose the graph has 'b’ branches, N nodes, (N-1) twigs in a tree and (N,-1) fundamental cut sets. Let 'i’ be the rows and 'j’ be the columns of the cut-set matrix.

• aij = +1 for the twig of selected fundamental cut set
• aij = +1 if the links have the same current direction as that of the selected twig
• aij = -1 if the links have opposite current direction as that of selected twig
• aij = 0 if the twigs and  are not a part of the selected fundamental cut set

Procedure For Writing Fundamental Cut Set Matrix:

Consider first f-cut set C1

• Branch 5 (twig) = +1
• Branch 3 (link) = -1

Consider second f-cut set C2

• Branch 6 (twig) = +1
• Branch 4 (link) = -1
• Branch 3 (link) = +1

Consider third f-cut set C3

• Branch 1 (twig) = +1
• Branch 2 (link) = +1
• Branch 4 (link) = +1

Fig 3  Fundamental Cut Set Matrix

Fundamental Cut Set Matrix and KCL

Q.Ib = 0

Where Q = cut set matrix

           Ib = branch current vector

Fig 4 KCL and Cut Set Matrix

          -i3 + i5 = 0

          +i3 - i4 + i6 = 0

          i1 + i2 + i4 = 0  

Fundamental Cut Set Matrix and KVL

Vb = QT.Vt

Where Vb = branch voltage vector

          QT = transpose of the cut-set matrix

          Vt = tree branch voltage vector

Fig 5 KVL and Cut Set Matrix

v1 = vt6

v2 = vt6

v3 = vt5 - vt1

v4 = vt6 - vt5

v5 = vt1

v6 = vt5