Ideal Independent Sources

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Electrical Circuits, Electrical Circuits Articles

1) Ideal Independent Voltage Sources
An ideal independent voltage source is a two-terminal circuit element where the voltage across it
a) is independent of the current through it
b) can be specified independently of any other variable in a circuit.
There are two symbols for ideal independent voltage source in circuit theory:

Symbol for Constant Independent Voltage Source

General Symbol for Independent Voltage Source

The battery cell symbol is usually used for constant voltage sources but the other one is more general and can be used for both variable and constant voltage sources.
The v-i plot for an ideal independent voltage source is shown below.

v-i plot for ideal independent voltage source

In a circuit, voltage across elements which are parallel with voltage sources are equal to the voltage of the corresponding voltage sources. This is a useful rule in solving circuits. For example, check out the following problem.

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2) Ideal Independent Current Sources
In contrast to ideal independent voltage sources, an ideal independent current source is a two-terminal circuit element where the current passing through it
a) is independent of the voltage across it
b) can be specified independently of any other variable in a circuit.
There is one symbol for ideal independent current source in circuit theory:

Symbol for Ideal Independent Current Source

The v-i plot for an ideal independent current source is depicted below.

v-i plot for ideal independent current source

Similar to voltage sources, the charactristic of ideal independent current sources can also be used to solve circuits (or a portion of a circuit) where an element is in series with a current source. Lets consider the following solved problem.

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Comments

27 responses to “Ideal Independent Sources”

November 3, 2010

khurram abbas

what is meant by loop analysis?

Reply
1. November 3, 2010
  
  Dr. Yaz Z. Li
  
  loop analysis (also known as mesh analysis) is an alternate to the nodal analysis. Both are used to solve circuits. In the loop analysis, the main unknowns are loop currents and other unknowns are written in terms of loop currents. KVL is used to obtain a set of equations where all unknowns are loop currents and by solving the set, loop currents can be found. A simple solved problem (one loop) is posted here. I am planning to post more and publish an e-book for mesh analysis as the one that I have published for the nodal analysis.
  
  Reply
November 30, 2010

Gajendrasingh Rajput

What about dependent sources?

Reply
1. November 30, 2010
  
  Dr. Yaz Z. Li
  
  The value of a dependent sources is proportional to another voltage or current in the circuit. For example, if a dependent voltage source is $2V_{x}$ this means that the voltage of that source is two times the voltage labeled as $V_x$ in the circuit. Solving circuits with dependent sources are usually trickier than the one without.
  
  Reply
November 30, 2010

Gajendra Rajput

how to convert circuits contining idependent and dependent sources

Reply
1. November 30, 2010
  
  Dr. Yaz Z. Li
  
  We do not convert dependent sources to independent ones in analyzing circuits. Instead, we try to find the variable that the dependent source depends on, with respect to main unknowns and knowns, and use it in the analysis.
  
  Reply
December 20, 2010

arnoldseeker

if a branch has only a voltage source connected to it then the resistance is not defined
how can i analyze this branch “nodaly”?!

Reply
1. December 21, 2010
  
  Dr. Yaz Z. Li
  
  If one of the voltgae source terminals is connected to the reference node, the voltage of the other node can be easily determined by the voltage source. Please download my e-book, this is thoroughly explained there. You may need to use supernode, if none of the terminals of the voltage source is the reference node.
  
  Reply
December 20, 2010

arnoldseeker

sorry but why cant we use the superposition principal in the case of dependent sources analysis ?!

Reply
1. December 21, 2010
  
  Dr. Yaz Z. Li
  
  In superposition, you have to turn off all sources except one and find the summation of the contributions. If you keep a dependent source on and turn off all other sources, the output will be zero. Superposition is based on considering the circuit as a system with inputs and outputs. Inputs are independent sources and outputs are unknowns that we want to determine. Dependent sources are not inputs. They simply relates some voltages/currents to other voltages/currents. In other words, they are part of the system itself, similar to a resistor.
  
  Reply
January 8, 2011

reddy

nice work .. its so clear and btw thanks

Reply
January 8, 2011

reddy

nice work……..

Reply
June 20, 2011

abhilash sharma

how to treat the current sources while solving circuits also the voltage sources as well ?

Reply
July 9, 2011

chacha

what can you say about DC resistance of the diode over this linear portion?the linear portion is a silicon diode

Reply
1. July 11, 2011
  
  Dr. Yaz Z. Li
  
  Which portion?
  
  Reply
September 5, 2011

anup

sir, what will be the time constant for series and parallel RLC ckt?

Reply
January 23, 2012

shaurya

what exactly are current sources?I was told that current sources are the one which supply current in a circuit and voltage sources are the one that supply potential difference across a circuit.What I am confused with is that the current sources should have a potential difference across them as no current can be drawn without pot difference,so a current source should also be a voltage source,then whats is the difference between current and voltage sources.
Thank You

Reply
1. January 23, 2012
  
  Dr. Yaz Z. Li
  
  The difference is that a voltage source keeps the voltage constant but a current source keeps the current constant. So, a voltage source does not care about the current, its duty is to keep the voltage as required. The current passing through a voltage source can be negative, zero or positive, it may vary by time but the voltage is constant for sure. A current source is the dual. Voltage across a current source can be positive, zero or negative or it can vary by time but the current is fixed. Do not try to find a simple physical element which matches with a current source. For a voltage source, a battery is a perfect match but there is no such a element which matches current sources that perfect. Some elements behave like current sources in some operating point.
  
  Reply
March 2, 2012

sumi

Sir,
I have doubts regarding testing of power supplies with a constant voltage source (o- 30V)and constant current source (0 -25A). .When I connect a shunt of 50A and 75mv across a constant current source and vary from 1A to 20A I get the required mV across the shunt. What should the reading on the constant voltage source read?
Can you answer me on this email
sumi

Reply
May 13, 2013

vimala

can dependent and independent voltage sources be added to one

Reply
November 21, 2013

ganesh

sir,current is nothing but flow of electrons.that is current is produced by voltage but voltage source and current source are used in a circuit at a time please explain.

Reply
1. November 21, 2013
  
  Yaz
  
  Please refer to definition and do not mix up physical power sources with ideal independent sources which are models. You can model power sources with independent sources plus other elements depend on their operating point. As I said, for now think about ideal independent sources as models and do not take them equivalent to batteries or other physical sources.
  
  An ideal independent voltage source is a two-terminal circuit element where the voltage across it
  a) is independent of the current through it
  b) can be specified independently of any other variable in a circuit.
  
  Reply
November 21, 2013

ganesh

current is produced by voltage .but why current source is indepedent of voltage as well as conversly

Reply
1. November 21, 2013
  
  Yaz
  
  current passing through a 2V voltage source can be zero, 1A, 2A, 1000A, … but its voltage is always 2V. So, voltage is independent from current.
  
  Reply
November 5, 2014

dushyanth

Current source has infinity resistance right?
But in the problems above current seems to flow through it how is it possible?

Reply
1. December 14, 2014
  
  Yaz
  
  That way of looking on sources could be misleading. Current source keeps current passing through constant equal to the source current value. Of course current passes through a current source.
  
  Reply
July 4, 2015

zaid khan farar

sir whats about to solve the configuration if one resistor is series with other parallel resistor ……how can we solve it through current divider rule???

Reply