Monday, August 27, 2012

Direct current 5 (voltage & current sources)

Voltage source & current source

In this post we wil discuss about both ideal and practical voltage sources and current sources. First we will discuss about voltage source. But before that we have to study about the internal resistance to understand the different of a ideal and practical source.

Internal resistance:

The materials which are used to made the source are not pure conductors (as they are not ideal) . Therefore there exists a resistance. due to this resistance, a voltage drop occurs in the source itself. So when there is a current flow from the battery, battery itself drop some voltage and rest  is feed to the circuit. This resistance is called 'Internal Resistance' of a source and it can be shown as a series connected resistance with the source.

E.M.F : 

We saw that there exists a voltage drop in the source itself and it changes with the current draw from the source. So the voltage across the source changes with the current. When there is no current flow from source, there is no internal voltage drop. The voltage across the source in this time, is called the E.M.F value of the source.

Voltage source :

Ideal Constant - Voltage Source: 

When there is no internal resistance , the output voltage of the voltage source remains constant whatever the change in load current as there is no internal voltage drop. Such a voltage source is called an 'Ideal voltage source' . In practice , none such ideal constant voltage source can be obtained. But, smaller the internal resistance of a voltage source, closer it comes to an ideal voltage source. 

Practical Voltage source:

All the voltage sources that we are using has an internal resistance. Therefore we have to make sure that we are adding the internal resistance to the account when we are doing calculations. 

Current source: 

Ideal Constant - Current Source :

When the internal resistance is infinity in a source, the internal voltage drop goes to infinity and there is no output voltage. It gives a constant current as the output.

Practical Current Source :

 In practical current source ,internal resistance is very high compared to the external resistance of the circuit    but not infinite.
 Consider 6V battery with Mega Ohm internal resistance and a load resistance with 20K to 200K. 

when load is 20K,

from ohm's law,
I = (V/R) 
I = (6V / 1.02MOhm)
I = 5.9 micro A

when load is 200K

from Ohm's law
I = (V/R) 
I = (6V / 1.2MOhm)
I = 5 micro A

you can see though the load resistance increases 10 times, current is only varying by 0.9 micro Amperes. Such a small value is negligible and therefore we can consider this source is a constant current source. 

Pabindu lakshita
B. Sc (Engineering undergraduate)

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