# Ideal DC Voltage Source

In this article you will learn about What are the Ideal and Stiff DC Voltage Sources? An ideal DC voltage source is a voltage source that provides the constant voltages to the load. Its simple example is a perfect battery whose internal resistance is assumed to be 0 (RS=0). Consider the following figure where an ideal DC voltage source is shown with a variable load resistance ranging from 1Ω to 10MΩ. If you connect the voltmeter across it then it will show the load voltages equal to 10V which is exactly equal to source voltages.

The output can be represented graphically as shown in the figure below:

Now consider a case where we assume that source has some internal resistance RS=1Ω. Then there will be some voltage drop across it. In order to evaluate those voltages we will apply ohm’s law V=IR. For calculating the current assume that RL=1Ω. Now the two resistances are in series with the same magnitude so voltages across them will be divided since I=10/2=5A. So voltages across RL=(5)(1)=5V. Now if you start increasing the load resistance, then the voltage drop across it will start increasing. Lets assume that RL=100Ω now. In this case I=10/101=99mA. So load voltages will be approximately equal to source voltages (9.9V). With further increase in load resistance we can see that the effect of internal resistance starts vanishing. Or simply we can say that the load voltages becomes equal to source voltages. The figure below shows the graph between load voltages and source voltages.

## Stiff Voltage Source

It can be noticed that there is a point on the graph after which the load voltages become equal to source voltages (constant). In other words, the load voltages does not come close to the ideal value until the load resistance is large enough than the source resistance. From graph it can be seen that when RL=100Ω then load voltages become equal to source voltages or when 100RS<RL or RS<0.01RL.

Stiff Voltage Source: RS<0.01 RL

This boundary gives us the following equation.

RL(min)=100 RS

Which means if load load resistance is 100 times of source resistance, then the impact of source resistance can be ignored. As long as the load resistance is greater than 100RS then the voltage source is stiff.

A well regulated power supply is a good example of stiff voltage source. This stiff voltage source graph with its condition is shown in the figure below.