An ideal voltage source has no internal resistance, that is it supplies a voltage \(U_0\) (called source voltage) which is independent of which load resistor \(R\) is connected to the terminals of the voltage source. That means the voltage at the resistor \(R\) is always the voltage \(U_0\). Look at the following Ohm's law:

Formula anchor$$ \begin{align} U_0 = R \, I = \text{const.} \end{align} $$

To keep the voltage \(U_0\) constant, the current \(I\) may become arbitrary - depending on the choice of resistor. This can result in very high currents that damage the circuit.

A practical voltage source, on the other hand, has an internal resistance \(R_{\text i}\), which limits the current \(I\) through the load resistor. The application of the Kirchhoff's voltage law results in a voltage \(U\) at the resistor \(R\), which does not necessarily have to correspond to the source voltage \(U_0\) anymore:

Formula: Voltage of a practical voltage source

Formula anchor$$ \begin{align} U ~=~ U_0 - R_{\text i} \, I \end{align} $$

So that a real voltage source corresponds as far as possible to the ideal voltage source, the internal resistance \(R_{\text i}\) must be chosen as small as possible, so that the second term in 2 is almost zero: \( U \approx U_0 \).

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