<P> The internal resistance of an ideal voltage source is zero; it is able to supply or absorb any amount of current . The current through an ideal voltage source is completely determined by the external circuit . When connected to an open circuit, there is zero current and thus zero power . When connected to a load resistance, the current through the source approaches infinity as the load resistance approaches zero (a short circuit). Thus, an ideal voltage source can supply unlimited power . </P> <P> No real voltage source is ideal; all have a non-zero effective internal resistance, and none can supply unlimited current . However, the internal resistance of a real voltage source is effectively modeled in linear circuit analysis by combining a non-zero resistance in series with an ideal voltage source (a Thévenin equivalent circuit). </P> <P> A near to ideal voltage source has been contemplated with a balanced post office box (PO box) network consisting of four diodes sourced by a balanced supply from centre tapped transformer and capacitive filter with ground open thus creating a positive and negative alternating reference level . If one of the points of equal potential of the balanced PO box is supplied with a non ideal voltage source the other point (opposite) raise to the same potential at virtual ground condition and this raised voltage at this point of virtual ground creates an almost ideal constant voltage source . Like ideal constant current source, an ideal constant voltage source causes nearly zero power supplied to the load, hence reflection from load to source is minimum which causes much better representation of waveform . </P> <P> Most sources of electrical energy (the mains, a battery) are modeled as voltage sources . An ideal voltage source provides no energy when it is loaded by an open circuit (i.e. an infinite impedance), but approaches infinite energy and current when the load resistance approaches zero (a short circuit). Such a theoretical device would have a zero ohm output impedance in series with the source . A real - world voltage source has a very low, but non-zero internal resistance & output impedance: often much less than 1 ohm . </P>

Internal and external characteristics of dc generator theory