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Voltage source + current source: description and differences

In order to supply circuits in electrical engineering / electronics with energy, voltage sources and current sources are required. The properties of current sources and voltage sources, the differences between voltage source and current source and how to work with them are discussed in the following sections. This article belongs to our electrical engineering division.

Let's start with the basics of this area: whether it's a mobile phone, PC, car or television. None of this can be used without voltage and current. We therefore need voltage sources and / or current sources for operation. Let's take a quick look at what that means.

  • Voltage source: A voltage source is an electronic component or an electrical circuit with two connections that form an electrical tension supplies.
  • Power source: A power source is an electronic component or circuit with two terminals that form an electrical electricity supplies.

Circuit symbols:

A corresponding circuit symbol is required to draw voltage sources and current sources in circuits. These are sometimes drawn with and sometimes without connections. The connections are also shown in the two following graphics.


Difference between voltage source and current source

A voltage source supplies voltage, a current source supplies current. "Wait a minute" some will think now. According to Ohm's law, a current can only flow if a voltage exists. Then why is there suddenly a difference between a current source and a voltage source?

The following background: In order to be able to operate circuits correctly, you need a constant voltage, for example always 12 V. This must happen regardless of whether there are 5 or 10 components connected to the voltage source and accordingly require a little more or less current. A voltage source thus has the task of making a certain voltage available regardless of the load. Logically this is only possible within certain limits, no source can supply infinite electricity.

In the case of the power source, it is the other way round: Here the current should only depend slightly on the electrical voltage at its connection points. Ideally, the current is independent of the connected consumer. Here, too, there are of course technical limits to the load-bearing capacity.

Ideal and real voltage source and current source:

An ideal voltage source always has the same voltage regardless of its load at the connection terminals, hence the name ideal. The internal resistance of such a source would have to be RI. = 0. For physical reasons, however, you cannot build a voltage source without an internal resistance. A real voltage source therefore always has an internal resistance greater than zero.

A current source is called an ideal current source if its internal resistance is infinitely large. That too cannot be implemented in reality. Real power sources always have a finite internal resistance.


Further topics

We have dealt with the basics of the voltage source and current source here. However, there are other topics that are interesting in this context. We would like to briefly introduce these here and link the appropriate articles:

  • Open circuit voltage / source voltage: The open circuit voltage is the electrical voltage measured at the terminals of an unloaded voltage source. More on this in the article no-load voltage.
  • Terminal voltage: The terminal voltage is the voltage that is present between the positive pole and the negative pole of a voltage source. More on this in the article terminal voltage
  • Equivalent circuit diagram and Internal resistance: An equivalent circuit is an equivalent circuit that behaves electrically in exactly the same way as the original electrical circuit. More on this in the article equivalent circuit diagram.
  • Short circuit current: An electrical short circuit is an almost resistance-free connection between the two poles of a voltage source, which causes a voltage between these parts to drop to a value close to zero. More in the article short circuit current.


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