Current source (circuit theory)

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In circuit theory and network analysis in electrical engineering, a current source is an active two-pole device that supplies an electrical current at its connection points . As an essential property, the current strength depends only slightly or (in the case of the model as an ideal electrical component within the scope of the circuit analysis ) on the electrical voltage at its connection points. The current strength is ideally independent of the connected consumer. Power sources can supply alternating current or direct current that is constant over time ; in technical usage they are also referred to as constant current sources.

Current and voltage sources have mutually opposite properties and, as ideal components, are independent models. A real power source is described in that the source model is operated together with at least one passive component. In the case of a real linear current source, this is an ohmic resistance parallel to the source model.

Circuit symbol of an electrical power source.
Standardization of the left circuit symbol meanwhile internationally in IEC 60617-2: 1996 and DIN EN 60617-2: 1997
This circuit symbol always stands for the model of the ideal, load-independent power source.

General

The voltage source is known from everyday experience; it can be clearly explained physically. On the other hand, explaining the power source physically should not be possible without further ado; it results from a mathematical model. In the context of electrical network analysis, the current source is the counterpart of a voltage source: When viewed as a two-pole, it delivers

  • the voltage source has a certain electrical voltage regardless of the current strength applied,
  • the power source a certain current strength independent of the built up electrical voltage.

Any arrangement of linear voltage and current sources and resistors in the form of an electrical circuit can always be fully described to the outside as a two-pole by only one current source with an internal resistance . This relationship is also referred to as the Norton theorem and plays a role in electrical circuit analysis , since it allows complicated circuits to be reduced to simplified equivalent circuits , which are then more easily accessible for analysis.

In the equivalent circuits, the current source is always considered to be independent of the voltage. In reality, this behavior can only be achieved approximately; then further components have to be inserted in the equivalent circuit diagram to better describe the reality . In the simplest case, an ohmic internal resistance lying parallel to the power source is used for this. In the case of technically used power sources, attempts are generally made to keep the internal resistance as large as possible.

The load on a source is understood to be an electrical consumer connected to its connection points - in the simplest case an ohmic resistor. The consumer converts the electrical power supplied by the source into a heat flow or other power. An unloaded source does not deliver any electrical power to a consumer if at least one of the two factors in the equation is zero:

  • In the case of a voltage source, which by definition always delivers, the unloaded state is achieved through . This case is known as idling .
  • With a power source, which by definition always delivers, the unloaded state is achieved through . This case is known as a short circuit .

The power and power loss inside the power source depends on how the source is technically implemented and has nothing to do with its basic behavior. The terms "ideal power source" and "real power source" used here are used in the same sense as they are customary in the specialist literature.

behavior

Characteristic curve of an ideal (in red) and two linear (in turquoise) power sources and for comparison of a real solar cell (in green)

overview

Equivalent circuit diagram of a linear current source (with consumer)

The output current strength of a current source as a function of the voltage generated at the terminals is graphically displayed as a characteristic curve .

  • In the case of an ideal power source, this is a horizontal straight line according to the definition; it is shown as a red line in the characteristic diagram on the right.
  • A real power source provides a falling characteristic curve, in which the current strength decreases with increasing voltage.
  • A linear current source is an ideal current source with the short circuit current and an internal resistance as described in the next picture. The output current is given by
Accordingly, the characteristic curve is a falling straight line. The inclination becomes smaller the larger it becomes; it is drawn in the color turquoise.
  • A non-linear power source is, for example, the solar cell with its strongly curved characteristic curve; this is drawn in the color green. In its flat area (with low terminal voltage) the solar cell behaves more like a power source; in the steep range (with low current output) it rather (but not as pronounced) assumes the behavior of a voltage source.

In addition to the independent current source with a fixed short-circuit current, there is the controlled current source , the short-circuit current of which is a function of an external variable. A voltage or a current is connected to separate input points for control.

Ideal power source

Characteristic curve of a power supply unit with adjustable voltage and current limitation as well as characteristics of two ohmic consumers

The ideal power source is the borderline case of a linear power source with an internal resistance . So that the current of an ideal current source can flow, it builds up a correspondingly high voltage depending on the resistance of the consumer.

An idle one must not give rise at an ideal power source! She lets go; flashovers occur. With current transformers , for example , there is actually a risk to life if a plug connection is opened secondarily or a line is otherwise interrupted!

This is usually different with an electronic power source: Here the terminal voltage cannot exceed the internal supply voltage with which the source is supplied from its power supply unit. There is no danger if the internal supply voltage is a so-called extra - low voltage .

With the appropriate equipment, a laboratory power supply unit has a rectangular characteristic with adjustable voltage limitation and adjustable current limitation. From the moment the current limit is reached (in the characteristic diagram when the smaller of the two resistors is loaded) it behaves like a constant current source. With a larger resistance, more voltage is built up for the same current. If the voltage limit is reached (in the picture with the steeper straight line), the device behaves like a constant voltage source.

In the model of the ideal power source, the available electrical power is assumed to be infinite. For a technical device, however, the power or voltage output is limited; the current can collapse if a limit specified in the data sheet is exceeded. Wherever the model property cannot be met, equivalent circuits are used on a case-by-case basis . This allows a real power source to be modeled using an ideal power source (such as a linear power source).

Linear power source

In the extreme case of a short circuit , the entire current of the source flows through the output terminals. With increasing load resistance, the terminal voltage rises up to the limit of no-load operation; then the terminal voltage assumes a value at which the entire source current flows through the internal resistance.

The bigger it gets, the bigger it gets . An idling real power source can destroy itself. Some power supplies may therefore only be operated under load.

Equivalence of the linear voltage and current source

Linear current sources are equivalent to linear voltage sources (ideal voltage source with internal resistance connected in series ). Which term is used depends on which ideal form the behavior of the source is viewed more closely. The following equations can be converted into each other; the left describe the voltage source, the right the current source.

Due to its source resistance, the electrical power that can be transmitted is limited to a maximum value. This is dealt with with the linear voltage source .

Efficiency

The efficiency of a power source results from the ratio of the power supplied to the consumer to the power generated by the power source. With the ideal current source, the output current is equal to the generated current , the voltage is the same at the source and consumer anyway; so that is the efficiency in this case .

In the case of the linear current source, part of the generated current is lost and the output power is less than that generated. For and is

.

Another equation applies to the loaded linear voltage source based on its equivalent circuit diagram (see here )

.

The highest possible degree of efficiency is achieved

  • when using a power source and
  • when operated with a voltage source.

The two efficiencies add up to 100% . If a loaded linear voltage source with the degree of efficiency is considered to be an equivalent current source (as in the previous chapter), the degree of efficiency changes .

Counting direction

In the case of a passive component or consumer, the reference direction of the current intensity should relate to the polarity of the voltage. This consumer meter arrow system, which is widely used in electrical engineering, as in the picture above, ensures that the voltage and current have the same sign. A positive current from a to b generates a positive voltage from a to b on the consumer . If one of the two arrows reversed, a minus sign would have to be inserted into Ohm's law.

The consistent use of the signs in the entire circuit is achieved by counting arrows on the generator as in the picture. Because inside the active component or the source, the current flows against the voltage. A positive current strength (in the picture in the source from bottom to top) generates a positive voltage (from top to bottom) at the consumer .

Parallel and series connection of power sources

If more current is to be supplied to the consumer than the source can supply, current sources with the same polarity or phase position may be connected in parallel.

Example: A battery can be charged more quickly with current sources connected in parallel (not voltage sources!).

The series connection of power sources is dangerous. Since exactly the same current flows through all sources, but the sources are not set exactly the same, one source can build up an impermissibly high or inverse voltage at the other source. A typical example is the series connection of many solar cells: If a cell is more heavily shaded, it receives a high inverse voltage under load and can be destroyed. For this reason, cells or modules connected in series are provided with protective diodes connected in antiparallel to them from a certain number.

Applications

A typical application example is a constant current charger with a defined or adjustable end-of-charge voltage. The consumer resistance can be zero, since current sources are designed to be short-circuit-proof. Other consumers that require a power source for operation are light-emitting diodes, laser diodes and gas discharge lamps. Depending on the performance and efficiency requirements, series resistors, switching regulators ( step-down regulators ), electronic or conventional ballasts and for cold cathode tubes also resonance and stray field transformers are used.

Elementary circuit

A very simple current source can be produced by connecting a voltage source in series with a resistor as a source resistor (series resistor) and the consumer . If the supply voltage is much greater than the voltage occurring at the consumer, a source resistance is required that is much greater than the maximum consumer resistance. If the load (the consumer resistance) changes, this has only a minor effect on the current. However, this source has a very poor degree of efficiency , since almost all of the energy supplied by the voltage source is converted in the source resistance. If, for example, the current strength is to be allowed to change by a maximum of 1% as a result of the load change, then

be. The use of an inductive or capacitive reactance as a series resistor (operation of gas discharge lamps with a so-called conventional ballast (ballast choke)) offers a remedy against the high loss with AC voltage .

Electronic power sources

These current sources are described under constant current source . Up to a certain voltage, you can generate that almost horizontal current-voltage characteristic. They are used for measuring and oscillator circuits as well as for timing elements.

Power converter

Current transformers are special transformers for potential-free measurement of large alternating currents. They generate a secondary current that is ideally proportional to the primary current. This is almost the case with terminating resistors (referred to as burden) from zero to a maximum value, the so-called nominal burden.

Further examples

Components such as solar cells , photodiodes or bipolar transistors and IGBT behave like current sources in certain areas of their characteristic curves. The reverse current of photodiodes is proportional to the luminous flux falling over many orders of magnitude .

Transmitters often supply a current as an output signal. These can be current measurements, temperature measurements or other measured variables from which a proportional current is generated. Currents such as the 4… 20 mA signal used for this purpose and used in industrial systems have advantages over voltages in terms of transmission: the voltage drop in long cables and interference-prone potential references at the feed point have no effect on the signal. With the 4… 20 mA signal, a line break can also be detected if the minimum value 4 mA is not reached.

Energy source / energy sink

The power sources discussed so far are energy-emitting. Now there are cases in which it makes sense to expand the model of the ideal current source in such a way that its characteristic curve is not limited to one quadrant, but rather continues to the left in the figure above into the area of ​​negative voltage. The ideal source model is always able to act as both producer and consumer; this is usually not the case with real sources.

Since the model of the linear source is based on the model of the ideal source, the characteristic curve of the linear source increases with power consumption with the same slope to the left to values ​​greater than the short-circuit current.

Equivalent circuit diagram of a bipolar transistor with a linear current source in the path from collector C to emitter E.

An example of the application of the comprehensive model of the current source, which is operated here exclusively in the consumer direction, is the bipolar transistor according to the equivalent circuit diagram opposite . In this the current flows against the usual direction for a given voltage direction of a current source. This is therefore energy-consuming. It is only part of the equivalent circuit diagram and does not exist. The energy absorbed leads to the transistor heating up. Thus, and can form the transistor is in an appropriate circuit to operate, the feeds an actually existing energy source.

Another example is the measuring transducer with a current interface , which allows an impressed current to pass as a measure for a measured variable (temperature, pressure, etc.) and acts as an energy sink that must be supplied by a supply device.

literature

Individual evidence

  1. DIN EN 60375: Agreements for electrical circuits and magnetic circuits , 2004, No. 8.2.1
  2. a b IEC 60050, see DKE German Commission for Electrical, Electronic and Information Technologies in DIN and VDE: International Electrotechnical Dictionary - IEV. , In the “Network Theory” section, entries 131-12-21 and 131-12-23
  3. a b Wilfried Weißgerber: Electrical engineering for engineers 1: DC technology and electromagnetic field. Springer Vieweg, 11th edition, 2018, p. 44
  4. ^ Heinrich Frohne, Karl-Heinz Locher, Hans Müller: Moeller Fundamentals of Electrical Engineering . Teubner, 20th ed., 2005, p. 34
  5. a b Reinhard Scholz: Fundamentals of electrical engineering: An introduction to direct and alternating current technology. Hanser, 2018, p. 115
  6. ^ Karl Küpfmüller, Wolfgang Mathis, Albrecht Reibiger: Theoretical electrical engineering: An introduction. Springer, 16th ed., 2005, p. 27
  7. ^ Rainer Ose: Electrical engineering for engineers: Fundamentals . Hanser, 5th ed., 2014, p. 38
  8. Steffen Paul, Reinhold Paul: Fundamentals of electrical engineering and electronics 1: DC networks and their applications. Springer Vieweg, 5th ed., 2014, p. 62
  9. Ulrich Tietze and Christoph Schenk: Semiconductor circuit technology
  10. ^ Ralf Kories and Heinz Schmidt-Walter: Taschenbuch der Elektrotechnik
  11. a b Rainer Ose: Electrical engineering for engineers: Fundamentals . Hanser, 5th ed., 2014, p. 41
  12. DIN EN 60375, No. 6.1
  13. MAX9934 - High-Precision, Low-Voltage, Current-Sense Amplifier with Current Output and Chip Select for Multiplexing. Maxim Integrated, accessed September 10, 2018 (datasheet).
  14. ^ Heinrich Frohne, Karl-Heinz Locher, Hans Müller, Thomas Harriehausen, Dieter Schwarzenau: Moeller Fundamentals of Electrical Engineering. Vieweg + Teubner, 21st edition, 2008, p. 41
  15. ^ Heinrich Frohne, Karl-Heinz Locher, Hans Müller, Thomas Harriehausen, Dieter Schwarzenau: Moeller Fundamentals of Electrical Engineering. Vieweg + Teubner, 21st edition, 2008, p. 41