Current source
Current and voltage sources have mutually opposite properties. This article discusses the properties of ideal and real linear current sources.
- 3.1 Basic circuit
- 3.2 Electronic power sources
- 3.3 Current transformer
General
As part of the electrical network analysis the counterpart is a voltage source which supplies a predetermined voltage independent of the current drawn as a two-terminal network. An arbitrary arrangement of voltage and current sources and linear ohmic resistances in a two-terminal network in the form of an electric circuit can be outwardly always fully described by only one power source with an internal resistance. This relationship is also known as the Norton theorem and plays in the electric circuit analysis is relevant, as thus be reduced to simplified equivalent circuits complicated circuits the analysis more accessible then.
A current source is referred to as an ideal power source when the internal resistance is infinitely high. This can be achieved in reality only an approximation, real power sources always have a finite internal resistance. When used in technology power sources is attempted as a rule to keep the internal resistance as large as possible.
As the load of a source is meant an at their connection points connected electrical consumers, such as in the simplest case an ohmic resistor. This is supplied from the power source for example, in heat or other forms of energy.
An unloaded spring is no electrical power P = ab to a consumer U ⋅ I, if at least one of the two factors in the equation is zero:
- With a voltage source, which by definition always a U ≠ 0 delivers it reaches the unloaded state by I = 0, this case is referred to as idling.
- With a current source which supplies by definition always an I ≠ 0, it reaches the unloaded state by U = 0, this case is referred to as an electrical short circuit.
The implemented within the current source power and power loss depends on how the source is technically realized, and has nothing to do with their basic behavior. The terms " ideal current source " and " real power source " are used in making sense of the same, as is common in the literature.
Behavior
As with any technical device, the following statements are valid only in a limited area, for which the respective device specification is valid and not in case of overload.
Overview
The output current I of a current source as a function of the resulting voltage at the terminals is represented as a curve.
- In an ideal current source, this is according to the definition of a horizontal straight line, shown in the characteristic diagram on the right as a red line.
- A real current source supplies a falling characteristic at which the current decreases with increasing voltage. A linear current source, the curve is drawn in the color turquoise, can be described in the equivalent circuit diagram of an ideal current source, and an internal resistance in parallel. The characteristic is a descending straight line. The consumer current flowing is given by
- A non-linear current source, the characteristic curve is drawn in the color green is, for example, the solar cell with its strongly curved characteristic. In its flat region ( at low clamping voltage) the solar cell behaves more like a current source; in the steep region (at low current drain ) it takes more (but not as pronounced ) 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 is a function of an external variable. A voltage or current is connected to at separate connection points.
Ideal power source
The ideal current source is the limiting case of a linear current source having an internal resistance R i → ∞. Thus, the power of an ideal current source can flow, it builds depending on the resistance RV consumer on a correspondingly high voltage.
In an electronic power source is usually different: Here, the terminal voltage of the internal power supply voltage does not exceed, at which the source is supplied from its power supply. There is no danger, when the internal supply voltage is a so-called low voltage.
With the appropriate equipment, a laboratory power supply has a square wave with adjustable tension controls and adjustable current limiting. It behaves from reaching the current limit ( in the characteristic picture when loaded with the smaller of the two resistors) as a constant current source. With a greater resistance more voltage is developed for the same current. If the voltage limit is reached (in the picture with the steeper line), the device behaves as a constant voltage source.
In the model of an ideal current source, the available electric power is assumed to be infinitely great. However, the power or voltage output is limited for a technical device; is exceeded, to be specified in the data sheet limit the current to collapse. Where it comes to the impossibility of the model property, in some cases equivalent circuits are used. Through this can be a real current source can be modeled by an ideal current source ( such as a linear current source).
Linear power source
In the limiting case of the short circuit with = 0, the entire current of the source of the output terminals flows. With increasing load resistance, the terminal voltage increases to the limiting case of idling; Then, the clamping voltage decreases to a value at which all of the source current flows through the internal resistance.
Will be greater, the greater. It may be possible an idle real power source destroy themselves too. Therefore Some power supplies may be operated only under load.
Linear power supplies are linear power supplies ( ideal voltage source with a series- internal resistance ) are equivalent. Which term is used, depends on to which the behavior of the ideal shape of the source is considered in more detail. The following equations can convert into each other; the left describe the voltage source, the right power source.
Due to its source resistance, the transmitted electric power is limited to a maximum value. This is treated in the linear voltage source.
Counting
With a passive component or load, the reference direction of the current is related to the polarity of the voltage. This " consumer - Bepfeilung " as in the picture above you can reach that voltage and current have the same sign. A positive current I from a to b generates the consumer a positive voltage from a to b. In a reversal of the two arrows would have to be inserted into the Ohm's law with a minus sign.
The consistent use of the sign in the entire circuit achieved by a "producer - Bepfeilung " in the picture. Because the interior of the active component or source flows against the current of the voltage. A positive current I ( pictured at source from bottom to top ) produces a positive voltage at the load (from top to bottom).
Parallel and series connection of power sources
To the consumer more power than can be supplied to the supply source so may power sources having the same polarity and phase are connected in parallel.
Dangerous is the series arrangement of current sources. Since all sources are traversed by exactly the same current, the sources are not set exactly the same, a source can establish an unacceptably high or inverse voltage to the other source. A typical example is the series connection of many solar cells: When a cell is more shaded, it gets under load, a high inverse voltage and can be destroyed. Therefore, you know series connected cells or modules from a certain number, each with anti-parallel to them protection diodes.
Realizations
Consumers which require a power source for operation, are light-emitting diodes, laser diodes, and gas discharge lamps. Depending on the power and efficiency requirements for this series resistors, switching regulator ( buck regulator ), electronic or conventional control gear and for cold-cathode tubes and resonance and reactance transformers are used.
Elementary circuit
A fairly simple current source can be prepared from the series connection of a voltage source with a resistor as a source resistor ( resistor) and the consumer. When the supply voltage is much larger than the voltage appearing at the load, it needs a source of resistance, which is much larger than the maximum load resistance. When changing the load ( the load resistance ), has only a minor influence on the flow. However, this source has a very low efficiency, since almost all of the energy supplied by the voltage source is implemented in the source resistance. If the current instance may change due to load change by a maximum of 1%, it must
Be. Remedy against the high loss offers at AC voltage, the use of an inductive or capacitive reactance as a series resistor ( operation of gas discharge lamps with a so-called conventional ballast ( ballast inductor ) ).
Electronic power sources
These current sources will be described with constant current source. They can produce those nearly horizontal current-voltage characteristic to a predetermined voltage. They are used for measurement and oscillator circuits as well as for timers.
Current transformer
Current transformers are special transformers for floating measuring large AC currents. Generate a secondary current, which is, ideally, proportional to the primary current. That's almost given at termination resistors ( called load) from zero to a maximum value, the so-called nominal load.
Other examples
Components such as solar cells, photo diodes or bipolar transistors and IGBT behave differently in certain areas of their characteristics such as current sources. Thus, the reverse current of the photodiode is proportional to the incident thereon luminous flux over many orders of magnitude.
Transmitter often provide a current as an output. This may hamdeln to current measurements, temperature measurements, or other measurements from which a proportional current is generated. Streams such as the well-known and it is therefore in industrial plants implemented 4 ... 20 mA signal offer in transmission over voltages advantages: the voltage drop over long cables subject to interference potential emoluments at the entry point have no influence on the signal. In the 4 ... 20 mA signal can also be detected, a line break when the minimum value of 4 mA is not reached.