Voltage source

The voltage source is referred to in the circuit theory of electrical an active two-terminal network, which supplies between its connection points, an electrical voltage. As an essential feature, this voltage does not depend only slightly, or in the model of an ideal voltage source in the network analysis of the electrical current from which the source is removed. The supplied voltage is ideally therefore independent of the respective connected loads. Supplying the voltage source is a time- constant DC voltage, it is also referred to as a constant voltage source.

A voltage source is preferably used as an electrical energy source that provides, depending on the connected load electric current, but not (see below) should not be confused with a power source. It also occurs as a supplier of electrical signals.

A voltage source can also be an object that can only produce an electric field, while only briefly or capable in non-recoverable scope for current output.

• 3.1 Basics
• 3.2 Ideal and real voltage sources
• 3.3 Characteristics
• 3.4 Parallel and series connection of voltage sources 3.4.1 Series connection
• 3.4.2 Parallel connection

Context

As part of the electrical network analysis is the counterpart to the power source of the two-pole of a current source, which supplies a certain electric current regardless of the voltage applied to the terminals of the power source voltage. 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 voltage source with an internal resistance. This relationship is also known as the Thevenin 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 voltage source is referred to as an ideal voltage source when the internal resistance of 0 Ω. This can be achieved in reality only an approximation, real voltage sources always have an internal resistance, which is larger. When used industrially voltage sources is attempted as a rule to keep the internal resistance as small as possible.

Kinds of voltage sources

DC and AC voltage sources

DC voltage sources maintain their voltage value constant in time. DC voltage sources operating as energy sources are, for example, dry batteries, accumulators and power supplies. - Further, there are voltage sources, which are not very reliable, but as the signal source or sensor are significant, such as thermocouples.

In contrast, an AC voltage source generates a voltage varying periodically with time. These sources include, for example, generators, transformers, signal generators and also network devices. And the electric power supply system constitutes an AC voltage source - In addition, there is little reliable sources, here, for example for transmission of messages or for receiving audio signals and measuring, e.g., microphones.

Asymmetric and symmetric bipolar power supplies

Has a DC voltage source, two ports, one of which is declared the general reference or ground potential, so it is called an asymmetric voltage source.

For a bipolar power supply simultaneously a positive and a negative voltage with respect to a common ground potential, for example 15 V and - 10 V, is provided. Are the positive and negative voltage also in their equal amounts, there is a symmetrical power supply. This allows in particular a balanced signal transmission, which is disturbance -tolerant compared to the transmission of asymmetric signals. A simple example of a symmetrical voltage source is a transformer with center tap.

Controlled and regulated voltage sources

In addition to the independent voltage source, with a fixed source of voltage, there is the controlled voltage source, the source voltage is a function of an external voltage or current, which is connected thereto to separate connection points.

Instead of using such an external actuating variable, the regulated voltage source is fed via a control circuit so that the output voltage is adjusted to a predetermined desired value. This interference can be compensated by changing the current load, fluctuations in the mains voltage or temperature drift. With some laboratory power supplies, the voltage at the load input (instead of source output ) can be controlled, which also influences the cables and connectors are balanced. It is irrelevant whether it is at the regulated voltage source is a DC or AC voltage source.

Regulated power sources are sometimes referred to as a constant voltage sources or, when they are particularly precise, as the reference voltage sources. In contrast to the model of the ideal voltage source, however, the deliverable power is limited. If the current exceeds a certain limit, the voltage can collapse abruptly.

Technical descriptions

Basics

In this article, the following terms are used:

• Initial or terminal voltage Ukl
• Designated source voltage U0, formerly known as electromotive force or electromotive force (EMF) U0 is the maximum voltage that may be present at the terminals
• Short-circuit current IK is the current which the source delivers, when the resistance across the terminals is equal to zero
• Called internal resistance Ri as a source resistor RQ
• Designated consumer resistance RV, even as the load resistance RL

Furthermore, the following counting direction is used:

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 following equivalent circuit can be reached that the voltage and current have the same sign. A positive voltage Ukl from a to b created in the consumer a positive current I 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 is achieved by a "producer - Bepfeilung " in the picture. Because when the circuit is closed flows inside the voltage source against the current of the voltage. For consumers connected a positive voltage supplies this a positive current I.

Ideal and real voltage sources

As an ideal voltage source, a voltage source is referred to, which always delivers, regardless of the downstream load the same voltage. Terminal and source voltage are therefore always the same, the power supply of the source is assumed to be infinite. But since this is impossible to achieve in practice, equivalent circuits for real voltage sources are used in technical calculations for the most part. Thus, the linear voltage source can be formed by a series circuit consisting of an ideal voltage source and a resistor, the internal resistance. This can be traced to the actually applied terminal voltage of the source in calculations of the influence of downstream loads. The stronger the source is loaded by the consumer, the lower the level present at the terminals of voltage. The internal resistance limits the maximum current that can flow in case of short circuit (RV = 0). The short circuit current is then calculated to be

The maximum current is thus larger, the smaller the internal resistance of the source. In practice, the internal resistance of a voltage source will vary, for example, the internal resistance of a new battery is much lower than that of a consumed.

For the reasons listed here, it is therefore desirable to keep the internal resistance of voltage sources as energy sources as small as possible, in any case much smaller than that of the consumer. For sources with only limited abgebbarer performance, such as signal sources are other aspects, see below in the Performance section.

Characteristics

The terminal voltage of a voltage source to the source voltage as a function of the current drawn can be graphically represented as a curve, as shown in the adjacent figure. There are presented several characteristics of different voltage sources in different colors. The maximum current, also referred to as short-circuit current is referred to as:

• In an ideal voltage source, shown as a red line, this is a horizontal straight line. In this case, the internal resistance of 0 Ω, the voltage does not change.
• A real voltage source with an internal resistance of greater than 0 Ω provides a falling characteristic, in which the current decreases with increasing voltage in different ways. Shown are two options: For a linear source gives an inclined straight line, shown in the color turquoise. Linearly attribute refers to the internal resistance, which is a linear ohmic resistance in this case.
• A non-linear spring, the characteristic is shown in green, has a curved characteristic. An example of such a source is a solar cell. Only in the shallow region of their characteristic, this source can be considered as a voltage source, in the steep area approaching the short-circuit current behavior is described adequately by a current source. Each individual case requires a special circuit replacement, so that nonlinear cases below untreated.

Parallel and series connection of voltage sources

Series

For series connection of multiple power sources, the total voltage is the sum of the voltages of the individual voltage sources. Similarly, the total source impedance is equal to the sum of the internal resistances. The current is the same for all sources in the frequency, phase angle and amplitude.

Parallel connection

If more power is needed by the consumer as a single source can provide, so that a parallel connection of voltage sources is required, this is only possible with real sources. These source resistors must be simulated by external resistors connected in the lines if necessary. They have to be so large that its voltage drop by the voltage across the load is smaller than the smallest of the open-circuit voltages. In addition, make sure that all parallel-connected voltage sources with respect to the voltage

• Have the same amount,
• At DC have the same sign ( polarity ) or AC voltage the same phase,
• Are ungrounded or grounded to the same pole; with more than one ground point can currents flow (see ground loop ).

If these points are not taken into account, this results in a most undesirable current flow between the sources. Depending on the current level or design of the voltage sources, this can lead to the destruction of individual sub- sources. These criteria can also corresponding electronic protection circuits monitor and control the voltages of the individual voltage sources met.

The total voltage of a plurality of parallel connected voltage sources is dependent on the source voltage and the internal resistance of the individual power sources as described above. The total current results from the sum of the currents of the individual sources. To calculate the internal resistance of the individual voltage sources to transform into power sources (see further below ), the resistance can then be calculated as a parallel circuit of the individual internal resistances.

Performance

The radiation emitted by a real voltage source power is calculated from the product of the current and the terminal voltage. Idle is this capacity is zero because no current flows through the load. Also in the case of short circuit no power is delivered, because although a high current flows, but the terminal voltage is zero. The current is limited in this case only by the internal resistance.

Between these two extremes ( open circuit and short circuit) of consumer resistance is the source from a power that is greater than zero. Thus, the source delivers maximum performance for a given value of the load resistor. In this case we speak of power adjustment. This occurs when the load resistance RV is equal to the internal resistance Ri of the power source. The maximum power is then calculated as

The power dissipation of the internal resistance Ri

Is almost completely converted into thermal energy. It is therefore partly responsible for that eg batteries during discharging or in batteries at load to warm up. If a short circuit, the converted heat energy on the internal resistance, the voltage source and located in the vicinity of objects damage, destroy or put on fire by heat.

Transformation voltage source ↔ power source

According to the Thévenin and Norton theorems, each linear voltage source can also be regarded as a linear current source. Which term is used depends on, at which the ideal form, the behavior 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.