Describing the structure and explaining how technical devices work

Describing the structure and explaining how technical devices work

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In order for an electrical current to flow in a circuit, an electrical voltage must be present. Colloquially, this is provided by a "power source"; In electronics, however, a distinction is made between "voltage sources" and "current sources":


In order for an electrical current to flow in a circuit, an electrical voltage must be present. Colloquially, this is provided by a "power source"; In electronics, however, a distinction is made between "voltage sources" and "current sources":

An ideal voltage source supplies a constant voltage value, regardless of the size of the current that is drawn from the source.

diagram of an ideal voltage source

An ideal power source always delivers the same current strength; the voltage required to deliver this amperage is automatically regulated by the power source.

diagram of an ideal power source

Both types are idealized models that cannot exist in reality: If, for example, an arbitrarily large current could actually be drawn from an ideal voltage source, the power provided would also be arbitrarily large according to the formula P = U \ cdot I. In the case of real voltage sources, the voltage therefore decreases, as explained below in the excursus: Internal resistance of real voltage sources is described in more detail, with increasingly larger currents; Likewise, real power sources cannot supply any large voltage values ​​that would be necessary with large load resistances in order to maintain the nominal current intensity.

circuit symbol of an ideal voltage or current source

The ideal models are nevertheless used because of their simplicity, since they can often describe the real processes well enough; If contradictions or (in the case of calculations) unrealistic values ​​arise when using these simple models, these must be improved as described in more detail at the end of this chapter.

Voltage sources with direct voltage

There is an excess of electrons at the negative pole of a voltage source, and a shortage of electrons at the positive pole. Both states are generated or maintained by processes inside the voltage source.


Batteries have chemical energy stored inside and are able to release it in the form of electrical energy. Discharged batteries, whose stored amount of energy has been used up, must be taken to a recycling center or thrown into specially set up containers. [1] In this way, the components of the battery can (largely) be recycled, and at least far fewer toxins enter the environment.

circuit symbol of a battery or an accumulator

Batteries cannot be recharged and therefore have significant disadvantages compared to accumulators, both from an economic and an ecological point of view. They are normally only used in mobile areas when a comparatively higher storage capacity and / or a slightly higher power output are absolutely necessary.


Accumulators (also called "accumulators") are "rechargeable batteries". When charging, electrical energy is stored in the form of chemical energy by converting substances inside the battery. When discharging, the chemical process takes place in the opposite direction and electrical energy is released.


Lead accumulator:

In the uncharged state the plates are made of lead sulfate (\ ce {PbSO4}). When charging, the positive electrode reacts to lead oxide (\ ce {PbO2}) and the negative electrode to lead (\ ce {Pb}). Diluted sulfuric acid is used as the electrolyte.

The voltage per cell is about \ unit [2] {V}. In commercially available lead accumulators, six cells are usually connected in series, so that a voltage of \ unit [12] {V} can be tapped at the connections.

Nickel-iron accumulator:

Diluted potash lye is used as the electrolyte; the voltage per cell is about \ unit [1,2] {V}.

At low currents, batteries and accumulators can be viewed as ideal voltage sources as a good approximation.

Power supplies

For stationary applications (wired) power packs have several advantages over batteries or accumulators: They do not have to be replaced and they always reliably deliver the desired voltage (without any discharge phenomena).

DC power supplies ("DC" or "Direct Current") usually consist of a transformer, a (bridge) rectifier, a voltage regulator and a few capacitors. Depending on the type of built-in voltage regulator, power packs provide a fixed or adjustable output voltage.
AC voltage sources

Household sockets provide an alternating voltage of \ unit [230] {V}, whereby the permissible amperage is usually limited by fuses to \ unit [16] {ampere} - a maximum electrical power of \ unit [230] {can therefore be achieved. V} \ cdot \ unit [16] {A} = \ unit [3680] {W}. The electrical voltage is generated in power plants by means of generators (or by means of solar cells and inverters) and - after voltage adjustment - transmitted to the respective locations via (high) voltage lines.

circuit symbol of a power supply unit with alternating voltage

While electrical experiments with "mains voltage" are life-threatening (!!) due to the high electrical power, the alternating voltage of the mains (\ unit [230] {V}) can simply be set to a lower voltage using a transformer. Depending on the design, transformers can either be located directly in the devices (for example in televisions, radios, etc.) or in the form of separate power supplies (for example in notebooks).

A suitable AC power supply unit ("AC" or "Alternating Current") should always be used for electronic tests with alternating voltage.
Power sources

Just as an ideal voltage source always delivers the same nominal voltage, an ideal current source always delivers the same nominal current; the voltage required for this is automatically regulated by the power source. As can be guessed, real power sources are more difficult to realize than voltage sources in practice.

Some laboratory power supplies can be used as a current or voltage source within certain limits; if the current source mode is used, the desired nominal current can also be set, for example \ unit [1] {A}. If the power source then determines that it is currently only delivering \ unit [50] {mA}, it continues to increase its voltage internally until the desired current level is reached or technical limits are reached.
Excursus 1: Internal resistance of real voltage sources

If a circuit is closed, the current - regardless of the type of voltage source - must always flow through it. Real voltage sources have their own electrical resistance, which is called "internal resistance" R _ {\ mathrm {i}} - in contrast to the connected consumers, which are called "external resistance" R _ {\ mathrm {a}} (or " Load resistance R _ {\ mathrm {L}}).

model of a real voltage source with the open circuit voltage u_0 and the internal resistance r

The total resistance Rges of a circuit is equal to the sum of the internal resistance of the voltage source and the external resistance:

Rges = Ri + Ra

As is usual with a series connection of resistors, part of the total voltage drops across the internal resistance and the remainder across the external resistance. The proportion of the total stress in the external resistance depends on the proportion \ frac {R _ {\ mathrm {a}}} {R _ {\ mathrm {ges}}} of the external resistance in the total resistance. [2] The external resistance is usually much greater than the internal resistance, and thus the proportion of the external resistance in the total resistance is almost 100%; consequently almost 100% of the total voltage drops across the external resistance.

Terminal voltage and open circuit voltage

The “terminal voltage” U of a voltage source is the voltage that is applied between the two terminals (connections, poles) of the voltage source; this voltage is identical to the voltage that drops across the external resistance of the circuit.

The terminal voltage assumes its maximum value when the external resistance is infinitely large: In this case, almost all of the voltage across the external resistance drops and almost no voltage across the internal resistance. However, since no current can flow with an infinitely large external resistance, this maximum voltage value is also known as the “open circuit voltage” U_0.

The open circuit voltage therefore corresponds to the voltage value of a voltage source when no consumer is connected. This value can be measured approximately with a voltmeter, as this does not have an infinite resistance value, but it has a very large resistance value.

diagram of the terminal voltage of a voltage source as a function of the internal resistance ri and the flowing current i

If a consumer with a finite resistance is connected to the voltage source, a current intensity I = U0 Rges is established, which flows through both the consumer and the voltage source. At the internal resistance Ri of the voltage source, the voltage Ui = R i I falls according to Ohm's law; the terminal voltage is thus reduced by this amount compared to the open circuit voltage. In this case, the following applies to the terminal voltage U:

U = U0 - Ri I.

The lower the external resistance of a circuit, the higher the current I; this results in a reduction in the terminal voltage U.


How large is the terminal voltage U of a voltage source compared to its open circuit voltage U_0 if the external resistance R _ {\ mathrm {a}} is equal to the internal resistance R _ {\ mathrm {i}} of the voltage source?

If Ri = Ra, the current strength I follows for the:

I = U0 Rges = U0 Ri + Ra = U0 2 Ri

If you insert this value into the above formula (1), the terminal voltage U is:

U = U0 - Ri U02 Ri = U0 U02 = 12 U0

In this case, the terminal voltage has dropped to half the open-circuit voltage.

In the event of a short circuit, the external resistance drops to almost zero; the current I is then only limited by the usually very low internal resistance of the voltage source. The current intensities that occur here can be so great that the voltage source can be destroyed by the thermal effect of the current; there is a risk of fire in the event of a short circuit.

In the event of a short circuit, batteries and accumulators can release all of their stored chemical energy within a few minutes. In practical applications, for example in cars, the risk of fire in the event of short circuits is often prevented by fine fuses: The wires built into them glow quickly when the currents are too high and thereby interrupt the circuit.

In the case of power supplies, the manufacturer must specify a load limit that specifies the current strength a power supply can deliver over a longer period of time. If the load limit is exceeded very clearly over a longer period of time or for a short period of time, overheating can result in short circuits in the power supply unit, which can destroy the power supply unit and cause the main fuse in the distribution box (FI switch or fuse) to "blow out". In specific applications, it is therefore advisable to use a power supply unit that is also designed for somewhat larger currents.

Excursus 2: Internal resistance of real power sources

An approximation model with an ideal source and an internal resistance can also be set up for real power sources. In this case the equivalent circuit is as follows:

model of a real power source with the open circuit voltage u0 and the internal resistance ri

In this case, the current from the (ideal) current source is divided: One part flows through the highest possible internal resistance Ri of the current source, one part through the usually comparatively low load resistance Ra. If I0 denotes the current supplied by the ideal current source and I denotes the current in the rest of the circuit, the following applies:

<a href="/wiring-diagram-gallery/1.html"><img style="max-width:580px;" src="../wirimg/1.png" alt="1"> </a>

According to Ohm's law, the following applies to the voltage U applied to the load resistor:According to Ohm's law, the following applies to the voltage U applied to the load resistor:

<a href="/wiring-diagram-gallery/2.html"><img style="max-width:580px;" src="../wirimg/2.png" alt="2"> </a>

If one again assumes the current strength I flowing through the load resistor as a variable, the term Ri I0, which is only relevant for the interior of the current source, can simply be written again as U, and thus a formula is obtained which is identical to equation (1) of a voltage source; when using this model, the U (I) characteristic curve of a real current source is identical to that of a real voltage source.

The main difference between a voltage and a current source is how large the external resistance Ra the source is designed for:

A real voltage source has no power loss when the load resistance Ra is infinitely large. A voltage source is therefore preferably operated in no-load operation or at low currents.
A real power source has no (internal) power loss if the external resistance Ra is infinitely small or the internal resistance Ri is comparatively infinite.

The advantage of the two above models for voltage and current sources is that the sources represented in this way can be connected in parallel or in series, even with different values, without this resulting in practical contradictions; the models therefore represent real voltage and current sources a lot more realistically.


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