## Electronic Components Symbols Voltage and Current Sources you Must know

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On: 15 Agu, 2020

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Electronic symbols overview

The various components are represented by corresponding symbols in electronic circuit diagrams. The following list is intended - without claiming to be exhaustive - to provide an overview of some of these "compo...

Electronic symbols overview

The various components are represented by corresponding symbols in electronic circuit diagrams. The following list is intended - without claiming to be exhaustive - to provide an overview of some of these "components".

Voltage and current sources

In order for an electric current to flow in a circuit, an electric 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.

An ideal power source always delivers the same amperage; the voltage required to deliver this amperage is automatically regulated by the 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, drops, as in the excursus below: Internal resistance of real voltage sources is described in more detail, with increasingly larger currents; in the same way, real power sources cannot supply voltage values of any size, which would be necessary with large load resistances in order to maintain the nominal current intensity.

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 corrected - 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

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 get into the environment.

Batteries cannot be recharged and therefore have considerable 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

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 runs in the opposite direction, and electrical energy is released.

Examples:

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 to one another 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 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 always reliably deliver the desired voltage (without 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] { 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.

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 in practice than voltage sources.

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 internally increases its voltage 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}}).

The total resistance R _ {\ mathrm {ges}} of a circuit is equal to the sum of the internal resistance of the voltage source and the external resistance:

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 called the "open circuit voltage" U_0.

The no-load voltage 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 is too large.

If a consumer with a finite resistance is connected to the voltage source, a current intensity I = \ frac {U_0} {R _ {\ mathrm {ges}}} is established, which flows through both the consumer and the voltage source. At the internal resistance R _ {\ mathrm {i}} of the voltage source, the voltage U _ {\ mathrm {i}} = R _ {\ mathrm {i}} \ cdot 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:

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

Example:

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 R _ {\ mathrm {i}} = R _ {\ mathrm {a}}, then it follows for the current strength I:

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

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 over a longer period of time or for a short time and therefore very clearly, short circuits can occur in the power supply unit due to overheating, 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 looks like this:

Model of a real power source with open circuit voltage U_0 and internal resistance R _ {\ mathrm {i}}.

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

If one again assumes the current strength I flowing through the load resistor as a variable, the term R _ {\ mathrm {i}} \ cdot I_0, which is only relevant for the interior of the current source, can simply be written again 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 R _ {\ mathrm {a}} the source is designed for:

A real voltage source has no power loss when the load resistance R _ {\ mathrm {a}} 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 R _ {\ mathrm {a}} is infinitely small or the internal resistance R _ {\ mathrm {i}} 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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