Voltage adjustment

If an electrical energy source and an electrical consumer are directly connected to one another in an electrical circuit , then voltage adaptation is understood to mean the condition under which the maximum electrical voltage at the consumer is determined. It is also understood to mean an action through which the maximum voltage at the consumer is reached.

Circuit with internal and external resistance. The source can frequently by the equivalent circuit diagram will be described in a linear voltage source, the consumer , for through the input resistor or the input impedance of a subsequent device. B. an amplifier.

This adaptation is described by the ratio of the internal resistance of the source (or its output resistance ) and the load resistance of the sink (or input resistance of the consumer). In the circuit shown with a linear voltage source , voltage adjustment is implemented when the voltage drop across its internal resistance is significantly smaller than the voltage drop across the ohmic load . This is the electrical current strength common to both resistors . In short, voltage adjustment applies to ${\displaystyle R_{\text{i}}}$ ${\displaystyle R_{\text{a}}}$ ${\displaystyle R_{\text{i}}}$ ${\displaystyle I\cdot R_{\text{i}}}$ ${\displaystyle R_{\text{a}}}$ ${\displaystyle I\cdot R_{\text{a}}}$ ${\displaystyle I}$

${\displaystyle R_{\text{i}}\ll R_{\text{a}}\;.}$

Devices connected to the power grid and almost all electronic devices with their own energy source are operated in this way.

General properties

With the open circuit voltage of the voltage source and with the current intensity , a terminal voltage is generated in the circuit shown ${\displaystyle U_{0}}$ ${\displaystyle I}$  ${\displaystyle U}$

${\displaystyle U=I\cdot R_{\text{a}}=U_{0}-I\cdot R_{\text{i}}\,.}$

With the indicator of the voltage adjustment it becomes ${\displaystyle R_{\text{i}}\ll R_{\text{a}}}$

${\displaystyle U\approx U_{0}\,.}$

Most voltage sources occurring in practice behave like the model of the linear source when used as intended; for these, the voltage adjustment applies regardless of the current strength or load.

When the voltage is adjusted, the power that can be delivered by the source must be significantly greater than that actually delivered. ( Please note: deliverable power within the meaning of this article should not be confused with nominal power . The deliverable power, for example from generators or accumulators, is a multiple of the nominal power; however, an attempt to call it up would lead to a significant overload of the source.) Source can only generate so little power that it should be used as fully as possible, so this requires power adjustment .

In a more general view, the source with its internal resistance can be viewed as the output of a device with its output resistance, which is connected to the input of another device with its input resistance . If the following device does not noticeably load the first device or if the voltage does not drop noticeably, then there is voltage adjustment. ${\displaystyle R_{\text{i}}}$ ${\displaystyle R_{\text{a}}}$

The efficiency of a linear voltage source

${\displaystyle \eta ={\frac {I^{2}R_{\text{a}}}{I^{2}R_{\text{a}}+I^{2}R_{\text{i}}}}={\frac {1}{1+R_{\text{i}}/R_{\text{a}}}}}$

approaches 100% when the voltage is adjusted. The almost always sought-after goal of high efficiency can be met by adjusting the voltage.

Applications

Energy technology - hallmarks of adaptation

Usually electrical devices such as irons, light bulbs or television sets are built for a fixed operating voltage of 230 V, which may fluctuate by a maximum of ± 10%. For electronic devices such. B. Cell phone or computer, comparable conditions apply at lower voltages around 5 V. This can only be achieved if the energy source has a very low internal resistance. The high power that can be delivered by the source allows several consumers to be connected in parallel, up to a limit in the technical design.

If a consumer draws 15 A from a 230 V socket, and if the consumer voltage should not drop by more than 2% or around 5 V due to this load, then the sum of the source , line and plug resistance must be less than 0.3 Be Ω.

The disadvantage of voltage adjustment is the risk of destructive overcurrent in the event of a short circuit , which is why the overcurrent must be switched off by fuses .

Sound engineering - adaptation of microphones and speakers

In audio engineering and hi-fi technology, the common connection between devices is voltage adjustment:

The ratio of the two resistances is the damping factor caused by the device through its input resistance. ${\displaystyle R_{\text{a}}/R_{\text{i}}}$ ${\displaystyle D_{\mathrm {F} }}$

${\displaystyle D_{\mathrm {F} }={\frac {R_{\text{a}}}{R_{\text{i}}}}}$

At each interface , the output resistance of the source and the input resistance of the load form a matching damping . In particular, the damping factor for the adaptation damping at the interface from the power amplifier to the loudspeaker must be observed. Voltage matching maximizes the magnitude of a voltage signal in transmission from the source to the load. This transfer takes place almost unloaded when idling.

In sound engineering, the nominal impedance of a loudspeaker is typically 4 Ω in Germany, and often 8 Ω in the USA and Japan. The output impedance of the amplifier is in the same order of magnitude as the cable resistance between amplifier and loudspeaker (<0.1 Ω). Therefore, the value of rapidly becomes smaller. ${\displaystyle R_{\text{a}}}$ ${\displaystyle D_{\mathrm {F} }}$

Rule of thumb: a circuit is voltage matched if the load impedance is at least ten times greater than the source impedance. (However, in some cases it can only be reached three times, as may be the case with 600 Ω microphones and some MC phono systems .)

In systems with very long lines (e.g. telephone ), no voltage adjustment is selected, but power adjustment in order to be able to cover the greatest possible distances and to avoid frequency distortions and echoes (reflections).

literature

• Horst Steffen, Hansjürgen Bausch: Electrical engineering: Basics. Teubner, 2007
• Stefan Weinzierl: Handbook of audio technology. Springer, 2008

Individual evidence

1. ↑ IEC 60050, see DKE German Commission for Electrical, Electronic and Information Technologies in DIN and VDE: Internationales Electrotechnical Dictionary - IEV. IEV number 702-07-14.
2. ↑ Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr: Handbook of Tonstudiotechnik . 8th edition. de Gruyter, 2018, ISBN 978-3-11-028978-7 , p. 498 f . ( limited preview in Google Book search)
3. ↑ Hansjürgen Bausch, Horst Steffen: Electrical engineering, basics. Teubner, 5th ed., 2004, p. 98 f ( limited preview in the Google book search)
4. ↑ Dieter Zastrow: Electrical engineering: A basic textbook. Vieweg + Teubner, 17th ed., 2010, p. 66 ( limited preview in the Google book search)