# DC voltage - Gleichspannung

A direct voltage is an electrical voltage , the instantaneous value of which does not change over a longer observation period. It has the same sign and the same amount at all times. In an expanded sense, the term DC voltage is also used when the DC component has priority in a periodically changing voltage [1] [2] and when the voltage does not change polarity .

Internationally, the abbreviation for identification is DC for direct current which uses direct current means, but also for DC voltage is used (as well as AC for AC / AC ).

Circuit symbol according to [3] for an ideal DC voltage source with the electrical voltage${\displaystyle U}$

## generation

Galvanic cells and solar cells are voltage sources that generate direct voltage . Furthermore, direct voltage can arise from a source with only a low load capacity, but which can be significant as an electrical signal , for example thermal voltage .

A voltage created by rectification and smoothing

The electrical voltage generated in power stations by electrical generators due to a rotary movement, however, is an alternating voltage. DC voltage can be generated from this by means of rectifiers . This primarily creates a mixed voltage , the mean value of which is referred to as DC voltage. [4] [5] With smoothing capacitors , alternatively with larger currents or with clocked power supply units with upstream coils , the superimposed alternating component can be reduced considerably, so that only a small residual ripple remains; at the same time the constant component can almost reach the peak valuethe original AC voltage can be increased. The greater the capacitance of the capacitor or the inductance of the coil, the lower the peak-valley value of the superimposed alternating voltage. The associated voltage made available to a consumer can now be viewed as direct voltage. In particular when used in communication systems, only a very small residual ripple is allowed so as not to disturb the sensitive electronics. In audio technology , if the smoothing is insufficient, a mains hum can be heard.

Depending on the requirements, electronic voltage regulators can be used to further "smooth". High-quality voltage regulators are controlled with reference voltage sources (which in themselves are not very loadable) .

Furthermore, direct voltage arises, for example, through friction and the formation of thunderstorms ; but this cannot be used technically.

## Notation

The symbol for the physical quantity "electrical voltage" is that${\displaystyle U}$; [6] [7] if there is a risk of confusion with alternating or mixed voltage , a direct voltage is identified by a horizontal line as an index to the symbol , ie${\displaystyle \textstyle U_{-}}$. [4] [5] The unit symbol V for volts should not be given a label. [8] For a DC voltage of 12 V is

${\displaystyle U_{-}=\;12\;\mathrm {V} }$

to write. If the dash cannot be used, the index DC is derived from the Anglo-Saxon language area - for both current and voltage, i.e. here${\displaystyle U_{\text{DC}}}$. The addition to the volt as a unit symbol${\displaystyle \mathrm {VDC} }$ or ${\displaystyle \mathrm {V\;DC} }$is not allowed even according to international standards. [9]

In certain contexts (preferably in the case of digital technology components ), DC voltages or the connections for them are also marked with

• ${\displaystyle U_{\text{SS}}}$for reference potential or ground ⏊
• ${\displaystyle U_{\text{CC}}}$ or ${\displaystyle U_{\text{DD}}}$ for positive voltage against ⏊
• ${\displaystyle U_{\text{EE}}}$ for negative voltage against ⏊