Mixed flow

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As a mixed stream is called an electric current , in addition to a DC component and an AC component contains. It can also be described as a current fluctuating periodically around a mean value or equivalent value other than zero .

Left: alternating current - right: mixed current

For example, mixed current is created by rectifying an alternating current. The first picture shows a sinusoidal current on the left ; it is an alternating current because its mean value is zero. Rectification produces the mixed current shown on the right , which consists of a direct current with the level and an alternating current that fluctuates around this level. This proportion is zero on average over time. The level of the direct component is called the rectification value in this context .

Time course of a pulse duration modulated square wave signal

Electromagnetic direct current generators also generate a current, the strength of which fluctuates with the position of the rotor in the machine; but a switching process takes place on the commutator after every half revolution. Sinusoidal alternating voltages arise in the excitation windings in the rotor, which are "rectified" at the commutator. As a result, there is no change in sign and a mixed current is created.

By means of smoothing capacitors or smoothing chokes , the direct component can be increased and the alternating component reduced, but never completely avoided; there remains a residual ripple .

Exemplary voltage-time curve for serial transmission

Furthermore, mixed currents arise in many applications of pulse duration modulation and signal transmission . Examples:

  • By changing the pulse duration with a constant period , an equivalent value is created proportional to the duty cycle .
  • With data transmission according to RS-232 with 8 data bits and 2 synchronization bits, the positive or negative voltage predominates depending on the information to be transmitted and depending on the pause between the characters (between the stop bit and the new start bit), and thus also a positive or negative direct component of the mixed current. Some line codes are deliberately designed to be free of equal components.


  • Heinz Meister: Electrotechnical basics . 9th edition. Vogel Buchverlag, 1991, ISBN 3-8023-0528-0 .

Individual evidence

  1. DIN 40110-1: 1994 alternating current quantities ; Two-wire circuits .