A current whose direction does not change is called direct current, the corresponding voltage is called direct voltage. A direct current and also a direct voltage can have a constant value, but also change their value without changing the polarity. Circuits in which direct current flows are called direct current circuits.
Constant DC current and constant DC voltage
The direction of the current or polarity that remains the same is characteristic of a direct current or a direct voltage. In the simplest case, the current strength or the voltage is constant (Fig. 1). Neither the amount nor the direction change. Such a direct current occurs in a circuit when a direct voltage source is used as the electrical source in the relevant circuit, for example, a battery, an accumulator or a power supply device with direct voltage.
Other forms of direct current and direct voltage
However, one speaks of a direct current or a direct voltage when the amount of current or voltage is not constant, but the polarity does not change. A typical example of this is a so-called pulsating direct current (Fig. 2), which arises, for example, when an alternating voltage is rectified. Other forms of direct current or direct voltage are also possible. The decisive factor is that the polarity remains the same.
DC circuit and AC circuit
The direct current circuit and the alternating current circuit can be characterized by the same physical quantities (voltage, current, resistance, power). The same laws also apply to ohmic resistances. However, coils and capacitors behave differently. More detailed information can be found under the relevant keywords.
Direct current and alternating current can also produce the same effects. For this purpose, the RMS value is introduced into the DC circuit: This is the value that an alternating voltage or an alternating current would have to have in order for it to produce the same effects as a corresponding direct voltage or a direct current.
Example: The effective value of the mains voltage is 230 V. This mains voltage – an alternating voltage – has the same effects as a direct voltage of 230 V.
Direct current of constant strength (shown in blue) and alternating current (shown in red): In contrast to direct current, the polarity of alternating current changes.