Pulsed DC (PDC) or pulsating direct current is a periodic current which changes in value but never changes direction. Some authors use the term pulsed DC to describe a signal consisting of one or more rectangular ("flat-topped"), rather than sinusoidal, pulses. Pulsed DC is commonly produced from AC (

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...

) by a

half-wave rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inver ...

or a

full-wave rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inver ...

. Full wave rectified ac is more commonly known as Rectified AC. PDC has some characteristics of both

(AC) and direct current (DC) waveforms. The voltage of a DC wave is roughly constant, whereas the voltage of an AC waveform continually varies between positive and negative values. Like an AC wave, the voltage of a PDC wave continually varies, but like a DC wave, the sign of the voltage is constant. Pulsating direct current is used on PWM controllers.

Smoothing

Most modern electronic items function using a DC voltage, so the PDC waveform must usually be smoothed before use. A

reservoir capacitor A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inv ...

converts the PDC wave into a DC waveform with some superimposed

ripple Ripple may refer to: Science and technology * Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid ** Ripple, more generally a disturbance, for example of spacetime in gravitational waves * Ripple (electri ...

. When the PDC voltage is initially applied, it charges the capacitor, which acts as a short term storage device to keep the output at an acceptable level while the PDC waveform is at a low voltage. Voltage regulation is often also applied using either linear or switching regulation.

Difference from AC

Pulsating direct current has an average value equal to a constant (DC) along with a time-dependent pulsating component added to it, while the average value of alternating current is zero in steady state (or a constant if it has a DC offset, value of which will then be equal to that offset). Devices and circuits may respond differently to pulsating DC than they would to non-pulsating DC, such as a battery or regulated power supply and should be evaluated.

Uses

Pulsed DC may also be generated for purposes other than rectification. It is often used to reduce electric arcs when generating thin carbon films, and for increasing yield in

semiconductor fabrication Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pre ...

by reducing electrostatic build-up. It is also generated by the voltage regulators in some automobiles, e.g., the classic air-cooled

Volkswagen Beetle The Volkswagen Beetle—officially the Volkswagen Type 1, informally in German (meaning "beetle"), in parts of the English-speaking world the Bug, and known by many other nicknames in other languages—is a two-door, rear-engine economy car, ...

. Pulsed DC is also commonly used in driving light-emitting diodes (LEDs) to lower the intensity. Since light-emitting diodes cannot be reliably dimmed through the simple reduction of voltage as in an

incandescent bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...

, pulsed DC is used to produce many rapid flashes of light that to the human eye are indiscernible as individual flashes, but are seen as a lower brightness. Since an LED has no traditional filament to stress, this also has the added effect of prolonging the lifespan of the LED by reducing its on-time. This can sometimes be seen on videos where an LED or lamp assembly composed of LEDs is filmed at a frame rate very close to - but not exactly - the same as the pulsed DC frequency, which causes the lamp to slowly and occasionally fade in and out as the LEDs fall out of synchronization with the video frame.

References

Bibliography

* Electric power Electric current {{electronics-stub