Decoupling capacitor

A smoothing capacitor, also known as a filter capacitor, a load resistor in parallel with the specified capacitor reduces the ripple of the rectified voltage requested after a rectifier circuit.

Operation

Is parallel to the consumer behind a rectifier, a capacitor, it will be charged almost to their peak during the rising course of the supplying rectified AC voltage up. At the negative supply voltage, the stored charge can not flow back through the rectifier; it is made solely to the consumer. With a suitable amount of capacitance while the voltage at the capacitor only slightly. Thus, increases by the use of the capacitor of the DC component of the mixed voltage, and their alternating component is also reduced; the voltage is " smoothed ".

Therefore considered to be equal in voltage, a smoothing capacitor acts as a storage of electric energy that can be taken by a consumer him. Considered ac default, a smoothing capacitor, an AC resistance, which the DC voltage ripple voltage according to its impedance behavior towards mass derived.

Note: As a result of the charge storage may also be some time after the separation of the power supply voltage or a high amount of energy in the capacitor remain.

Dimensioning

The capacitor in the DC power supply circuits must, depending on the requirements of the circuit often have a value of several mF and is therefore usually carried out as an electrolytic capacitor. The smaller the load resistor, the greater the capacity should be, in order to achieve sufficient smoothing.

In a bridge rectifier (also B2U rectifier ), the smoothing capacitor is always a half - power period, that is at 50 Hz, every 10 ms, charged. If the voltage during this time? T fall just to DELTA.U, then the capacity is calculated according to the formula:

Such a small voltage variation causes a small current flow angles, and high current pulses to an upstream transformer is often not designed. In this case, is provided with a capacitor smoothes smaller capacity. Depending on the requirements on the quality of the DC voltage is further filtered by a second smoothing capacitor in an RC element as a low pass; or smoothing is refined with a now more normal electronic voltage regulator.

With the smoothing are both high current pulses as well as additional smoothing step voltage losses and thus to loss of power, so that these processes are limited to rather small supply currents ( rough guide: up to 1 A). To achieve a small ripple even at a higher current, in addition to the increase in the capacity and an increase of the frequency is possible. Switching power supplies operate at a much higher frequency than the mains frequency, and allow a much smaller dimensions of the capacitor for the same ripple.