Smoothing capacitor

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A smoothing capacitor , also known as a filter capacitor , is a capacitor placed parallel to the load resistor , which, behind a rectifier circuit, reduces the residual ripple of the rectified voltage .

Working method

Circuit for rectification and smoothing
T = transformer secondary winding
G = bridge rectifier
C = smoothing capacitor
R = load resistance

If there is a capacitor behind a rectifier parallel to the consumer, it will be charged almost to its peak value during the rising course of the rectified AC voltage . If the supply voltage drops, the stored charge cannot flow back through the rectifier; it is only given to the consumer. With a suitable size of the capacitance, the voltage across the capacitor drops only slightly. Thus, the use of the capacitor increases the DC component of the mixed voltage and its AC component is reduced at the same time; the tension is "smoothed".

In terms of DC voltage, a smoothing capacitor acts as a store of electrical energy that can be drawn from it by a consumer. In terms of alternating voltage, a smoothing capacitor is an alternating current resistor which derives the alternating voltage superimposed on the direct voltage according to its impedance behavior to ground.

Caution: As a result of the charge storage, a high voltage or amount of energy can remain in the capacitor for some time after disconnection from the power supply.


The smoothing capacitor reduces the ripple of the voltage, so that only a residual ripple remains. However, it also causes short current draw peaks in the supply and in the rectifier

In DC power supply circuits, the capacitor must often have a value of several mF , depending on the requirements of the circuit , and is therefore usually designed as an electrolytic capacitor . The smaller the load resistance, the larger the capacitance should be in order to achieve sufficient smoothing.

When a bridge rectifier (also B2U rectifier) of the smoothing capacitor is always after half mains period , ie every 10 ms charged at 50 Hz. If the voltage should only decrease by Δ U during this time Δ t , then the capacity is calculated using the formula:

For Δ U = 0.1 V and Δ t = 10 ms, the required capacity of the smoothing capacitor for a current of I = 10 mA is C = 1000 μF. With a half-wave rectification, this value must be doubled (half the frequency).

Such a small voltage fluctuation results in a small current flow angle and thus high current pulses which load the power network with harmonic current . In order to keep the current flow angle within limits, a capacitor with a smaller capacity is used for pre-smoothing. Depending on the requirements to the quality of the DC voltage is connected to a second smoothing capacitor in a RC-member , or better in terms of the losses, an LC element as a low pass further filtered ; or the smoothing is refined with an electronic voltage regulator that is more common today .

The smoothing is associated with both high current pulses and additional smoothing stages, voltage losses and thus power losses, so that these methods are limited to rather small supply currents (rough guide value: up to 1 A). In order to achieve a low residual ripple even with a higher current, in addition to increasing the capacity, a series connection of a choke to the load or an increase in frequency is possible. Switching power supplies work with a much higher frequency than the mains frequency and allow a much smaller dimensioning of the capacitor with the same voltage of the residual ripple. However, switched-mode power supplies also require a DC voltage for operation; Power factor correction filtering can be used as a further measure to reduce the harmonic current .


  • Dieter Nührmann: The complete workbook electronics, two volumes . Franzis-Verlag, 2002, ISBN 978-3-7723-6526-3 .