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What is Smoothing capacitor?

A capacitor connected in parallel with the output of a rectifier or similar circuit to reduce the voltage ripple and bring the output closer to DC. It charges while the voltage is high and supplies current to the load while it is low. Large aluminum electrolytic capacitors are used in mains-frequency rectifiers, and ceramic capacitors among others at the outputs of switching power supplies. For full-wave rectification the peak-to-peak ripple can be estimated as load current ÷ (2 × supply frequency × capacitance).

Read the beginner explainer: AC vs DC Explained: Impedance, Phase and Power Factor

Japanese name: 平滑コンデンサ

Smoothing capacitor appears in 3 practice questions across 1 topic, including Power Electronics Circuits.

Field: Power Electronics Circuits

Try it: questions about Smoothing capacitor

A sinusoidal AC voltage of 100 V RMS is full-wave rectified with an ideal diode bridge. Without a smoothing capacitor, which of the following is closest to the average output voltage?

  1. AAbout 90 V
  2. B100 V
  3. CAbout 141 V
Show answer and explanation

Answer: A. About 90 V

The peak of the sine wave is Vm = √2 × 100 ≈ 141 V. The average of the full-wave rectified waveform is (2/π) × Vm = (2√2/π) × V ≈ 0.90 × 100 ≈ 90 V. 100 V is the RMS value of the full-wave rectified waveform, the same as that of the input. With half-wave rectification the negative half cycles are missing, so the average is half of this, about 45 V.

Smoothing capacitor in the explanations

Questions that mention Smoothing capacitor (3)

  1. A sinusoidal AC voltage of 100 V RMS is full-wave rectified with an ideal diode bridge. Without a smoothing capacitor, which of the following is closest to the average output voltage?
  2. Which of the following correctly describes the output of a three-phase full-wave rectifier that rectifies three-phase AC with a bridge of six diodes?
  3. Which of the following correctly combines what can happen when an induction motor driven by an inverter is decelerated quickly by lowering the frequency with the countermeasure?

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Related terms

Rectifier circuit RMS value Three-phase AC Induction motor Inverter Back EMF Boost converter Buck converter Buck-boost converter Continuous conduction mode

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