How to choose a power filter capacitor

Written by j.t. barett
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How to choose a power filter capacitor
Filter capacitors remove noise from DC electrical power. (capacitors image by naolin from

A capacitor is an electronic device that stores an electric charge. It also has the effect of slowing down voltage changes, which is useful for filtering noise from electric power. Most DC power supplies have a set of filter capacitors that reduce low-frequency electrical hum and noise. The capacitors also serve as an energy buffer, allowing the power supply to handle surges in power demand. Engineers choose electrolytic capacitors for power supply filtering, as they are rugged, inexpensive and provide large capacitance values for effective filtering.

Skill level:

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Things you need

  • Power supply schematic
  • Capacitor distributor catalogue
  • Calculator
  • Pencil and paper

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  1. 1

    Skim the capacitor distributor catalogue and look for electrolytic capacitors for power supply filtering. Note that the range of capacitance generally falls between 200 and 100,000 microfarads.

  2. 2

    Examine the schematic for the power supply in which you plan to use the filter capacitor. Note the DC voltage values in the power supply.

  3. 3

    Multiply the largest DC voltage value in the power supply schematic by 1.5 to obtain a minimum voltage rating for the filter capacitor. Write down this value.

  4. 4

    Note the capacitance value specified in the power supply schematic. Write down this value.

  5. 5

    Find capacitors in the catalogue that match the values you've written down. Note that more-expensive capacitors are rated to work at higher temperatures. If heat will be a problem for your power supply, choose a capacitor with a better temperature rating.

Tips and warnings

  • If the power supply schematic does not specify a capacitance value for the filter capacitor, choose the largest value capacitor your budget will allow that will also fit in the supply's enclosure.
  • You can use a filter capacitor with a greater capacitance value than that specified in the schematic, as this only improves its filtering ability.
  • Electrolytic capacitors are usually polarised, that is, they have a positive and negative terminal. Always check the polarity connections when you wire the capacitor into a power supply.

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