Conceptual Questions

Discussion in 'General Electronics Chat' started by NY10, Jul 30, 2010.

  1. NY10

    Thread Starter Member

    Jul 30, 2010
    Hi, all

    What does multiple capacitors in parallel do ?

    For example,
    +3.3V supplying to the capacitors in parallel (10u, 4.7u, 1u, 1u, 0.1u, 0.1u, 0.1u, 10n, 1n)

    My guess is that since there are multiple capacitors, these will store a lot of energy.

    I would like to know everything about this concept.

    Thank you very much.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. bertus


    Apr 5, 2008
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  4. sage.radachowsky


    May 11, 2010

    Hi NY10...

    If you see all those on a circuit board, and they're all from the 3.3V to the Ground, a lot of them are for "bypass". The 10 uF and 4.7 uF may be for general purpose pulse response. The smaller ones are probably for local distribution to a component.

    "Bypass" is sort of a strange term. It means placing a capacitor near a component or a part of a circuit that may use large pulses of current intermittently. It provides a local supply of power for the pulses. For example, if there is a microcontroller, then the logic inside it is sometimes switching very fast. It may draw current in spikes, and it is good to have a capacitor near the device. If it is near an op amp, then it's helping the op amp respond quickly to the changes in the inputs, when it would draw large currents.

    Sometimes it's good to put a smaller and larger capacitor in parallel, to be able to respond to different time period pulses. A smaller capacitor (nF) may respond faster to very high frequency pulses, while a larger one (uF) may respond with more power to a slower pulse.

    The more caps in parallel, the more current can be supplied, because the effective series resistance (ESR) is reduced.

    They can be sprinkled around a circuit board to help provide pulse current, and to limit the effects of the resistance of the circuit board traces. Traces can have significant resistance, like several mOhms per inch. It depends on the thickness and width.

    They also help to filter out the noise on the Vcc supply to other components, as they buffer the current pulses. In combination with the resistance of the circuit traces, they form an effective RC low-pass filter that reduces high frequency noise and ringing from inductive flows over the traces.

    All good circuit boards have bypass caps thoughtfully place.
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