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Capacitor Sizing - To Big/Small

Discussion in 'General Electronics Discussion' started by Sadlercomfort, Oct 28, 2015.

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  1. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    Hi Guys,

    I need some help finding information about capacitors and sizing. I'm trying to learn about 'Failure Modes and Effects Analysis' reports, currently writing about capacitors and how under-sizing and over-sizing can effect the circuit.

    I'm trying to find out how the value of C1 effects the circuit (See attached).


  2. shrtrnd


    Jan 15, 2010
    Threw me for a minute there. I thought you were going to ask about physical size/dielectric composition of caps. (They've been getting substantially physically smaller over the years)
    You just want to know about capacitor electrical characteristics.
    I'll let others here give you electrical calculations. Did you check the 'Resources' heading at the top of this page?
  3. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    There's a lot on the Internet about the dielectric failure modes, however this to be more specific to material sciences.

    I just to know the basic effects of having a larger capacitance than required or a lower capacitance. As these are the two fault modes I'm looking into. Talking more about how this might effect the operation of the circuit or even damage it.

    I'm not sure there's anything in the resource section
  4. davenn

    davenn Moderator

    Sep 5, 2009
    what specifically are the 2 fault modes you are referring to ?
  5. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    One: A capacitor has a low capacitance value due to age or whatever

    Two: A capacitor has a high capacitance due to the error of the designer

    Each fault will effect the circuit differently, I need to identify how and why.
  6. Torch


    Jul 26, 2012
    Hi Sadlercomfort,

    The circuit is a voltage regulator system and it designed to keep a constant reference output. Ideally we want the input to not change too rapidly as this would lead to poor regulation of the output. The capacitor is designed to filter out high frequency(rapidly changing) signals to allow proper functioning of the internal regulation circuit. The chosen capacitor value will allow for rapidly changing signals to be filtered out.
    The first extreme would be choosing the value of capacitance too low, which would mean some rapidly changing signals will still pass through to the regulator circuit and lead to an unstable output. The second extreme is seen from this graph:
    capacitors are not ideal and actually start to behave as inductors for very high frequency signals, this depends on the value as well as type of the capacitor. Choosing a to high value will actually cause the opposite effect of what the capacitor is chosen to do. Leading to again an unstable system. Hope I was clear enough
    duke37 likes this.
  7. duke37


    Jan 9, 2011
    The graph shows that the larger capacitors are only effective at lower frequencies. The dip depends on the capacitor proper and the length of the leads. The leads should be as short as possible.

    In your diagram C3 is placed further away from the regulator than C2. Shift it as close as possible to reduce lead length.
  8. Sadlercomfort

    Sadlercomfort Ash

    Feb 9, 2013
    Thanks Guys,

    This makes alot of sense. :D
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