A question about power supply capacitors

Discussion in 'General Electronics Chat' started by jaygatsby, Dec 19, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Another person just posted a thread about his broken PC power supply. I noticed in his pics that there were gigantic (physically) capacitors in the supply. This reminded me that I have seen big capacitors in many power supplies.

    I'm in the middle of building a power supply of my own. All the documentation I've referenced so far has suggested that I use a few different size capacitors as filters. These were all small capacitors, nowhere near what I see in computer power supplies.

    Why the big caps?

    Thank you
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Very basically, the size of the caps are determined by the input frequency, the output current, and allowable amount of ripple voltage. This is a deliberate simplification, as the actual calculations can be pretty complex, and your average hobbyist won't have all of the parameters available to them.

    If your output is drawing a significant amount of current and your input is 50Hz or 60Hz, you will need a significant amount of capacitance to get the ripple down to an acceptable level.

    You might be tempted to say, "Well, I want ZERO ripple!" which sounds cool, until you realize that you will need an infinitely large capacitor to get there. You won't have any ripple, but you won't get any voltage out of it, either. An infinitely large capacitor would look electrically like a dead short.
     
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  3. spinnaker

    AAC Fanatic!

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  4. SgtWookie

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    Jul 17, 2007
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    I'd mentioned both 50Hz and 60Hz.
    I started with 60Hz and 3000uF per each 1A of output current.
    50Hz is 6/5 the capacitance of 60Hz circuits, so for a 50Hz circuit, you'd need 3600uF for 1A of output.

    Your mileage will vary, as transformer output impedance, rectifier used, etc. etc. will also affect the ripple.
     
    Last edited: Dec 20, 2011
  5. crutschow

    Expert

    Mar 14, 2008
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    A voltage regulator (linear or switching) requires relatively small capacitors to filter any high frequency switching noise or to insure circuit stability. But to filter the rectified 50-60Hz power requires very large filters of 1000μf on up.
     
  6. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    So I put the capacitors for the regulator in parallel with the output of the bridge rectifier, and also in parallel with the large filters?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Yes. The regulator capacitors are typically needed to stabilize the circuit and should be placed physically close to the input pin on the regulator.
     
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  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This point is strongly emphasised in the datasheets of some regulators.

    Referring back to an earlier thread on capacitors, there is nothing optional about the common return for such capacitors. The connections should go from the capacitor to the regulator by as short a route as practicable. Long, thin, wandering connections on whichever side of the capacitor add inductance and resistance in series, and can cause problems.

    Reservoir capacitors for the rectifier output are sometimes physically very big, and have their own internal resistances, long internal path lengths as well as possibly quite long connections to the regulator, which may not be very near. These are among the reasons why additional small bypass capacitors may be required close to the regulator. Their capacitance values may appear insignificant compared to the thousands of microfarads of the reservoir, but at higher frequencies the smaller capacitor may have an important effect.
     
  9. JMac3108

    Active Member

    Aug 16, 2010
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    Another factor not mentioned, and one that affects the size of power supply capacitors significantly, is voltage rating. Probaly was not mentioned because everyone replying assumed an off-line AC-DC power supply. But there are plenty of DC-DC power supplies out there operating on much lower votages and using smaller capacitors.

    When it comes to capacitors, voltage rating definitely drives the size.
     
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