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Capacitor-selection

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Mar 31, 2007.

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  1. For my next project I've just built a PSU using a small 9V-transformer.
    Hooking it up to a Graetz-bridge for rectification, I measure 9.5 volts
    over Vcc/GND (i.e. over "+" and "-" on the bridge) using my DMM.

    Now I'm going to add filter caps. What I'm unsure of here is if
    10v-ratings will work, or if 0.5 volts is a way too small safety margin. I
    have big (~4000uF) 10v-caps in my junkbox, and since the junkbox serve a
    purpose (using older but working components) I feel that I want to pick
    those instead of new 16v-rated caps instead.

    So, will 10v-ratings work or will it be a too narrow margin?
     
  2. Chris

    Chris Guest

    Hi, Rikard. You know what? Your peak rectified voltage is already
    more than 10V. The DMM is reading the average DC, not peak. Once you
    add a cap you'll see close to the peak voltage, which will go up
    substantially.

    It looks like you'll have to spring for the higher voltage caps.
    Sorry.

    Good luck
    Chris
     
  3. A 9 volt AC sine wave swings from +1.414*9 to -1.414*9 or
    +-12.7 volts. A 9 volt transformer under no load may
    produce something like 110% of 9 volts (the rated voltage is
    produced at full rated current load), so those peaks might
    by more than a volt higher than that, approaching 14 volts.
    And sometimes the line voltage will be a bit higher than
    normal. The rectifier will waste somewhere between a volt
    and 2 volts of the transformer output, and the capacitor
    will tend to charge up to the peak of the rectified output,
    under light load. I would go with at least a 15 or 16 volt
    capacitor (little cost or size penalty for using a 25 volt
    unit), at about 4000 uF per ampere of expected output
    current. Expect to get a DC output current of only about 50
    to 60% of the AC current rating of the transformer secondary.
     
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