Full wave rectifier - smoothing capacitor value?

Discussion in 'General Electronics Chat' started by drabina, Mar 6, 2012.

  1. drabina

    Thread Starter Member

    Oct 24, 2008
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    I am building a simple full wave rectifier with smoothing capacitor. I am using the following transformer to drop the input voltage:

    in: 120V 65W
    out: 12V 4170mA AC

    The rectifier bridge is 50V 25A (I also have 400V 8A). The power supply is going to be used for 18W guitar amplifier based on this schematic:

    schematic

    Since I am still a noob when it comes to electronics, I have have no idea what value the smoothing capacitor should have.

    Anybody can help me out?

    Thanks.
     
  2. jimkeith

    Active Member

    Oct 26, 2011
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    C = 0.7 *i /ΔE /F = 0.024F or 24,000uf

    Assuming 1V peak to peak ripple and 120hZ ripple freq
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  4. drabina

    Thread Starter Member

    Oct 24, 2008
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    Thanks for replies. So if my math is correct, according to Jony130's equation I am getting:

    C = (2A * 1/120Hz) / 1V = 0.016F

    Is that correct? The amp draws 2A max to produce 18W.
     
  5. jimkeith

    Active Member

    Oct 26, 2011
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    Actually, the capacitor may be reduced by the 0.7 factor--jony130's method assumes instant charging--in my method, it is assumed that the transformer supplies the current for 30% of the cycle and the capacitor supplying current for the remaining 70%.
     
  6. drabina

    Thread Starter Member

    Oct 24, 2008
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    Thanks again for the replies. I know now how to calculate the smoothing capacitor value! The problem is that the price of 24,000uF cap is somewhere close to $35. I may be better reusing one of my switching power supplies that I have in my project box.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You don´t need a 24000uF cap, you can use for example 5 4700uF caps in parallel. For 35V they should cost like $5 total.
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But you don't need such big capacitor, simply buy 6800uF.
     
  9. jimkeith

    Active Member

    Oct 26, 2011
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    In agreement with Jony130, the value is not all that critical--I would add that it does no harm to go larger as well.
     
  10. drabina

    Thread Starter Member

    Oct 24, 2008
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    I have few 2200uF caps and maybe one 3300uF. I can probably add up to over 10,000uF with what I have in my parts bin.
     
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