Need help with filtering out AC voltage after rectifier.

Discussion in 'The Projects Forum' started by AndrejaKo, Dec 21, 2010.

  1. AndrejaKo

    Thread Starter New Member

    Sep 6, 2010
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    I have a 220 V 50 VA to 2* 12 V 2.1 A transformer followed by KBL10 (1 kV, 4 A) Graetz bridge rectifier. After rectifier, I get around 10 V of AC, which I'd like to eliminate. After doing some research in the Internet, I'm thinking about using a choke to block the AC part of the voltage.

    I'm thinking right now about using Radiohm 42H22 30 00 or 42V25 30 00 chokes (3 A, 1.2 mH, 70 mΩ per winding). Would they be good enough for my use? If not, what inductance should I use, or even better, how would I calculate needed inductance? I'm pretty new to that field.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    What size of filter capacitor do you have on the DC line?

    You need at least 470u or more to bypass that AC signal to ground and leave you much steadier DC
     
  3. AndrejaKo

    Thread Starter New Member

    Sep 6, 2010
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    I have 2200 μF, but I'm thinking about replacing it with 3300 μF.​
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Are you reading the AC ripple with a load attached or with no load on the line?
     
  5. AndrejaKo

    Thread Starter New Member

    Sep 6, 2010
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    I'm getting it with no load attached.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Attach a resistor that draws about 50% max amps of the supply and recheck the ripple.
     
  7. AndrejaKo

    Thread Starter New Member

    Sep 6, 2010
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    I'm getting around 0.155V of AC, so I guess I was panicking for no reason.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    I'm a little confused as to why there would be much AC ripple at all without a load. Shouldn't the cap be acting as a peak detector, and basically hold steady?
    Where is the 10v ripple coming from?

    The ripple will increase under load, depending on the RC time constant of the filter cap plus load. Under enough load, the cap will have little effect on the size of the ripple, which will just be the incoming pulses.
     
  9. AndrejaKo

    Thread Starter New Member

    Sep 6, 2010
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    The first location at which the ripple shows is at the rectified output. I was thinking about blocking it between capacitor and rectifier
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic?
    That would make the things easier to explain.

    Bertus
     
  11. wayneh

    Expert

    Sep 9, 2010
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    So the "ripple" was actually just the RMS (meter read) DC voltage coming off the naked (unfiltered, no cap) rectifier? That's pretty much what you'd expect and isn't anything to worry about.
     
  12. AndrejaKo

    Thread Starter New Member

    Sep 6, 2010
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    Yes. OK, I'm relieved now. Thanks!
     
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