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bridge rectifier help

Discussion in 'General Electronics Discussion' started by madmurg, Feb 7, 2012.

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  1. madmurg

    madmurg

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    Feb 7, 2012
    I have just completed a bridge rectifier which I want to use to power my arduino from a 9v ac power supply. I have fitted a 6800uf 10v capacitor across the pos and neg. when testing it I found that I have 19v dc across the pos and neg. any ideas to what I have done wrong. I have not tried it with load on yet as I dont want to fry my arduino. am I right in thinking when I put load on the voltage will decrease.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Your AC source has probably an open circuit voltage (RMS) of 14 V. The capacitor stores the peak value which is 1.4*RMS (minus ~1.2 V for the rectifier diodes). The voltage will drop if you load the circuit. However, I'd recommend to bring down the voltaeg to a level that is more digestible for the arduino. I'd use a simple linear regulator (type 78xx where xx stands for the voltage - I don't know what input voltage the arduino expects) plus some stabilizing 100nF capacitors. Its easy, look it up in the 78xx's datasheet.

    Regards

    Harald
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi madmurg
    welcome to the forums :)


    and speaking of the voltage out of the bridge rectifier. Asa rule of thumb, its a really good idea to have the voltage rating of the electro cap at twice the DC voltage out of the bridge. else the electro cap is likely to go bang.

    Dave
     
  4. madmurg

    madmurg

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    Feb 7, 2012
    But i am testing at the terminals where the a/c comes in and i am getting 9v a/c surely this would be 14v as well on open circuit.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    is that with or without the bridge rectifier connected ?


    D
     
  6. madmurg

    madmurg

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    Feb 7, 2012
    that is with the bridge rectifier connected
     
  7. madmurg

    madmurg

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    Feb 7, 2012
    attached is a scan of my circuit
     

    Attached Files:

    • vvv.jpg
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  8. davenn

    davenn Moderator

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    Sep 5, 2009
    OK ... Using a resistor divider is not a good way to go especially for supplying a uPC
    there is absolutely NO voltage regulation and the uPC will NOT react well. Time for you to go to plan B .....

    here is a really simple regulated 5V supply using the 9VAC plus or minus a few volts
    that I have drawn up for you....

    [​IMG]

    that should be really straight forward for you :)

    cheers
    Dave
     

    Attached Files:

    Last edited: Feb 9, 2012
  9. madmurg

    madmurg

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    Feb 7, 2012
    Thanks i will try this out
     
  10. BobK

    BobK

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    Jan 5, 2010
    Or you could pick up a wall wart that outputs 5V at a flea market for $2 like I do :)
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    but there's no fun in that ;) where's your sense of experimentation and adventure, Bob ?? hahaha


    Dave
     
  12. BobK

    BobK

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    Jan 5, 2010
    Well, I got tired of building power supplies when I was in high school, over 40 years ago. I still have a +- 12 V supply I built back then that uses zener diodes and a pass transistor for each supply and it still works fine. I am a programmer at heart (and by profession) so I spend most of my time using microcotrollers for inappropirate things.

    Bob
     
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