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why it become 20vdc after bridge rectifier while supplied 18vac

Discussion in 'General Electronics Discussion' started by janeliza, Mar 4, 2015.

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  1. janeliza


    Mar 4, 2015
    hello. i have a question. i built a power supply whose desired to have an output of regulated 12vdc, the problem is when i've done the assembly, when i test it the output becomes 20vdc right after bridge rectifier, while i've supplied it with 18vac. what shoul i do to make its output 18vdc or lil bit lower.? also when it passes thru regulator, it doesnt change the output voltage. i really hope for your help.
  2. davenn

    davenn Moderator

    Sep 5, 2009
    welcome to the forums :)

    doesn't sound correct, it should be higher ... if working correctly you should be seeing ~ 25V - 28V

    the regulator obviously isn't wired correctly

    This may be the cause of the first problem

    show us your circuit and be prepared to also show a pic or 2 of your actual construction
    so that we can possibly see a wiring error

  3. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    The regulator is either connected around the wrong way or the "common"terminal is not connected to 0v or the regulator is damaged.
    The 12v regulator can deal with 15v to 25v.
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