AC -> DC conversion phenomenon

Discussion in 'The Projects Forum' started by surfline, Feb 28, 2010.

  1. surfline

    Thread Starter Active Member

    Aug 12, 2009

    I am building a power supply and I have a weird effect happening on the input part:

    I am using 120V , 60Hz input to a step down transformer (stepping down to 30 volts) and then a diode bridge to a 10,000uF filter cap. This serves as the input to the rest of my power supply circuit.

    The input to the diode bridge is 30 volts and the output is 40 volts.. I am using a diode bridge IC. I tested the transformer and the diode bridge outside of my circuit and it worked perfectly without a 27 volt output (predicted from voltage drop of diodes). But how am I getting 40 volt output? Could it have something to do with my enormous 10,000uF cap?

    See attached schematic.


  2. beenthere

    Retired Moderator

    Apr 20, 2004
    That sounds about right. The AC voltage is measured as RMS (root mean square) so AC power calculations will relate to DC power. The effect is that the AC voltage measurement is less than the actual peak voltage of the waveform - the factor is .707 X peak - for the RMS.

    After rectification, the filter capacitors can charge to the peak voltage, less the conduction drops across the diodes in the bridge. Peak is 1.404 times the RMS voltage, or 42.12 volts. The diodes probably drop right at 700 mv, so the voltage ion the filters should be right around 40.72 volts.
    Last edited: Feb 28, 2010
  3. surfline

    Thread Starter Active Member

    Aug 12, 2009
    Thanks! Makes perfect sense now.