partial full-wave rectification

Discussion in 'General Electronics Chat' started by Zuma, Mar 17, 2011.

  1. Zuma

    Thread Starter New Member

    Mar 12, 2011
    4
    0
    If I have an AC voltage source that periodically falls to 0volts, is there a method to full-wave rectify only the AC portions of the wave?

    For example, if the input wave looks something like this:

    Code ( (Unknown Language)):
    1.  /\    /\    /\             /\    /\    /\             /\    /\    /\  
    2. /  \  /  \  /  \-----------/  \  /  \  /  \-----------/  \  /  \  /  \-----------
    3.     \/    \/                   \/    \/                   \/    \/
    Is there a simple method to achieve this:

    Code ( (Unknown Language)):
    1.   ____________               ____________               ____________
    2.  /            \             /            \             /            \  
    3. /              \-----------/              \-----------/              \-----------
    Instead of this, which full-wave rectification gives:

    Code ( (Unknown Language)):
    1.   ____________               ____________               ____________
    2.  /            `--__         /            `--__         /            `--__
    3. /                  ``------/                  ``------/                  ``------
    I suppose I'm thinking of a capacitor that just maintains the voltage from one peak of the AC to the next, but then dies off almost instantly, so that it doesn't continue into the 0volt range.

    I just had a thought; I might be able to use logic gate(s) to give the capacitor a low-resistance path to ground, once the input drops below a certain voltage.

    That's a bit more complex than I had in mind though, so I'd be interested to hear if there is a simple way to achieve this result.
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    OK...........from square 1 :

    Is the first graph representing your o' scope trace for an AC source to be fed to a rectifier ??

    You should be showing an unbroken sinewave in either 60 or 50 Hz based on what part of the world you are in

    We'll go from there.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    I don't think your third trace shows full wave rectification of the first.

    I think this is what happens if you add a capacitor to a full wave rectifier.

    Full wave rectifiers do not incorporate capacitors by themselves.
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Do you have a load resistor across the cap?
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Generally this is called demodulation of a amplitude modulated wave. This should help you find more info. You are on the right track for a simple demodulator.
     
    Zuma likes this.
  6. billbehen

    Active Member

    May 10, 2006
    39
    1
    What if you don't use a cap at all, then overamplify with an op-amp to sky the voltage as soon as you have any rectified signal at all? The op-amp will just max out at its positive input voltage. This assumes you don't need a high power output....
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    It looks like you just want a diode, unless you want to maintain the voltage, in which case a cap + suitably chosen resistor will work well.
     
  8. Zuma

    Thread Starter New Member

    Mar 12, 2011
    4
    0
    Thanks for the replies, and sorry for my confused terminology. "Demodulation of a amplitude modulated wave" is exactly what I was after.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the time scale on your plots?
     
  11. Zuma

    Thread Starter New Member

    Mar 12, 2011
    4
    0
    I had no particular time scale or frequency in mind. Well, I was working with a frequency of 100Hz, but thinking of the general case of a digital-like signal, where the "high" sections are a sine wave, and I want to make it more digital-like. "Demodulation" put me on the right track.
     
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