Rectification on Multisim

Discussion in 'The Projects Forum' started by seanbein, Jan 11, 2012.

  1. seanbein

    Thread Starter New Member

    Mar 16, 2010
    21
    0
    my input ranges from 160vrms~270vrms

    Hi all, based on the picture posted, I have 2 questions to ask:
    1.Where should the ground be?in pic 1 or pic 2?
    2.This is intended for a smps with flyback converter. The output of this rectification will be fed to the primary of flyback transformer.On my transformer design, which voltage should I look to?the Vp-p (eg:160*root two)
    or the vdc which is the output of this rectification?this is confusing because the vdc is about 100v on my channel B oscilloscope regardless of input 160~270 and the measurement probe indicates 49.5 at the second picture where the ground is changed.
    [​IMG][​IMG]
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,031
    Be careful with "ground". Usually, the ground in pic1 becomes ground (the lowest voltage pole) of the DC side. This isolates the DC side from the AC side. But problems arise when connecting "grounds" from different devices that may not share true earth ground. Oscilloscope ground is different than device ground, and current can flow. Bad.

    Something's wrong if your DC voltage is not changing with the AC voltage, unless maybe the sim knows something about your diodes. Maybe they can't handle that voltage?
     
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  3. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,226
    Without a mains transformer the actual earth ground will be as shown in pic2, but the DC reference ground (common) is as shown in pic1.

    The measurement probe voltage will change with a change in ground connection since it is referenced to that ground point. The oscilloscope reading should not change (assuming it has an isolated differential input capability). But the output voltage should change with a change in the input voltage as wayneh noted.
     
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  4. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,031
    Oops, missed that. No isolation without a transformer.
     
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  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    3N249 is made in several versions, rated from 50V VRRM upwards. http://www.datasheetcatalog.org/datasheet/vishay/3n252.pdf You need to have a VRRM (PIV) of at least twice the peak value, plus some margin.

    It is suspicious that your output DC voltage is so low, but even more suspicious that the AC input current is displayed as 14.9kAp-p (!) Something is obviously wrong.

    Note that your "AC" supply also seems to have a DC value, which seems very odd - is this deliberate, and if so, why?
     
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  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Yes: Pretend "oscilloscopes in simulation programs often have inputs that behave as if they were fully isolated from each other - differential inputs, effectively.

    Real oscilloscope inputs are normally single - ended, and share a common reference, firmly connected to mains ground.
    The sort of measuring set up shown in the diagram in practice would result in a short-circuit.
     
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  7. seanbein

    Thread Starter New Member

    Mar 16, 2010
    21
    0
    hi,i have replaced the rectification diodes with 1n4004 in practical and there is no problem with the amplitude changing with varying ac input.however, my output is only a half bridge rectified waveform.any suggestion on what to look at?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Connect the oscilloscope to only one output at a time, not both input and output. Do not connect the input common.
     
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