:: Help with rectifier ::

Discussion in 'General Electronics Chat' started by suby786, Oct 30, 2006.

  1. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    when i wire up the 1st ckt with the 4 diodes, and connect my osciliscope to the out put pins... (no resistor or caps ) just to the scope... it doesnt come out with a full wave rectified but only a half wave, which i get if i just use 1 diode in series...which is the 2nd image

    whats going on?

    im using a variac to supply 12v AC down from 240ac...

    All i want is a fully rectified signal... i dont even know where to GROUND my caps... if ground it to the EARTH LEAD it sends my signal down to zero and 1 diode gets VERY HOT...

    HELPPPPP
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Your diode arrangement is a full-wave bridge. It is valid for producing a positive and a negative voltage from the secondary of a transformer. But, there's a gotcha. The transformer secondary must have a center tap. That center tap becomes the ground reference.

    If your transformer has no center tap, then all you can do is half-wave rectify it. One diode will do the trick.

    Your arrangement shorts the transformer to ground (earth) every half cycle.
     
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    You should be able to scope full-wave rectified voltage on the DC output pins.
    Connect your scope ground to the pin called 0v and the probe to the pin VS-1.4V and you will see the rectified output. But make sure that your scopes input ground isn´t connected to the mains ground, or you will again short the transformer.

    It is also much better to use isolation transformer or normal 12V transformer to do the measurements, as there are no problems with shorting to ground.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    You can get full-wave rectification with his bridge and a non-center-tapped transformer. You can't get positive and negative voltages without the center tap.
    The transformer secondary must be isolated from GND. I suspect that one side of his input is also connected to GND, such as a non-isolated Variac.
     
  5. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
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  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Isolated variacs are available, but the non-isolated ones are more common. The user should always assume the transformer is not isolated unless he has specific knowledge that it is isolated.
     
  7. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    the file attached is exactly how ive wired it...plus a 1000u and 100n capacitors

    im not using a variac now directly...i got myself a 240v primary and 9v secondary isolating transformer mounted on my board now..

    so anyway on my scope (its 1 of those fancy 1's loaned 4rm my uni) it only shows a HALF rectifed signal, and builidng upon what sum1 said about my scope SHORTING it to ground may mean it MAY be full rectifying..

    i took the circuit in2uni and tried it on an OLD scope... and it seems that it is full rectifying it, which makes sense as my 5v regulator is producing 5v and im taking the ground off where it shows in this diagram... from the join of 2 diodes... and it works pretty well...
     
  8. EngineerJoe

    Member

    Nov 4, 2006
    15
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    The ground clip of most oscilloscopes is connected to earth ground through your power cord. The reason you can only see half of the full wave is because there is another ground in the circuit which shorts out one (or one pair) of the diodes (consequently, the hot diode).

    The other ground is probably within the stepdown transformer. If it was made with a single coil (an autotransformer), it will be a three wire terminal device. It may have a 4th wire that is common to one of the other three. An ohm meter will tell you if you have isolation between the input and output connections of an unenergized transformer. Try drawing your circuit with a ground connection at one end of your low voltage transformer output and then draw another ground connection to the + or - terminals of your full wave rectified output. This is what happens when you connect the ground clip of the oscilloscope.

    The amplifiers used in most oscilloscopes are called single ended amplifiers meaning that they require a ground "reference" of some sort. What you may want to use is a device that has a differential input amplifier or just get a different transformer. Magnetic isolation refers to independent coils or windings on the same transformer core where there is no electrical connection between coils. Autotransformers are cheaper to make.
     
  9. vikram_kumar

    New Member

    Nov 2, 2006
    3
    0
    we r not connecting the load resistance thats y we get half of sin wave
    connect the circuit fig1 we get full sinusoidal
     
  10. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    i tried that...putting a resistor there for now...as my load is a 5v regulator...i tried that... im gettin what i require from the circuit anyhow...but im unsure on what exactly my circuit is doing...i just wish i could see the theory and these images come 2life on my scope lol...

    apparently you can get enough of an rms from a half rectified signal to power my regulator...but its obv better to have it full rectified...
     
  11. haditya

    Senior Member

    Jan 19, 2004
    220
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    forgive the intrusion..but how does a non-isolation xmer like a variac really affect the circuit operation.. ??
     
  12. haditya

    Senior Member

    Jan 19, 2004
    220
    0
    aaa..i think figured out what happens on using a non-isolation xmer and on connecting a probe (of the CRO).
    the cro probes are connected to 2 points across the output. however the ground probe also connects to supply ground throught the cro power cord. this then causes the mains to short out durin a half cycle
    is this explaination correct?
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Yes, the scope ground shorts diode D1 in the attached drawing, which means that diode D4 shorts out the variac during a half cycle.
     
  14. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    so the conclusion is...

    that i cant actually be able to see the actual signal... and it MAY be doing what its suppose to...but i cant see it due to this CRO issue?

    so at the end of the day my circuit will work just fine...without a CRO connected?
     
  15. PAB

    New Member

    Jun 21, 2007
    6
    0
    sorry i know this is slightly off topic but i am new to this forum and have not worked out how to start anew topic.

    What i am after is the formula to calculate the output voltage of a polyphase rectifier.

    Eg. I have a 3 phase PRI (415VRMS) supplied transformer @ 50Hz, and SEC output of 380VRMS on a DELTA Tap, and 380RMS on a STAR Tap.

    This gives me 2 sets of 3 phases with a phase difference of 30degrees (if you impose the star winding over the delta).

    Once (SCR) rectified there will be 12 pulses @ 220VRMS (311peak).

    HOW DO I CALCULATE THE RMS recitiied voltage for this circuit (I understand there will be a small ripple voltage between waveforms.
     
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