Full Bridge rectifier working as half bridge

Discussion in 'General Electronics Chat' started by Tahsina Hossain, Apr 20, 2015.

  1. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    Hello there, I am using a full bridge rectifier (as shown in picture) at the inverter output but my full bridge rectifier is behaving as a half bridge. I tried to test the diodes through multimeter, it seems to work fine.

    Whenever I use the oscilloscope probes to check the voltage drop in the shunt resistor (to check my output current) it shows me only one side is getting rectified.
    upload_2015-4-20_10-20-52.png

    Untitled.png

    Please help, I am really getting stuck in here.

    Best Regards,

    Tahsina
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Your circuit is wrong. The 0.156 ohm resistor should go to the centre point (a-k connection) of the first two diodes. Not the junction of the two cathodes.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    Does the inverter actually produce AC, i.e. does the output change polarity?

    It says "Unipolar output" ??
     
  4. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    Hi yes, the output of the inverter is unipolar AC.
     
  5. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    I am using the shunt resistor to measure the output current of the inverter, If I connect it centre point (a-k connection) of the first two diodes shouldent it behave like a load?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    What is "unipolar" AC? Sounds like pulsing DC to me.
     
    DerStrom8 and gerty like this.
  7. uwed

    Member

    Mar 16, 2015
    64
    17
    In case you use both voltage probes of the oscilloscope at the same time, and they are probably not differential probes, and in case you put the probes' ground onto 2 different points, then this will short-circuit some components, which would change the circuit behavior. It would look like some diodes don't work properly as long as the probes are attached to the circuit.

    This is just a speculation because I don't see how you attach the probes and what kind of probes you have. However, check it to be sure. Maybe perfom the same measurements with only a single probe attached to see if something is different then.
     
  8. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    Hi, that's a great observation, I have kept the grounds at the same point, I am using a common ground of the inverter and I am taking the probe grounds in there, I am using math function to see the drop across the resistor, is that right? I have a very complicated diagram but I will post a photo of the real circuit today.

    Please let me know if I am doing anything wrong.

    Best Regards,

    Tahsina
     
  9. uwed

    Member

    Mar 16, 2015
    64
    17
    upload_2015-4-20_19-16-56.png

    Your circuit including parameter values look OK. I did a little simulation, see curves above, and it looks a lot different what you have on the oszi. Is your capacitor really connected? What do you mean by "uipolar AC source"? The source usually provides a sine wave, going from -25*sqrt(2) to +25*sqrt(2). Or did I misunderstand something?
     
  10. uwed

    Member

    Mar 16, 2015
    64
    17
    Or is the inverter at the input producing unipolar rectangular voltage pulses? It might be helpful to measure the inverter voltage waveform.
     
  11. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    Hi, I did the same simulations and it works fine in the simulations, the oscilloscope data is different. I measured the inverter output voltage and it is unipolar. What I am doing in my inverter circuit that is I am taking input voltage of 25 volts from a dc supply and using four IGBTs to produce unipolar AC output at the inverter output side, While in simulation it works fine but in the practical circuit looks like only two of the diodes are doing their job. I tested all the diodes and they are fine. I just dont know what am doing wrong!!

    I really appreciate your help and insights on the issue.
     
  12. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    So, you actually have pulsed DC rather than AC; This is why your bridge rectifier only conducts half of the time.
     
  13. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    Hi, so how should I make the changes to conduct the full wave.
     
  14. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    The bridge rectifier must be driven from a source that produces true AC, i.e. one that changes polarity.
     
  15. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    I am generating modified sine wave from my inverter output and it changes polarity.
     
  16. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    Ok, so what is your definition of "unipolar"?

    You have a single-rail 25VDC supply (is that correct?), how do you generate the negative half of the output waveform?

    Or, is the output configured as an H-bridge?
     
    Last edited: Apr 20, 2015
  17. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
    40
    0
    Hi all, thanks a lot for all the support and advice, I found out that I was measuring on a wrong ground. Now it works fine :D
     
Loading...