Fully controllable rectifier bridge - Gate triggering question

Discussion in 'The Projects Forum' started by timekeeper, Dec 11, 2016.

  1. timekeeper

    Thread Starter New Member

    Oct 24, 2007
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    Hello. I am designing a fully controllable rectifier bridge using 4 SCR Thyristors. I use an Arduino Nano to detect Zero Crossing and provide the ability to the user to select the firing angle. The user can select the firing angle via a rotary encoder and a small 16x2 lcd displays that current angle. The code is fully tested and I manage to get smooth triggering from 3 to 175 degrees (tested with my oscilloscope). And now comes my problem. I am trying to build the triggering circuit using MOC3021 as opto-isolator. 1.jpg
    In the schematic I have omitted the rest of the MOC3021. +5V is a logic HIGH from Arduino. I think I understand how to trigger Thyristor 1 and 2 (marked D1, D2) but when it comes to D3 and D4 I can't realise how to connect the opto isolator since I can't understand the potential difference. Is it right to say that almost all voltage will be dropped on the load (connected between TP1 and TP2) except for that small voltage drop on the Thyristor when it's on? If so, should I use a resistor like R2?

    Thanks in advance.
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi tk,
    Why do you think it is necessary to phase control the two upper SCR's.?

    Analyse the two possible current paths thru the bridge and you should be able show that only the two lower SCR's are required, the upper two could be regular diodes.

    E
     
  3. timekeeper

    Thread Starter New Member

    Oct 24, 2007
    8
    1
    Hey there ericgibbs, thanks for your answer. That is an option but that would not be a fully controllable rectifier bridge.

    I think I found my answer, I will try tonight to simulate it on my breadboard and write back.
     
    Last edited: Dec 16, 2016
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    It would fully control the output from the rectifier.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I agree with Eric and Albert.
    It's completely redundant to control more than two SCR's.
     
    Last edited: Dec 17, 2016
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are many applications where 4 quadrant is used instead of 2 quadrant, drive manufacturers use 4 quadrant for reversing and dynamic braking.
    Max.
     
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  7. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    A four quadrant bridge rectifier?
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The original servo drive, but mostly 3ph SCR bridge.

    KB Drive
    The KBMG Series of DC Drives are rated for 1/100 to 2 horsepower motors.* This chassis ultra-compact, full-wave regenerative drive is capable of operating DC PM or Shunt motors in a bidirectional mode. Its 4-quadrant operation provides forward and reverse torque in both speed directions. A 5k ohm potentiometer is included.
    Max.
     
  9. ericgibbs

    Moderator

    Jan 29, 2010
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    hi tk,
    This LTSpice shows your circuit operating as a full bridge rectifier.
    I have chosen values to suit the sim, you should compute your own circuit values.

    E

    EDIT:
    Sim now corrected;
     
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    Last edited: Dec 17, 2016
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  10. timekeeper

    Thread Starter New Member

    Oct 24, 2007
    8
    1
    Thanks to everyone for answering.
    Extra thanks Eric for your trouble, I will simulate it on my breadboard. Unfortunately Weekends are full because of my regular job, so I think I 'll give a try at Monday.
     
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