Thyristor

Discussion in 'Homework Help' started by Cerkit, May 7, 2010.

  1. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Hi. I am looking at a forced commutated thyristor circuit. In the diagram there is an anti parallel diode alongside the thyristor. Can someone explain the need for this diode?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    You forgot the diagram. :p
     
  3. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Ok attached.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I think your circuit is incorrectly drawn.
     
  5. t_n_k

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    This would be more likely to work.
     
  6. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Ok. My question is why is there a diode across the thyristor?
     
  7. t_n_k

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    Perhaps you didn't read the notes on my diagram in the preceding post. I thought they explained the function of the diode. Sorry if they aren't all that legible - the thing was done rather hastily. The notes explain that the diode conducts when the SCR current has fully commutated - the reverse (turn-off) current coming from the charged series inductor-capacitor combination commutates from the SCR to the diode. The diode then conducts until the turn-off current pulse falls to zero - at which stage both the SCR {+ LOAD} and the diode are off.

    If this is all that important to you I strongly suggest you convince yourself about how the circuit works. Drawing some diagrams with current and voltage waveforms often helps.
     
  8. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Ok. Well probably I don't understand how it fully operates. Can you explain to me more what happens when the switch is closed?
     
  9. t_n_k

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    This is presumably your homework so you should contribute as well ....
     
  10. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Ok. So the thryistor is fired and current flows through the load. Then to switch it off the switch is closed and current can flow through that route. The capacitor discharges. I guess I'm stuck there...don't see why the thyristor stops conducting at this point??
     
  11. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Look is it possible to explain to me please, this is not homework, I'm studying for an exam and don't understand. The notes say a lot but I can't figure it out.

    It says that closing the switch forces the anode of the thyristor to negative, charge is removed from the SCR and after a time Toff it can block forward voltage again.

    Can you explain all that in a different manner??
     
  12. JoeJester

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  13. t_n_k

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    There are many classes of forced commutation - the circuit I gave is only one simple option.

    Try Googling something like "forced SCR [Thyristor] commutation classes A, B, C, D, E, F".

    Since your notes probably refer to a particular circuit you might try posting a clearer schematic of the circuit you originally had in mind. Then we might be able to give more specific help.
     
  14. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Ok. I guess I realised which bit exactly I don't get. It says the discharge current from the capacitor opposes the load current from passing through the thyristor. How exactly does it do that?
     
  15. t_n_k

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    Understanding the finer points of thyristor commutation requires some careful thought.

    A simplistic answer to your question is consider the conducting thyristor as a low resistance carrying the load current. If a suitably charged capacitor is switched across (in parallel with) the conducting thyristor current will flow out of the capacitor in the opposite direction to the load current. If the opposing current magnitude from the capacitor exceeds the load current then the net thyristor current will fall to zero - causing the thyristor to come out of conduction into a blocking state.
     
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