Reverse conducting SCR?

Discussion in 'General Electronics Chat' started by Hagen, Feb 9, 2011.

  1. Hagen

    Thread Starter Active Member

    May 8, 2010
    30
    1
    While testing an SCR module I came across a situation I can't figure out. I have simulated the situation in Spice. Simply stated, I am connecting the gate of the SCR to the Anode, and applying DC to the anode and cathode, with a current limiting resistor in series. The thing is, when I apply negative to the Anode, and positive to the Cathode, I was not expecting current flow.
    Wrong..! What am I missing? Why does this "reverse biased" SCR conduct?
     
  2. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    It's a pnpn structure. With reverse voltage across anode/cathode but no bias on gate then acts like two reverse biased diodes (ie. only leakage current between anode and cathode). But if gate is biased then can act as transistor - you should be able to find a nice explanation showing the 'transistor' layout somewhere.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Here ya go

    That explains them well.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    Connecting gate to anode is not a normal operating mode. I think you are seeing the gate-cathode breakdown voltage, analogous to Vbe breakdown voltage in an NPN.
    In the datasheet of 2N5064 (the only datasheet I checked), this breakdown voltage is not even specified (at least, I couldn't find it).
    The Teccor Spice model uses a value of 5V. See ZGK below.
    Code ( (Unknown Language)):
    1. *SYM=SCR
    2. *SRC=2N5064;2N5064;SCRs;TECCOR; 200V 0.8A
    3. .SUBCKT 2N5064   1 2 3
    4. *    TERMINALS:  A G K
    5. QP  6 4 1  POUT OFF
    6. QN  4 6 5  NOUT OFF
    7. RF  6 4    400K
    8. RR  1 4    267K
    9. RGK 6 5    80
    10. RG  2 6    9.09
    11. RK  3 5    0.112
    12. DF  6 4    ZF
    13. DR  1 4    ZR
    14. DGK 6 5    ZGK
    15. .MODEL ZF   D (IS=3.2E-16 IBV=100U BV=200)
    16. .MODEL ZR   D (IS=3.2E-16 IBV=100U BV=200)
    17. [B].MODEL ZGK  D (IS=3.2E-16 IBV=100U BV=5)[/B]
    18. .MODEL POUT PNP (IS=320F BF=1 CJE=402P TF=102U)
    19. .MODEL NOUT NPN (IS=320F BF=100 RC=0.45 CJE=2.01N CJC=402P TF=3.74U)
    20. .ENDS
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    But I don't think it gives any insight as to how it would have transistor action if the gate was tied to the anode, and then the anode was reverse biased.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    True, I was only providing the "equivalent transistor hookup diagram" mentioned in the post I quoted.

    SCRs/DIACs/TRIACs are uncommon enough in most circuits that knowledge of their workings is a bit less than knowledge of inductors, usually less (e.g. not very good).
     
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