thyristor leakage

Discussion in 'General Electronics Chat' started by Zara Engineer, Jan 8, 2016.

  1. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hello everyone


    I have built a circuit to switch a thyristor* on and off .

    The circuit is like the circuit below.

    [​IMG]

    when the gate is on all is right and the device is latched when the gate voltage is removed , but the problem that if I remove the connection of anode (+5v)(with Vcontrol = 5V) the circuit continues to conduct with Voltage of cathode = 0.55V.(VR2).

    Maybe this voltage hasn't any effect but why that's happen? is there any leakage current from gate to cathode !

    *BT151 :
    [​IMG]
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
    Hello,

    A thysistor will keep conducting (once triggered) until the powersupply is removed.
    Normaly the thyristors are used in AC circuits.
    In DC circuits the thyristor can be used as a latch.

    Bertus
     
  3. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    hello,

    ok no problem that the thyristor causes a latch to the device , but my problem here that the gate still conducting and the anode is removed, the result that I found a cathode voltage = 0.55 , what's the reason for this leakage whatever it conducts AC or DC or the device is latching or not!

    thank you in advance
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    Have a look at the attached PDF.
    It will give you the theory and schematics for thyristors and triacs.

    Bertus
     
  5. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    hello ,

    My component is an SCR , I don't have any problem with the basics of scr , my problem is clear , what's the reason of leakage with no voltage applied on anode , voltage only applied on gate and source !

    thank you Bertus
     
  6. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    It looks like a sensitive gate. Desensitize it by inserting a 1 k Ohm or so resistor between gate and cathode.
     
  7. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    Hello Johann,

    How can I know this sensitivity , is it given on datasheet ? is it for all types of scr ?

    I didn't read about that in the scr basics !

    but if we take your suggestion in my circuit , it will be like this :

    circuit3.PNG

    if we do the calculation for circuit we have here a voltage divider : Vk=Vg*(100)/(470+1000+100) = 0.31 V.

    so ?
     
    Last edited: Jan 8, 2016
  8. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Hello Zara,
    I did not check out the specs for your scr, but basically you get scrs with "normal" sensitivity and some like c106D which is a so-called sensitive gate device. It almost triggers when the wind blows over the gate, but should be used in conjunction with a resistor between gate and cathode to prevent false triggering. When I read about your problem earlier on, It just crossed my mind, so I commented.
    Is the control voltage ever removed, or is it there all the time?
     
  9. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    hello again,

    sorry but I edited my reply above please reread it .

    no the gate voltage is all the time .
     
  10. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Try disconnecting on the cathote side instead of on the anode side. This will interrupt both the anode current and the gate current simultaneously and the scr should turn off.
    I mean, disconnect the ground instead of the anode.
     
    Last edited: Jan 8, 2016
  11. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Remember the gate to cathode is also a low resistance in parallel with the new 1k.
     
  12. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Rather connect your load between positive supply and the anode and take the cathode straight to gnd. This way there will not be any signal current flowing in the load.
     
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  13. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
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    hello Johann,

    Ok thank you very much, but about the sensitivity can you help more to understand this idea .
     
  14. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    It is all in the design of the actual scr. Please see page 40 of the following for detailed explanation:

    http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF

    Hope it helps!
    Johann
     
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  15. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Your scr, the BT151 is not a sensitive gate scr. The problem seems to be the position of your load with the constant trigger level that you have.
    As I have mentioned before, rather put the load on the Anode side. Idealy, when you interrupt the anode current to turn the scr conduction off, you should simultaneously remove the gate control also until you need to turn on the scr again.
     
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  16. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
    OK johann thank you .
    I will change the position of my load from low side to high side.
     
  17. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,767
    1,102
    If the control input is continuous then there will be a current flowing continuously through the gate/cathode junction of the scr.
     
  19. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    That is not leakage. That is intentional current that you are supplying through the gate terminal.
     
  20. Zara Engineer

    Thread Starter Member

    Feb 6, 2015
    53
    0

    whatever the anode ?
    is it a characteristic for thyristor only or for bipolar transistor , because I didn't read anything about that in the basics of transistor !
     
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