How do i not break an SCR?

Discussion in 'General Electronics Chat' started by drodgers, May 11, 2008.

  1. drodgers

    Thread Starter Member

    May 11, 2008
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    0
    Hi, I am building a coilgun and heard that SCRs were good for triggering them, i got a few 800V 16A (200A peak) SCRs from my local electronics store and rigged one up in essentially this circuit (apologies that it is in paint):
    [​IMG]

    I am only charging the caps to about 100v for testing but the SCRs keep breaking, I am rather new to SCRs so do I need a resistor in the gate connection? Do I need a bigger SCR? Is there some other switching device you know of that I can use?

    I don't know the inductance of my coil but here are some SCR stats:
    Igt - 80ma
    dV/dt - 100 V/us

    Thanks for any help you can give.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You have to use a current limiting resistor on the gate. Also, put diodes across the inductor to absorb any spikes created by it.
     
  3. drodgers

    Thread Starter Member

    May 11, 2008
    10
    0
    I plan to use the trigger at multiple voltages and so a resistor calculated for my max voltage would be too large for my tests and so forth. What would be the max current I should allow across the gate?


    Is this gate resistor the only problem or would the dV/dt for a small inductor at 400V also be too large for most SCRs?

    Thanks
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    You appear to have the SCR connected backward. Can't tell if what you draw is what you really meant though. The trigger appears to be misplaced.

    See:http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

    As a finer detail, put the coil on the anode side so the SCRs cathode connects directly to the capacitor. Thus, if you just flip the SCR, but leave the coil where it is, you will be OK.

    John
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    Here's a schematic. Don't forget the trigger supply and capacitor have to have a common ground. John

    Edit: On closer examination, it looks like you have the gate connected to the full capacitor voltage. Note that the maximum gate voltage is limited usually to 20V (see:http://pdf1.alldatasheet.com/datasheet-pdf/view/138680/MSK/MSK4890.html). That is possibly the main reason your SCRs stop working. I have changed the schematic to make that clear. With a 5 to 10 V trigger, you may not need the gate resistor. With higher voltage (for example 12V), it is a good idea to keep it.
     
    Last edited: May 11, 2008
  6. drodgers

    Thread Starter Member

    May 11, 2008
    10
    0
    Thanks, this explains what i have been doing wrong, no wonder I blew so many up :p
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Check out my blog, I use paint regularly for schematics with good results. I have a set of templates ready to use. Looks like I need to add an SCR though...
     
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