SCR and Coilgun

Discussion in 'General Electronics Chat' started by Joeshmoe, Mar 5, 2007.

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  1. Joeshmoe

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    Jan 12, 2007
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    I am building a coil gun and until now I have just been switching the current from he capacitors directly into the coil. I have look around and found that one way to improve the performance was to use an SCR. I had a schematic from a website but I cant seem to find it anymore. I was wondering if any of you could help me with the wiring or point me to the right place.

    The scr data
    http://www.datasheets.org.uk/specsheet.php?part=C155D
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    An SCR might do the trick, but they have dV/dT limitations which cause them to fry when the current builds too fast. An IGBT might be more durable.
     
  3. Joeshmoe

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    Jan 12, 2007
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    i understand that there are limits i posted the schematic in the first post i was just wondering how i should wire it.
    i found this http://www.coilgun.info/mark4/scr_wiring.htm
    but but scr has one more wire coming out from it and i dont know what the resistor it for
     
  4. beenthere

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    You might want to read the E-book here, Volume 3, chapter 7. All SCR's are three terminal devices. The resistor is to keep gate power dissipation within limits.
     
  5. Joeshmoe

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    Jan 12, 2007
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  6. beenthere

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    The body (and the stud mount) are the anode, The heavy red lead is the cathode, The small red lead from the off-center connector is the gate. The small-gauge white wire is anyone's guess. It may be electrically bonded to the other small red gate lead. SCR's are three terminal devices - the symbol visible on the body of the device is the symbol for an SCR and not some more exotic device.

    You're buyng a pig in a poke when you deal with Ebay. If you have a industry number identifying the SCR, look it up on line and see if the spec's match up with the seller's. Also, check prices from a catalog like mouser.com to see if it's a good buy. You might think about spending a few bucks more for a new SCR.
     
  7. Joeshmoe

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    Jan 12, 2007
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  8. beenthere

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    If you just switch the capacitor voltage directly to the gate, the SCR will be destroyed. The spec sheet gives a max voltage on the gate as 3 volts above the cathode potential, and a max current as 150 milliamps. The maximum for dV/dT is 100 volts per microsecond. The SCR is only rated for 400 volts across it.

    If you use this device, you will need to come up with a gate circuit that does not exceed the maximum voltage and current.

    You might be more successful if you can figure the coil discharge current. Then try looking at International Rectifier for an SCR that matches the requirement more closely. You will want to have an SCR that can handle more voltage and current than the coil will require. Pay close attention to the gate drive requirement.
     
  9. Joeshmoe

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    Jan 12, 2007
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    so how is it working here---http://www.coilgun.info/mark4/scr_wiring.htm-- is it the resistor that makes the difference?
     
  10. Joeshmoe

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    Jan 12, 2007
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    do I need to make a trigger circuit?
     
  11. Ron H

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    Apr 14, 2005
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    IIRC, the small white wire is the cathode reference for the gate. It's a little like a Kelvin connection. You provide the gate drive current through the small wires. It might otherwise be difficult to find a cathode connection that doesn't bounce several volts relative to circuit GND, due to the humongous current spikes.
     
  12. Joeshmoe

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    Jan 12, 2007
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    so how would I wire it up? i did a draw up in a previous post is that one good?
     
  13. Ron H

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    Do you know the inductance of your coil? How much voltage are you going to charge the cap to?
     
  14. Joeshmoe

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    Jan 12, 2007
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    i dont know the inductance of my coil and currently the max I can charge it is 310v but I want to see the performance at different voltages... also I have bigger caps I just need to make sure I dont over do the amperage and blow the scr(once i figure out how to use the scr)
     
  15. mrmeval

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  16. Ron H

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    The peak current will be a function of inductance and resistance. What is the geometry of your coil, how many turns does it have, and what is the wire size? Maybe we can figure out the resistance and inductance.
    Also - do you know what the equivalent series resistance (ESR) of your cap is?
     
  17. Joeshmoe

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    Jan 12, 2007
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    i dont know how many turns it has but it is 14 gage and i dont know the esr of my caps
     
  18. thingmaker3

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    May 16, 2005
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    If you know the gage of the magnet wire, the inner and outer coil diameters, and the length of the coil, you can use Barry's Simulator estimate the inductance.
     
  19. awright

    Well-Known Member

    Jul 5, 2006
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    I have to disagree with beenthere's and Ron H's description of the small wires on the SCR. The white wire goes to the off-center terminal on top of the SCR body and is the gate connection. The small red wire is called the "2nd cathode wire" and is common with the large red wire main cathode connection. The small red wire is redundant with the large red wire, but is brought out as a convenience for the designer as a reference connector to the SCR cathode. This allows direct connection of the trigger circuit to the cathode and gate via the two small wires, rather than having to bring a cathode connection from the stud or heat sink to the trigger circuit which would be a physical hassle and could cause undesired noise pickup and related problems.

    The di/dt limit for the SCR is related to the optimization of the trigger pulse. A high current SCR has a physically large die and gate structure to dissipate heat and to limit current density. As I recall from writeups in manuals from the early days of SCR development, the trigger current initially turns on an area of conduction nearest the gate connection. It takes a small but finite amount of time for the conducting region of the SCR to spread over the entire junction and the speed of this spread is related to the nature of the trigger pulse (i.e., its magnitude and rise time and source impedance) and the external circuit conditions. If the di/dt is too rapid due to the circuit parameters external to the SCR, excessive current can concentrate in the initial, less than complete conducting region, possibly damaging the die either instantaneously or incrementally over time even if the peak current is within the surge current spec.

    As others have indicated, you really should know what the current pulse shape and magnitude are to apply an SCR correctly. Pulse parameters can be measured with a properly frequency-compensated probe and fast oscilloscope measuring the voltage across a very low resistance non-inductive resistor in series with the main pulse. Application notes from SCR manufacturers (mine are by GE from 4 or 5 decades ago) can give you information on measurement techniques. Try to find an old GE or Westinghouse SCR Manual or check for app notes from current SCR manufacturers. Wire wound resistors will give a false peak current reading due to their inherent inductance. You might get away with a bunch of less than 1 ohm carbon resistors soldered in parallel between two drilled copper or brass plates. You want the peak voltage across the sampling resistor stack to be well below one volt.

    awright
     
  20. detentlatch

    New Member

    Oct 1, 2008
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    I think you will find that the small red lead is the Cathode, and the small white wire is the Gate. Having both small wires makes it easy to connect to the firing circuit, avoiding the need to make a (possibly awkward) connection to the cathode yourself. Oops, I see now that this has been addressed already. (yes, also avoids voltage sags due to current)

    Also, I think you'll find that this "maximum" gate voltage is actually a minimum! It's the maximum voltage that any particular device will require to fire. Been caught out on that one before myself. It's stated more clearly on this spec sheet:-
    http://www.datasheetarchive.com/pdf-datasheets/DataBooks/Document23559.pdf

    Rob Storey
     
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