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Driving SCRs for coil gun and EMF protection

Discussion in 'General Electronics Discussion' started by spencer2004, May 14, 2014.

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  1. spencer2004

    spencer2004

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    May 1, 2013
    Hello forum dwellers.

    I am currently in the early stages of designing and building a 450v 5 stage coil gun. I think that I have most aspects of the design figured out except for the switching of each coil.

    I plan to use SCRs for switching each coil as they have very high peak current ratings, the SCR that I plan to use is VS-70TPS12PbF which has a peak current rating of 1100A so it should be good enough for this application.

    http://www.vishay.com/docs/94391/vs-70tpspbf.pdf

    My first question is how do I drive the SCRs from a 5V signal? I don't know if I'm looking in the wrong places or something but I can't really find any information on how to do it. Also is it better to have the SCR between the coil and ground or between the capacitor bank and the coil?

    My second question is what is the best way to protect the capacitors and SCRs from the EMF caused by the coil? I know that placing a diode in anti-parralel with the coil can help with this but does it have to be a particular type of diode?
     
  2. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Well you have not supplied a circuit diagram so I can't see your capacitor bank. And you assume we all know what a 5 stage coil gun is. Well I don't. Can I ask first, how do you plan to turn off the device?. Just making sure you know you can't just remove the trigger. The gate is also going to draw a lot of current, have you thought about what other components you might need for this. You may also have to consider the maximum rate of change of device current as this can over heat the die inside before the device has had time to dissipate the heat. As long as you drive the gate of the SCR with the correct voltage and have enough current to trigger the gate then it should work. So you need to supply say 1.25V (1.5Max) and should be able to provide at least 100mA
    Adam
     
  3. spencer2004

    spencer2004

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    May 1, 2013
    Thanks for your reply.

    For those that don't know a coil gun fires a ferro-magnetic projectile through a barrel using electromagnets, the number of stages refers to the number of electromagnets used.

    This is roughly what the circuit will look like minus the protection diodes.

    [​IMG]

    Am I right in assuming that once each capacitor bank is fully discharged the SCR connected to it should turn off? If not I'll have to implement something that disconnects the capacitors or SCRs from the rest of the circuit.

    Do I need some sort of current limiting on the gate?

    The datasheet says that the SCR will need a maximum of 1.5V to trigger at 25C, should I drive them at 1.5V or can it be a higher voltage (e.g. 5V) as to make the drive circutry simpler?
     
  4. duke37

    duke37

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    Jan 9, 2011
    Rather than trying to drive at a specific voltage, you should drive at defined current (100mA). From 5V, you will have 5-1.25V available so use a 33R resistor.
    The SCR will turn off when the holding current drops below the threshold. This may occur when the coil resonates. The trigger must be shorter than the output pulse.
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I can't see how this will work. The voltage on the output (cathode) of the diode will be a function of the current being supplied by the diode so it won't be far from the high voltage supply as long as it can handle the maximum current passing through it. I don't know what diode you are using what type is it and what current can in handle. Also what's the specification of the coils, the inductance and the resistance of the coils?
     
  6. spencer2004

    spencer2004

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    0
    May 1, 2013
    If you are referring to the 5 SCRs in the circuit then the model number and a link to the datasheet is in my origional post.

    The coils are just going to be some 2mm (12AWG) enameled copper wire wraped around a pipe made of either glass, plastic or brass (haven't decided yet), I have no idea what the inductance is going to be. The wire has a resistance of around 5 milliohms per meter and each coil is going to be prehaps 5 - 10 meters long.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
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    Jan 21, 2010
    Here are some problems:

    1. If the high voltage source is connected the SCRs will not turn off (or the high voltage source will be damaged (or maybe both).
    2. If the high voltage source is turned off, the capacitors will be discharged below zero volts, possibly damaging them.
    3. The current is likely to be limited by the ESR of he capacitors
     
  8. duke37

    duke37

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    769
    Jan 9, 2011
    Surely, a conductive tube like brass will absorb the energy by eddy currents so the gun will have little push.
     
  9. spencer2004

    spencer2004

    17
    0
    May 1, 2013
    The high voltage source will be disconnected before firing.

    Placing diodes between each capacitor bank and it's corresponding coil should solve the negative voltage problem right?

    After looking in to it I have decided against brass for the barrel, it would work but it makes it around 25% less powerful. Glass is too brittle so I'm going to have to go with some sort of plastic barrel and just hope that it dosen't melt.
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I don't think you mentioned the supply would be switched off before I replied to your post o_O But thanks for the extra information.
     
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