Silicon Controlled Rectifier

[Kevin Sevcik]

Originally Posted by teh_pwnerer795

This year our robotics team could not afford to take part in FIRST Robotics. So we divide our team into two. Our group is building a EM Gun. Its going to be a 3 stage coil gun using #23awg copper wire. Our barrel is a aluminum hallow shaft measured at 3/8 of an inch. Im still trying to work out the numbers and schematics but everything takes time and research I am going to be using voltages around 350-400 and SLR's are apparently really good at controling and switching these high voltages and ampere's.

As far as your barrel material, you should remember that metals conduct and thus will generate eddy current opposing the magnetic field of your coil and robbing you of some power. There a few ways around this, but I'll leave that for you guys to discover.

Originally Posted by lukevanoort

While SCRs can work for you application, I'd only use them in a single coil coilgun design. MOSFETs or IGBTs are probably a better choice since you can turn them off. Also, don't forget to put a diode across each coil so that you don't fry your electronics from the current generated when the field collapses.

You could use SCRs if you powered each coil separately, which I think you'd be doing anyways. I think starting with SCRs would be a safer bet than IGBTs since they'll stay on until they've dissipated all the energy in the caps. This would reduce your rate of fire since you'd have to recharge from zero every shot, but you wouldn't have to worry about a charged cap wanting to kill you after a firing.

Also, it's highly important to understand the purpose and placement of the extra resistor and diode you'll be adding. The coil of the gun is basically an inductor, and will be trying to keep a constant current going through it. Once your cap is spent, the constant current from the coil will act to charge it in the negative direction, which is baaad for the polarized caps you'll be using. So the idea is to re-route this current through a diode and resistor to dissapate the energy stored in the coil. The circuit would look something like:

           SCR
 + --------|>-------
               |   |
      Resistor Z   3
Cap            |   3 Coil
         Diode ^   3
               |   |
 - -----------------

The size of the resistor depends on how quickly you want to dissapate the energy, and how large a voltage differential your circuit can handle. The maximum current going through it is determined entirely by your coil's inductance and cap's size. After that, Ohm's law will tell you what the voltage drop across it will be and how many watts it will dissipate at this peak current.