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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
So, i have a bank of 2 3900uf 400 volt capacitors /which will be in series so 1950uf and 800 volts and 600 joules.
How big should the diode and scr be? Also how many turns should i have for a coilgun? Or you could just give me the approximate current of the capacitors and i can get an scr for a little above that amount. This is the first coil gun i consider to be a coilgun so sorry if i sound stupid.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
It all depends on the resistance and inductance. Search "barry RLC sim" and play with that applet. my SCR is 1200v and 5200A surge, my diode is 1200v and 4500A surge(this is way overpowered), i wouldnt push that past 1kJ.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Saz43 wrote ...
Where do you get a diode like that? I've been searching all over and I can't find any above 100A.
Well i know that they are rated much higher, someone said 10x high than the rating. So those 100 amp diode may have pulse rating of 1000 or more. Check the data sheets.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Are you sure the 100A figure is not for continuous current? We are looking at surge current here, thats what you want. And I would bet the surge rating on a 100A diode is well more than what you need.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Ok I see, it's similar to the SCR where it can handle overcurrent in quick pulses. Is there a way to predict how much the diode will lengthen the current pulse through the inductor when you fire the coilgun? Will adding multiple diodes in series for safety reasons further lengthen the current pulse? Thank you!
Registered Member #222
Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96
Yes, you can estimate how the diode will affect the circuit. Generally, though, you can model the diode as a piece of wire when the diode is forward biased and as an open circuit when the diode is reverse biased. A step further would be to model it as a constant counter acting voltage source due to the voltage drop across the diode. You'll need spice or some other circuit simulator to get a more complicated prediction.
It's different if you are using the diode for clamping for reverse current protection. Yes, you can predict how it will affect the circuit assuming you know the parameters of the diode and coil. If the diode is used for this purpose, then it's probably sufficient to model the diode using short (or really small resistor) and open circuits depending on the situation of course.
Multiple diodes will cause a bigger voltage drop and the coil of the coilgun will see less voltage across its terminals, so yes, it will lengthen the pulse.
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Coilgun/pulse discharge capacitor bank questions.
