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S.C.A.R. 1000 (single coil assult rifle 1000j)

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PSCG

Wed Oct 05 2011, 07:10AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
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Posts: 136

Can i discharge the caps while still attached to the charging circute jus makin sure

I've seen in many coilguns that the capacitor charger is always attached to the capacitor bank without any problem. Also, if you have a good back EMF quenching system there should not be any problem.

ben5017

Wed Oct 05 2011, 10:59PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
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Posts: 156

Could you give me a little more detail on the back emf quenching system, it would be greatly helpfull

PSCG

Thu Oct 06 2011, 07:47AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
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The most usual back EMF protection is a fast diode connected antiparallel with the coil (else known as a flyback diode). Wiki has a perfect page about it. Check it out.

Be sure that the peak current rating of the diode is higher than the back EMF peak current and the voltage rating higher than your capacitor bank voltage (in your case, 450V). Back EMF peak current can be found using Barry's RLC simulator (the best i know). Just throw your data in and the graph will show you the rest.

Damping/quenching also helps minimize the suckback effect (projectile pulled back to the center of the coil after initial acceleration).

ben5017

Fri Oct 07 2011, 05:59PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
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Posts: 156

Thanks, I will most certainly look into those. any more sugestions for starting to optimize performance now that my scr should be delivered any min now?

Saz43

Fri Oct 07 2011, 09:08PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Forgive my unacademic language, but dude, sick coilgun!!! It looks like you've managed to top mine as the coolest looking portable coilgun.

PSCG wrote ...

The most usual back EMF protection is a fast diode connected antiparallel with the coil (else known as a flyback diode). Wiki has a perfect page about it. Check it out.

Be sure that the peak current rating of the diode is higher than the back EMF peak current and the voltage rating higher than your capacitor bank voltage (in your case, 450V). Back EMF peak current can be found using Barry's RLC simulator (the best i know). Just throw your data in and the graph will show you the rest.

Good advice- but I offer some corrections. The quenching diode doesn't need to be a fast type, since coilgun pulses are low frequency (100s of Hz). A standard rectifier will work just fine, and be cheaper (I can verify this, since I used a standard rectifier on my beast).

Also, Barry's simulator assumes an unaugmented RLC step response (no quenching diode or SCR). The result of the simulator is fully valid untill the capacitor voltage crosses zero, then the fact that you have a quenching diode and SCR in the loop changes everything. Instead of the current being driven back into the capacitor as indicated by the simulator (typical RLC ringing behavior), it shorts through the diode, which becomes forward biased as soon as the capacitor voltage crosses zero (not when the current waveform suggested in the simulator crosses zero). Then the current just decays exponentially due to resistance as it circulates through the coil and diode, instead of reversing back and forth in a neat sinusoidal ringing pattern. In simpler terms, the diode begins conducting as soon as cap voltage is zero. For highly underdamped coilguns, this can happen soon after peak current- just check "show capacitor voltage" on the simulator, find where it goes to zero, and this will give you a better idea of what current to expect. Therefore, to be safe, the best reccomended current tolerance of your quenching diode should match the expected peak forward current.

Good luck on your project, I for one and VERY excited to see it finished. Please do post a video that explains how you put it together and of course unleash its fury on some household objects! And of course be safe.

PS If you don't have a quneching diode, that may explain your exploding capacitors... Do you have any residual negative voltage on your capacitors after firing?

PSCG

Sat Oct 08 2011, 08:25AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
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Posts: 136

The quenching diode doesn't need to be a fast type, since coilgun pulses are low frequency (100s of Hz). A standard rectifier will work just fine....

That means that i was very stupid when i used a fast diode for quenching in my PSCG 410 A coilgun? Because i do not use a standard diode quenching topology.

I'm sorry if my information were not correct. I've been working on reluctance electromagnetic projectile acceleratiors for only 2 years and i'm still learning....

@Ben5017: The projectile loading system is bolt - action or automatic?

Saz43

Sat Oct 08 2011, 05:17PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

PSCG wrote ...

That means that i was very stupid when i used a fast diode for quenching in my PSCG 410 A coilgun? Because i do not use a standard diode quenching topology.

I'm sorry if my information were not correct. I've been working on reluctance electromagnetic projectile acceleratiors for only 2 years and i'm still learning....

Not at all- a fasts diodes will work just as well. They're more expensive but since diodes are cheap as dirt in the first place, it probably doesn't make much of a difference unless you're buying like 20 at a time. I hope your project turned out well.

ben5017

Mon Oct 10 2011, 11:28PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
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Posts: 156

where would I buy one of these?

PSCG

Tue Oct 11 2011, 06:42AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
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Posts: 136

Usually, 99.9% of the parts that i use for coilguns are salvaged from TV's, SMPS's, etc. But if you cannot find a suitable diode in your junk box, Ebay is the second thought. You can buy 5 600V 10A (and if i'm correct, >100A peak current) diodes for 7$. So, it's not a loss.

ben5017

Mon Oct 17 2011, 12:29AM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
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Posts: 156

Would these work ?

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