Capacitor Banks / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Capacitor Banks

1 2 next

Move Thread

LAN_403

Thomas W

Fri Nov 05 2010, 09:29PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Hi,

I'm getting a large amount of 400v 4700uf capacitors about 22 of them with under 20 milliOhms ESR for a Rail-Gun
Thing is that i don't know how i should set the bank up - whats the best configuration?

also i need a way of charging, has anyone got any ideas?
i would preferable like it to charge at the needed voltage and not rising over it as i wish to make it auto charging without me having to watch it then stop it before it gets over volted - i run at 240v mains at home, should i use a boost converter for that many capacitors or a voltage doubler then potential divider to take it down to 400v?

i may have made some mistakes and things in my post - please tell me what if i have :)

Thanks,
Tom Williamson.

ScotchTapeLord

Fri Nov 05 2010, 11:01PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I think for charging you should get a Variac and configure it to 120% voltage and then just full-wave rectify the output into your bank. This is the easiest solution, and it will allow you to charge your capacitors from 0V to slightly over 400. Obviously you wouldn't want more than 400V, so you could just adjust it once to the voltage you want the first time you use it, then never have to monitor the voltage again.

Thomas W

Fri Nov 05 2010, 11:05PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

hmm i think a variac is unforuantely over my budget im looking more into small lightish chargers like mabye a ZVS?

ScotchTapeLord

Sat Nov 06 2010, 01:53AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

You'd only need something like that. At my time of posting the bid was under 10 dollars- and it's for two. It's good to keep an eye out for deals like this one. You don't need a high-wattage one because it will only be on for seconds at a time.

Little ones don't weigh too much but there are certainly lighter solutions.

Rail guns are pretty complex, so in my opinion it's better not to spend too much time on the non-mechanical aspects.

GhostNull

Sat Nov 06 2010, 08:27AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

lol ScotchTapeLord, postage on that is $137.85 to me 0_o

You could rewind a MOT. You can probably able to pick up an old microwave oven for free and take it out. If you are thinking of this:

I will probably take too long to charge a capacitor bank of your size. You also would not be able to build a sufficiently powerful boost converter (at a reasonable price) that could charge your bank in a decent amount of time.

With the capacitor arrangement, for a rail gun, you would want to have a higher voltage for a very high current pulse so maybe put two in series to get 800V or 3 in series to get 1200V. When putting electroyic capacitors in series you need to remeber to use balancing resistors.

Thomas W

Sat Nov 06 2010, 09:08AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

i think i will go with your idea
i know a expert on transformers who should be able to help me :)
Ok one question:

I'm basically gonna be a noob with a large load of capacitors
i will go with 1200v and a MOT for charging but whats all that about balancing resistors?

GhostNull

Sat Nov 06 2010, 09:37AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Hansen, Barry "Parallel and Serial Capacitors"

Thomas W

Sat Nov 06 2010, 10:50AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Thanks,

also this:

Worse yet, if one capacitor is slightly leaky, it will gradually transfer its voltage to the others, possibly exceeding their voltage rating in turn. And if one of them punches through its dielectric barrier, it can then damage others in a cascading fashion. This is why series capacitors are generally avoided in power circuits.

kind of worries me :S

Pinky's Brain

Sat Nov 06 2010, 11:54AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

The next paragraph tells you how to fix it (basically overwhelm leakage current through the cap with leakage current through a resistor, minimizing the voltage rise depending on capacitor leakage current differences). Hell, if you're really paranoid add some TVS's across the balancing resistors, if one capacitor fails short it will blow the fuse.

Thomas W

Sat Nov 06 2010, 01:12PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

ah, thanks :)
also how hard would it acctualy be to wind the MOT to 1200v ?

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob