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Capacitor Bank Charging Question

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AdamF

Wed Feb 09 2011, 01:59AM

Registered Member #3603 Joined: Tue Jan 11 2011, 11:21PM
Location:
Posts: 11

Hello all,

I have decided to build a coilgun for my final AP Physics project. This won't need to be presented until late May, but I've decided to get into the planning to see what is feasible for me to actually do.

I want to use 400v capacitors for my bank. I've worked out a reasonable final energy for the projectile I have in mind and worked backwards to the energy stored in the cap bank (assuming 1-2% efficiency) and I will end up needing about 3200uF at 400v.

I believe I understand the coil construction theory as well as safety for the cap bank such as anti-parallel diodes, nice thick bus-bars, bleeder resistor(s), etc.

Before I go any further, though, I would like to figure out how I would charge such a capacitor bank.

Charging time is not an issue (relatively), and neither is portability. I want to stay away from a boost converter because the main project is the electromagnetic acceleration, not the electronics of a boost converter. I think I want to use a transformer to step up 110-120v AC to about 270AC and then bridge rectify that which should give me (270)(1.414)=380v. Is this correct?

Also, how should I go about getting a transformer? I will probably end up winding my own. Can I take a transformer out of a wall-wart and just rewind one of the sides? I know the ratio of the windings is the ratio of the step-up/down but what does the number of windings have to do with it (i.e. what is the difference between 1 primary 10 secondary vs 100 primary 1000 secondary windings)? I'm assuming it affects power-handling but I'm not sure. If I current-limit the charging side with a resistor will that effectively limit the current going through my home wiring? Also is it worth smoothing the output from the rectifier with a capacitor or is that pointless because I will be charging capacitors with it and nothing else?

Thanks!
Adam

Dr. ISOTOP

Wed Feb 09 2011, 02:24AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Just use a mains voltage doubler - that will give you ~320V peak.
You can compensate for the lower voltage by using a larger capacitance.

GhostNull

Wed Feb 09 2011, 07:19AM

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

Try searching this forum. I think there have been lot's of discussions on charging. I doubt you'll be able to get a voltage doubler to practically to charge the capacitors from mains.

It would be much cheaper and more effective if you took an old MOT (Microwave Oven Transformer) and rewound it. There are lots of people that rewind MOTs for their own purposes. Try Instructables:

or this forum.

I recall another person was doing a coilgun for their physics project. Just make a quick search, should be easy to find.

Best of Luck
-Ken

Barry

Wed Feb 09 2011, 02:40PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Look for a voltage doubling or "step up/down" transformer as used by overseas travelers. These are relatively common and convert between 110 - 220 vac. I got one at Vetco for $30. Comes complete with switch, fuse and standard power plugs.

Good luck, Barry
Some cereal is barley normal.

AdamF

Fri Feb 11 2011, 04:43AM

Registered Member #3603 Joined: Tue Jan 11 2011, 11:21PM
Location:
Posts: 11

Thanks for the suggestions guys!

I will probably end up using the MOT as I am on a budget and there are several free used microwaves being offered in my area on craigslist.

Another question: Will aluminum electrolytic caps be able to handle the ripple from AC rectified to DC? or should I plan on somehow smoothing the output of the transformer?

Turkey9

Fri Feb 11 2011, 07:37AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

The output from the transformer will be smoothed by your cap bank. It you add a smoothing cap, all you will be doing is adding another cap to your bank. These caps were designed to remove the ripple from large power supplies, so until you short them into your coil, you are doing exactly what they were designed to do.

Dave Marshall

Fri Feb 11 2011, 08:16PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Just a crazy question. Adam, you didn't happen to post about this one on Reddit did you?

Ripple isn't going to hurt your caps. Your only concern is overcharging them really. At 3,200uF, the ESR may be sufficiently high to keep you from blowing them to hell with back EMF, but you should probably still use some clamping diodes.

-Dave

AdamF

Sat Feb 12 2011, 12:11AM

Registered Member #3603 Joined: Tue Jan 11 2011, 11:21PM
Location:
Posts: 11

Wow yea I should have realized that...

No, this is the only place I've posted about it. I will be using clamping diodes. I may come back with some questions about anti-parallel because I'm not sure I understand that totally.

ScotchTapeLord

Sat Feb 12 2011, 03:03AM

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

Look up counter-electromotive force. The inductor keeps the current flowing even after the capacitor has discharged, draw it out and you should see that the inductor will try to charge the capacitor negatively. When it tries to do this, your clamping diode will become forward biased and present a short across the capacitor.

AdamF

Sun Feb 13 2011, 06:22PM

Registered Member #3603 Joined: Tue Jan 11 2011, 11:21PM
Location:
Posts: 11

Ok, that makes sense. But does that mean the cathode of the diode should be attached to the anode rail of the cap bank? That's mainly where I'm confused; in physics we talk about the flow of positive charge but then that screws me up when I think about diodes.

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