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Capacitor undervoltage?

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LAN_403

noobletmonger

Sun May 06 2007, 02:27AM

Registered Member #683 Joined: Sun May 06 2007, 02:00AM
Location:
Posts: 9

First off, this site is awesome! I have been browsing around here for a while and decided that I would like to make my own coil gun. I have done some research and I have a couple of questions.

I am thinking about purchasing some 360v 200uF photo flash capacitors for use in my first coil gun. I already have a power supply unit (from a camera) that charges a 330v 120uF capacitor just fine. My question is will using the power supply unit I have on the higher voltage capacitors cause any damage (to them or the power supply, forget me

)? My guess is no, but I want to make absolutely sure because I have never built a coil gun before. Also, if I decide to use multiple capacitors, I should connect them in parallel, correct?

Thanks for your help!

Barry

Sun May 06 2007, 03:26AM

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

Welcome to coilgunning.

I would expect that your HV power supply will work just fine to charge up the additional capacitance. It will charge up more slowly, of course, but it should work fine.

Connect the capacitors in parallel, and the total capacitance is simply the sum of the individual capacities. Just be sure to observe correct polarity (join all the +'s together). For more background on what happens when you connect capacitors together, read Multiple Capacitors on my web site, or Capacitor Banks in the HV Wiki.

Cheers, Barry
I used to have a handle on life but it broke off.

noobletmonger

Sun May 06 2007, 04:07AM

Registered Member #683 Joined: Sun May 06 2007, 02:00AM
Location:
Posts: 9

Thanks for confirming that with me. After I posted that I looked up "series and parallel circuits" on wikipedia and figured it out.

Here's a few more questions since I don't think it would be necessary to start a new thread:

To handle the back-EMF should I use a few 1N4007 diodes connected in series antiparallel to the capacitors? (I hope I used the right wording)
Do I even need to worry about back-EMF?
How do I figure out how many joules, volts, and amps my capacitor bank will have?
What type of mechanical switch should I use to discharge the capacitors?

Hopefully when this is complete it will fit inside an altoids tin. Probably not, but it would be cool!

Simon

Sun May 06 2007, 11:57PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Please do a whole lot more research before playing with those caps. They aren't that dangerous but only if you understand them.

Our own wiki is a good start. See
Coilguns and Capacitors.

noobletmonger

Mon May 07 2007, 09:37PM

Registered Member #683 Joined: Sun May 06 2007, 02:00AM
Location:
Posts: 9

Indeed. I have already read those articles and I am well aware of the dangers involved. "Playing" might be the word to use for someone who is not careful and does not take this seriously. That is not me.

I had once made a large Leyden jar out of a plastic "barrel" used to hold dog treats. It was about 1.5 feet tall with a diameter of 1 foot. This was last winter so the air was very dry and I would charge it up using my rubber-soled slippers. I had left it partly charged overnight and in the morning, not realizing it was still charged, I picked it up a gained a new respect for electricity.

Fully charged, the spark is about 1-2 inches long and can burn a small pinprick through a piece of paper.

Just a little story for you guys!

Simon

Tue May 08 2007, 12:31AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

A healthy respect for electricity is definitely healthy. The main thing you need is background reading.

If you read enough articles, look at other coilguns and tinker with simulators you should be able to answer your questions, especially this one:

wrote ...

How do I figure out how many joules, volts, and amps my capacitor bank will have?

Not to discourage you at all; get reading!

wrote ...

To handle the back-EMF should I use a few 1N4007 diodes connected in series antiparallel to the capacitors? (I hope I used the right wording)

For a small first capacitor this can't hurt. Many people add a small resistor in series with the diode. There have been lots of experiments with various ways to deal with back-EMF (including ways to recapture the energy) so this is an open question.

noobletmonger

Tue May 08 2007, 02:28AM

Registered Member #683 Joined: Sun May 06 2007, 02:00AM
Location:
Posts: 9

Thanks, I will definitely take your advice and get reading.

On a side note, all of the necessary parts will fit inside of an altoids tin. Maybe I will start a thread on it when I am done to get advice on improvements. Just waiting for all the parts to come in the mail!

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