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newbie question on caps

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LAN_403

cookster

Tue Mar 10 2009, 07:16AM

Registered Member #2017 Joined: Tue Mar 10 2009, 07:03AM
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ive seen caps with high capacitance and low voltage and caps with high voltage and low capacitance which is best to use for a coil gun and why.thanks for any help.

KLH

Tue Mar 10 2009, 05:07PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

First of all, DID YOU READ THE RULES? They state that you should do a preliminary search on your question on the HvWiki or on Google, or even Wikipedia. This is a really simple question that requires a really simple search on any of these (on coilguns and what kind of caps they use).

Anyway, to answer your question, you should use high voltage caps since the pulse time and main coil current is more appropriate for most projectile / coil combinations. You need to be more specific about how your coilgun is setup.

Be sure to do a search next time.

rp181

Tue Mar 10 2009, 08:36PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Any voltage and capacitance will work. It all depends on how it is setup (the coil). For higher capacitance, it is more difficult to get to work, as inductance and resistance needs to be low, and the coil long. Higher voltage is easier to get to work, and will almost work with any coil (How well it works is a different matter).

DYI

Tue Mar 10 2009, 09:32PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Hmm, I thought that high voltages had a habit of saturating projectiles quickly and making coils difficult to insulate, and that low voltages around 400V or so were easier to work with.

I am, of course, not an expert on the subject by any stretch.

rp181

Tue Mar 10 2009, 09:40PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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In this case, I think he was referring to High voltage as 400v, ad low voltage high capacitance to those 16v 120000uf capacitors.

Barry

Wed Mar 11 2009, 06:50PM

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

In theory, it doesn't matter. The potential energy available in joules is PE = 1/2 * C * V^2 and you can use any combination of capacitance and voltage to store the energy.

"All you have to do" is match the discharge time and energy density to the projectile. This can be done with careful design.

In practice, there are practical difficulties at the extremes of high voltage or high capacitance. The first concern is usually the cost of switches, since silicon devices get expensive at extremely high voltage or extremely high current.

Silicon devices are relatively common in the range of 40 to 400 volts. I wouldn't call either end of this range "extreme". You can build a successful coilgun either way.

Quite often, people find the price and availability of capacitors will dictate their choice of energy storage device. I'd say go with whatever you can afford and find, and buy as many joules as you can. Be sure to compare prices on the basis of price-per-joule <points to formula above>.

Cheers, Barry
The road to success is always under construction.

hotcrazyfruit

Thu Mar 12 2009, 01:53AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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What i always do, is look at my power source first. you want to get the most out of the capacitors, so you find a capacitor that is rated for slightly higher than your power supply can output (20-30 volts so you don't blow em up or boil them xD) THEN you go for capacitance. since capacitance is not a variable (once you have your capacitors) you should buy them adjusted to your power supply.

Barry

Thu Mar 12 2009, 02:30PM

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

Good idea. It's a system - all the parts had better be able to operate at the desired levels. Sometimes the SCR is the limiting factor, but it's a little harder to guesstimate the peak surge current expected.

By the way, I wouldn't hesitate to use a capacitor to its WVDC rating (working voltage d.c.). It has already taken into account the 10% safety factor, which is usually reflected in its "max voltage" rating which is usually printed on the can next to the wvdc rating.

With this much stored energy, it's always a good idea to test the leakage rate. That is, measure how well the cap stores a charge over time with nothing else connected to it. Because it's the leakage through the internal resistance that can generate heat and boil the electrolyte until it explodes hot acid in all directions.

Cheers, Barry
Lot's of people don't realise it but I'm photoshopped. My eyes were done using duplicate and flip horizontal.

hotcrazyfruit

Fri Mar 13 2009, 11:26PM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Barry wrote ...

. Because it's the leakage through the internal resistance that can generate heat and boil the electrolyte until it explodes hot acid in all directions.

ITS ACID!!!! omfg. i made a coil gun, with 18 camera flash capacitors, i meant to only charge them to 220, because its all i had at the time, but accidentally charged it to 460,(they are rated for 330WVDC) which resulted in immediate boiling and "detonation" of the capacitor bank , (which was the handle). it smelled absolutely horrible, almost indescribable. ill be more carful next time. :)

Barry

Sat Mar 14 2009, 02:10PM

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

hotcrazyfruit wrote ...

Barry wrote ...

Because it's the leakage through the internal resistance that can generate heat and boil the electrolyte until it explodes hot acid in all directions.

ITS ACID!!!! omfg.

hehe, I see the light dawns over the result of an experiment gone horribly wrong. Yeah, hence the root word "electrolyte". You've discovered that, in addition to leakage current, massive dielectric breakdown and internal arcing will also boil the acid. Next time get a video for us!

Electrolytic capacitors are made by etching a dielectric layer on aluminum foil. Each company's particular chemical bath is a closely-held secret cocktail of juices designed to gain advantages over other manufacturers. But it always starts with acid.

Cheers, Barry
The trouble with being punctual is that nobody's there to appreciate it.

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