AC capacitor question

Um -- what the heck are you talking about?

There are capacitors that are made primarily for service with zero DC bias, there are capacitors (polarized electrolytics) that are made for service _with_ a DC bias, and there are capacitors made for service attached to the electrical grid.

Open a Digi-Key catalog and eliminate the half-dozen most popular dielectrics used to build capacitors with and you'll still have entries to spare -- and that's not including oddball dielectrics, nor is it taking into account the wide variation in ways that capacitors are constructed _using_ these dielectrics.

I've seen a lot of different categories under which capacitors are sold, I don't recall ever seeing capacitors sold as "AC capacitors" or "DC capacitor".

Perhaps if you said something like "I have very little practical knowledge of capacitors and I want to select the type that's best for a rail gun" then folks would be able to help you out.

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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

What I meant is that DC capacitors are polarized and AC are bipolar. I would like to charge a, AC Capacitor with direct current (electrolytic capacitor ) , there just need the positive to zero. From what I can read from you, I will be able to do what I need to do. I am just saying AC capacitor because you can see VAC on these capacitors.

cheers

It will cost you quite a bit more, for lower performance. DC electrolytics pack a lot more storage into the same space and cost a lot less. AC electrolytics aren't intended for DC applications.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

"lerameur"

What I meant is that DC capacitors are polarized and AC are bipolar. I would like to charge a, AC Capacitor with direct current (electrolytic capacitor ) , there just need the positive to zero. From what I can read from you, I will be able to do what I need to do. I am just saying AC capacitor because you can see VAC on these capacitors.

** That is an oil filled film ( likely polypropylene) capacitor with two sections, 5 uF and 17 uF.

.... Phil

That looks like an engine start/run capacitor; while it is rated in VAC, that doesn't make "AC capacitor" a meaningful term.

Here in the US, DC is polarized and the rest of us are tending to get bipolar over it, but that has nothing to do with capacitors.

I'll reiterate -- ask what you really mean to ask. If you're looking for good caps for a rail gun, then ask that. If you know what the rail gun is going to ask from it's caps (bipolar voltage? Always unipolar? Quick discharge?).

I suspect that a rail gun needs to discharge its caps very quickly -- this is an issue with aluminum electrolytic caps; you can't just use any old 'lytic and expect it to deliver tons-o-current all at once. So normally storing lots of energy as DC would be a task for 'lytics, but if you _also_ need to discharge that energy lightning-fast, you may need to either carefully select your 'lytics, or use some other, far bulkier, cap.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

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Sure, your 'bipolar' caps are great at DC. Sometimes lots better than your 'DC' caps. DC or poalrized caps are great because they pack a lot of charge into a small volume. But you pay for this density with all sorts of odd behavior.

George H.

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e quoted text -

Hi Tim, Just wondering if you'd want the rail gun cap to discharge like a transmission line?? You've got to hold some energy as the 'slug' travels down the rail. You might even want to try and shape it.

George H.

An ac cap is just a cap, with the added advantage of being able to survive ac current and voltage. It pays for that with a much lower capacity_per_volume compared with the dc cap, but the voltage range is much higher. For a railgun, the problem is, that it forms a resonance circuit with your cap, and your design needs to address the problems that causes, like a blocking diode to stop the current from reversing, and swithes and diodes being able to stand very high currents and voltages.

You might consider an agn lead battery instead, which can supply a shortcircuit current of about 4000 amps.

Switching that will be very difficult, maybe a heavy mercury wetted swith might work.

To experiment, you can of course even close the wire- cicuit any way you like, but expect your wiring too burn and melt. Something has to melt of course because the battery can stand the short for only a few tenth of a sec.

You might switch the current by moving the railgun slidebar from an insulated part of the rail to the bare part of the rail. Again, be carefull, use thick cotton or wool protective clothing and welding handgloves. And tell us how things went.

quoted text -

I have no clue -- I've never messed with rail guns.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Basically I need about 6kv with 30uF, with a fast discharge. I am trying to build a DC tesla coil. as shown here:

I found on ebay this capacitor

Do you think it will work for me? or do I need an electrolytic capacitor?

k

Non-electrolytic capacitors are OK for Tesla coils, whether AC or DC is used.

Meanwhile, 30 or 60 uF sounds high to me for making a Tesla coil that is not extremely huge. That sounds high to me by about 3 orders of magnitude for this voltage range for a table-top size Tesla coil. And these huge capacitors can probably electrocute an elephant.

- Don Klipstein ( snipped-for-privacy@misty.com)

You are getting into something where the internal construction of the cap may matter more than type.

For a rail gun, you want all the energy out of the cap in one fast discharge - so the cap has to have low "external series resistance" and a low inductance type of construction - interleaved sheets of dielectric/plate work better than a cap with the dielectric and sheets wound up.

Large heavy braided copper connections - or copper pipe with large surface area, and short connections to minimize losses.

Caps designed for "commutation" use are a good bet - non polar and designed for relatively high voltage, high discharge rates.

RF caps another good choice - very low ESR/inductance, very high voltages. AND very expensive as a rule

Check out what Tesla coilers use - they have the same requirements, high voltage high discharge etc.. Do it yourself glass plate caps are fun - maybe too low in capacity for your aps, but they are inexpensive, big bulky, and can be made nearly indestructible. (a paper cutter will cut thin sheets of aluminum flashing material - to make some dynamite stacked foil caps)

Photoflash caps are a good choice - DC electrolytic designed for fast discharge at moderately high voltages.

Lastly, an ordinary electrolytic or HV cap will sometimes fail open when the lead connecting to the internal foil vaporizes at the connection (usually a spot-weld between copper and aluminum) commutating caps, photoflash, RF caps are usually designed with that in mind.

Rail gun or coil gun? Same type of cap requirements though.

These days AC caps can be made quite small - so a lot of the "motor run" caps are now non polar, non electrolytic, with good "dissipation factor."

Once again proving that Elephants have no business playing with high voltage! ;-)

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

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