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Aerovox Capacitor suitable for high current discharges?

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Dr. Shark

Fri Feb 24 2006, 11:04AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I have two Aerovox energy discharge capacitors that are rated at 250mfd 2.5kV and are of the kind that comes in a package similar to a paint can, i.e. a 10" x 4" x 4" box. I would like to use them for pulsed power stuff (induction coilgun), so basically I intend to discharge them into a near short circuit load and expect heavy ringing and reverse voltage.
The thing is, I have no data on the caps, and I have no idea if they can handle this kind of punishment. So I was wondering if anyone knew what this kind of capacitor is normally used for, and if the internal construction is prone to suffer from the mechanical stress that comes from quick discharges, or if they will be fine.

Steve Conner

Fri Feb 24 2006, 12:56PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If it says "Pulse discharge capacitor" or "energy discharge capacitor" or something similar on the nameplate, they should be designed to stand the stress of fast discharges. Looking at the terminals may give some clues as to the current and magnetic force levels the capacitor was designed for: if they are large hefty bolts on low-profile ceramic insulators then that would be quite encouraging, spade terminals sticking out of small plastic bushings would not make me optimistic

craigsarea

Fri Feb 24 2006, 01:17PM

Registered Member #130 Joined: Fri Feb 10 2006, 08:12PM
Location:
Posts: 15

Wouldn't the reverse voltage be a big problem though? You might need some circuitry to stop the reverse voltage destroying your caps. Or alternatively you could charge the caps to a lower voltage so as to keep the reverse voltage low.

Have you emailed Aerovox to ask if they have any data on the caps? If you have a serial number then they might be able to help you. I've emailed them before and they've always replied.

Marko

Fri Feb 24 2006, 02:36PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

With reversing polarity (ringing with col) problem should not be big for caps, but for coilgun that can cause losses.
Large dide in reverse will stop the ringing but it will have to dissipate a lot of energy...

Dr. Shark

Fri Feb 24 2006, 03:26PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Yes, they are "energy discharge capacitors" with M6 bolt connections, but I assume that even those come in different kinds: Things like big flashlapms (e.g. for lasers) usually use pulse caps, even though they require the pulse to be stretched to several 100us to avoid damage to the flash lamp. I am looking for much shorter discharges, ideally I think I should be using pulse caps and not discharge caps, but who can afford those anyway?

Protecting the caps with solid state devices is probably not going to work, it would be much better to use a triggered vacuum sparkgap, but they are nearly impossible to get. Cheaper to blow some caps

I'll email Aerovox about a data sheet, let's see if they respond...

Marko

Fri Feb 24 2006, 03:34PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

You can short that caps and hardly blow them, they are made for such kind of stress.
You have nothing to protect them as they will ring what they have (in coilgun) and discharge.
Inductive kick is not good for electrolytics as it gives them lots of currnet in wrong polarity.

Such capacitors are (usually) mechanicaly over-engineered to withstand magnetic stresses.

For triggering best idea is mechanical or triggered spark gap as voltage is high enough...

Dr. Shark

Fri Feb 24 2006, 03:50PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Triggering is definitely going to be a challenge, as I live in an apartment I can have zero noise, or someone will call the cops. I will need to construct some kind of sealed spark gap, which is going to be quite challenging if it has to survive the equivalent of .5g of high explosive detonating inside it. I think it will need to be rather large so the pressure from the shock wave drops to an acceptable pressure for common materials like PVC or acrylic.

Marko

Fri Feb 24 2006, 03:56PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Actually some stuffed material that supresses sound is best for your gap.
Use whatever you can find, rubber, ordinary cleaning sponge or so and well designed box should minimize the bang, if you need complete silence then you simply cannot do it in your apartment (coil gun makes eough noise itself)

keith

Fri Feb 24 2006, 06:22PM

Registered Member #148 Joined: Sun Feb 12 2006, 08:21AM
Location:
Posts: 8

joe doh wrote ...

I think it will need to be rather large so the pressure from the shock wave drops to an acceptable pressure for common materials like PVC or acrylic.

I think that as long as you don't entirely seal the container, it doesn't have to be that large. I have shrunk coins, which involves exploding a meter or so of awg 14 copper wire at 10-12kj, in steel containers no larger than 15cm square with no serious problems - I put 20-30kg of weight on top of the container, and all that happens is the container jumps up a bit to release the expanding gas.

Tesladownunder

Sat Feb 25 2006, 04:06PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Just to compare some data with larger pulse caps. The Maxwell I have 110 uF, 12 kV, 8 kJ is rated for 100kA discharge or fault at 80% reversal (typical into a coil system) for 30,000 shots or 1600h DC.
I have 2 of the same Aerovox caps as Keith but these are only rated at 20% reversal.
Running at no reversal gives about 40 times the lifespan as 80% reversal.
Mechanical switches work fine for me (Many shots at 3-4kJ so far with no problems.) Shown below.

Peter

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