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Shaun

Mon Aug 06 2007, 05:27AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hi everyone,

I have couple of MO caps that I have some uses in mind for, probably a nice little inductive Marx, but I'm wondering what DC voltage can I safely charge them to. They are all .8uF 2000VAC. Is it just 1.414xVac? I also thought I had read somewhere here that someone had charged them to around 8kV, which made me think it could be something else.

There's a small schematic printed on the side indicating a bleed resistor, but it doesn't show any form of diode which I have been told these capacitors contain. Can anyone tell me how this diode is arranged and if it will affect my Marx? And maybe a safe charging voltage?

...

Mon Aug 06 2007, 06:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

They are only rated for about 2kv, but they usually don't explode till almost 10kv... It is up to you to decide how high you want to go (keeping in mind that the energy stored goes up as voltage^2)

The diode should be a completely separate entity from the cap, I have never seen a MO cap with internal diode.

Reaching

Mon Aug 06 2007, 06:38AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

I have some MO caps with a shematic printed on with! internal Diode. So watch out for the shematic!
They can stand up high dc voltages but be careful. those thingys explode like bombs. i dont like the idea to overvolt them. they have a safe area and wont explode at say 3kv or so, but on 8kV? i have normal high voltage caps rated at 2 kv and i charged them up to 4kv until they died, but some caps of the same ratings died at 2.2kv...

So, its like a overclocked processor. you wont have fun with it

Dalus

Mon Aug 06 2007, 10:28AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Steve Ward used the up to 8Kv

Reaching

Mon Aug 06 2007, 10:49AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

but thats no proof for safely charging up MO caps to 8kV..

what if one xplodes? What if a shrapnel hits your eyes or so?
Hig voltage and especially the things we build are dangerous enough, so you must not provoke the fate.

but, do what you want, who cares

Dalus

Mon Aug 06 2007, 11:18AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

If you read the link you would know he also tested the breakdownvoltage. With his caps there was only a snap heard, so I would't thinks that that would explode. Nevertheless you should use a plexy blast screen when testing them.

Shaun

Mon Aug 06 2007, 03:34PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Thanks a lot guys, I think I'll stick to 5kV to be safe.

But about the diode, I did not find anything resembling an HV diode in the microwaves I got the caps from. I'm pretty convinced its inside the capacitor. How would this diode be arranged? Like, would it be in series with the capacitor, or anti-parallel, or some other arrangement? How could I test for it?

EDIT: OMG wow I had no idea you could crush cans and shrink coins a little with this! Screw the Marx, I'm gonna go for a can crusher! (I already have a Marx, but it only uses 2200pf 15kv caps; I wanted some more power hence the MO caps)

Steve Ward

Mon Aug 06 2007, 05:06PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

what if one xplodes? What if a shrapnel hits your eyes or so?
Hig voltage and especially the things we build are dangerous enough, so you must not provoke the fate.

Not to say you are "wrong" but consider the amount of energy these things store compared with say a real explosive. I think my cap stored around 32j at the time of failure. For that volume of capacitor, this is tiny and certainly wouldnt cause an explosion from internal failure (unless you were charging it from a high power source that would continue to heat the oil inside).

The real trouble comes in when someone makes a huge bank of these things, where if one fails all the rest of them dump their energy into it. Maybe i should do a test and hook up my 12kV 5kj pulse cap to a single MO cap

. In any case, i would recommend something like a 3/4" thick plywood container, possibly internally reinforced with some fiberglass and resin. The surrounding capacitors are likely to absorb most of the kinetic energy of a failure, but id want to be safe if i had actually made the 5kJ bank that i was planning on

Bjørn

Mon Aug 06 2007, 06:25PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Remember that some capacitors can store huge amounts of enery in the electrolyte if it boils. Even a tiny 16V 50uF capacitor can put out an eye if overcharged.

Most capacitors are designed to fail safely but some either explodes or turns into rockets.

Steve Conner

Mon Aug 06 2007, 06:36PM

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

Yes, but the stored energy in the MO cap isn't anywhere near enough to boil the oil inside it. The only risks are like Steve W. mentioned, if other capacitors discharge into it or a high current power supply follows through. (which is what usually makes electrolytics explode)

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