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Bjørn

Mon Aug 06 2007, 07:29PM

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

Measuring the voltage and calculating the stored energy based on capacitance is not a safe method since the energy might go somewhere else. You need to know how much energy is put into the capacitor to know if it has the possibility to fail in a really dangerous way.

When overvolting a capacitor you are not completely sure if it will react as specified in the datasheet so it is a very good idea to keep an eye on the total energy put into it. If not it might be unsafe.

Reaching

Mon Aug 06 2007, 10:50PM

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

the best thing you can do is to let your eyes away from MO Caps and People who overvolt them totally careless. its like you put 8 times 120v into a tv. what would happen?

A 2 kv rated capacitor is not manufactured to stand up 8kv(For a long time), remember thats 4 times the voltage!!

i dont know, but i thought there are serious members out there who take a look on this thread and say "wow, dont do those silly things".

im finished with this thread, it makes me angry to see those ...anyway

Shaun

Tue Aug 07 2007, 01:26AM

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

It seems I have sparked a small debate on the pros and cons of overvolting capacitors. But I was actually not planning on overvolting them, I just thought they were purposely derated to the 2000VAC, as other AC capacitors usually are, and that they would have a higher DC voltage rating. As I said, I will be charging mine to 5kV because I figure if they did well enough under 8kv, 5 will be fine.

I keep asking about the internal hv diode as well, but that seems to have fallen to the wayside in the wake of this small argument...

Steve Ward

Tue Aug 07 2007, 02:08AM

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

If there were an internal diode it would have to be strapped from one terminal of the cap to ground (or perhaps a 3rd terminal which would make it totally obvious). So uhh, just do a diode check from one terminal at a time to the case of the capacitor. I personally have never encountered a cap with internal diode (and ive never even heard of this). Almost all of them have an internal bleeder resistor, though.

Shaun

Tue Aug 07 2007, 02:20AM

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

Thanks Steve, appears to be no internal diode. Does that mean I accidentally threw it out? Or could there just not have been one? I harvested anything of interest from the microwave, MOT, MOC, magnetron, fans n' connectors, but I saw nothing resembling an HV diode.

CT2

Tue Aug 07 2007, 02:36AM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

The diode would have been connected directly to one terminal of the capacitor and then to the case, well that's where they usually are anyways. The diodes are little black rectangles and a few cm long. It is beside the capacitor in the picture.

sparky

Tue Aug 07 2007, 06:36AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I did an explosive test on several types of Mo caps a few years ago. On DC a 2kVAC rated MO cap can do about 6-7kVDC without failier. Push the caps beyond this level and you'll get an explosion. I tried 8 kVDC at 4kW and the cap tore itself a part --- VERY DANGEROUS!!

Shaun

Tue Aug 07 2007, 06:57PM

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

Too bad, I guess I must have thrown it out. Well, I guess those hundreds of 1n4007s I have will work just as well...

Matthew T.

Thu Aug 09 2007, 09:22PM

Registered Member #513 Joined: Sat Feb 10 2007, 07:30PM
Location: Pointe-Claire, Quebec, Canada
Posts: 19

Hello Shaun,

I think you will be fairly safe overvolting those microwave capacitors. I just tested one at 10 kV and it only died after charging and discharging 7 times. The failure wasn't very impressive. A little snap and a small bump on the side... nothing else. But I must agree that if you are to make a bank of these capacitors you should most definitely use a shield. And if there is a diode inside, it goes from one of the tabs to the metal case. Just ignore it.

Cheers,

Matthew

sparky

Fri Aug 10 2007, 01:59AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

Try this: put 2 MO caps together with the internal diode setup inside and push 8 kVAC into the two caps....then try and short them. Damn, watch out! Those caps will tear themselves a part. Now - I'm sure different brands will do different things - some are mylar while others are polyproplyene film with paper. Do be VERY VERY careful!

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