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Rebuilt Cap Bank - Now with a 200 Pound Pulse Discharge Cap

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LAN_403

rp181

Mon Dec 08 2008, 09:39PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Neil is talking about its expected lifetime. The capacitor will die eventually, and using a lower voltage will extend this. No warranty would cover expected damage.

El_Roberto

Tue Dec 09 2008, 12:44AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Hmm good point I didnt think of that. I re-read the first post and I see now why he is doing it, Only 3000 discharges, How many do you expect it will have at the voltage your using?

rp181

Wed Dec 10 2008, 12:22AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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At 50% charge voltage (5.5kV) the life doubles to 6000 pulses.
In any case, 3000 discharges is alot. Its not like the full capacity would be used every time. If it was, it would still last a year with ~10 full power discharges a day.

Sugestion: have you thought about attaching bleeder resistors?

Edit: look at what i found on the gaep site:
End-of-life is defined by 5% capacitance loss.
This was for the other capacitor line, it might be true for that one too.

Neil

Wed Dec 10 2008, 12:03PM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
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rp181 wrote ...

At 50% charge voltage (5.5kV) the life doubles to 6000 pulses.
In any case, 3000 discharges is alot. Its not like the full capacity would be used every time. If it was, it would still last a year with ~10 full power discharges a day.

Sugestion: have you thought about attaching bleeder resistors?

Edit: look at what i found on the gaep site:
End-of-life is defined by 5% capacitance loss.
This was for the other capacitor line, it might be true for that one too.

Actually, the life of capacitors with regard to charge voltage and reversal is not linear. By halving the voltage I am not just doubling the life of the cap, but multiplying that life by about 200 times. Its a logarithmic relationship.

The graph below means my 3000 discharge life has grown to 600,000 discharges.

This graph shows that the rated 10% reversal can now in fact get up to about 90% reversal for 50% voltage without effecting the life of the cap.

1228908823 1458 FT58983 Voltage Derating Vs Reversal

Not all my discharges are going to end up with 90% reversal so I could very well end up with well more than 600,000 discharges as the relationship between reveral and lifespan is also logarithmic. If you imagine the line in the graph below going back past the 10 percent to 5% (equivilant to 90% to 45%) than it looks like you could multiply your life by about 5.

Considering I spent a bit over $20,000 dollars all up on this, if I were to take it to 11,000 volt each time, and some how managed to retain my reversal to 10%, it would be about $66 dollars each time I discharged it. If you look at the graph for 80% reversal at full charge the multiplyer is 0.02, I could barely get 60 discharges off at about 330 dollars a go.

By halving the voltage, I will end up with an almost bullet proof cap, hopefully. That is why I went with a cap with such a large storage which allows me to do this with still some reasonable kJ storage, and lets me have an occasional play with it at higher storages.

That is my take on the graphs, if someone interprets them different I am very open to their explanation.

rp181

Wed Dec 10 2008, 12:23PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Ahh, i see. I read the graph wrong, i didnt see the scale correctly. It showed 2 at 50%, and said the multiplier, so i multiplied it =p

Neil

Sat Dec 20 2008, 07:15AM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
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Decided to go back out to the farm this weekend and try blow some more things up. I'm very pleased with the results

. Cranked it up a bit more this time, left me with ringing ears after I blew a full can of coke apart, unbelievably loud. Got out the ear plugs after that. Here are the videos for anyone interested.

Ripping open a full can of coke

Launching a can into the air and crushing it from the acceleration

Water vs 8000 Joules

El_Roberto

Sat Dec 20 2008, 08:43AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Wow cool, do you think you could put in the non-slowmo version? I really want to see these in real time too! Also how long does it take to charge to 8Kj? And what are you using to charge it?

PYLEMET-mk2

Sat Dec 20 2008, 09:23AM

Registered Member #1513 Joined: Sat May 31 2008, 12:08PM
Location: Russia Klin
Posts: 48

Neil, you could buy similar cap at least 5 times cheaper here:

if you need cheap SCR's you might find them too in Russia

rp181

Sat Dec 20 2008, 03:46PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Are those even pulse? Even if they are, GAEP has a very good reputation for the quality of its capacitors.

Neil

Sun Dec 21 2008, 09:08AM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
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Posts: 51

El_Roberto wrote ...

Wow cool, do you think you could put in the non-slowmo version? I really want to see these in real time too! Also how long does it take to charge to 8Kj? And what are you using to charge it?

Thanks, thats a good idea, I'll try to include actual speed as well as slowed down in my videos. Only took like a minute to charge, variac is only rated for 2 amps so kept it between 1 and 2 amps.

Llevellyn wrote ...

Neil, you could buy similar cap at least 5 times cheaper here:

if you need cheap SCR's you might find them too in Russia

Thanks for the suggestion. Actually I dont believe they are pulse rated caps, I was caught out by that very brand which I was using, type K75-40. See the blown up capacitor at the top of the page. Reading through that webpage in the catalogue I can't see any foil caps, just metalized film. My new one might have cost more but I have confidence in it lasting, as well as pumping out many many times more amps than my old caps.

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