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Registered Member #83
Joined: Thu Feb 09 2006, 12:15PM
Location: Warsaw, Poland
Posts: 5
Hi guys,
I have been busy for a while (3 years or so?), i was consumed by setting up my own (automotive electronics) business. Since it is up and running strong since couple of months, i have finally some time to get back to my hv hobbies.
As some of you might remember, i was experimenting with a 6kV, 2.2kJ 120uF pulse cap setup. I've got outstanding results with can crushing (or tearing apart actually;) and limited success with coin shrinking.
Recently got a hold of a set of 9pcs Siemens pulse caps (each rated 21uF at 1700v). Next week probably will get another 10.
Basic stored charge calculation comes out at 303J per unit. Now my question, what you think would be the best topology for good results on coin shrinking? I can basically:
- connect all 9 in parallel to get 168u at 1.7kv, - two chains of 4 caps in parallel, 10.5u at 6.8kv, - Four chains of 2 caps, 42u at 3.4kv, - three chains of three caps, 21u at 5.1kv, 2.7kJ.
From what i understand higher the voltage, faster is the resulting shock wave (Better Di/Dt) but i am not exactly sure if it has any drawbacks on the performance?
My second question is the switch, some of you might remember that my experiments back in 2005 have ended in a dangerous and devastating switch failure, which unexpectedly exploded, making an uncontrolled dump of the charge right into a low voltage solenoid... Now i dont want to make the same mistake again so i am considering something else instead of a solenoid-controlled mechanical switch; any suggestions here? Triggered spark gap perhaps?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
I would say a triggered spark gap because that's what fastmhz uses for his 24kJ cap bank and so far I think it's working well. You would want a higher voltage for the bank so that the gap doesn't have to be too small.
Registered Member #83
Joined: Thu Feb 09 2006, 12:15PM
Location: Warsaw, Poland
Posts: 5
My old setup initally employed the TSG, however i was having troubles with reliability - it was a little unreliable, sometimes it just refused to fire. Perhaps i could spend some more time on the design, anyway i have converted to a mechanical switch and it worked fine until the day it has fallen apart.
I am currently not planning to get anywhere close to 24kJ, but the target charge should be around 5-6kJ if the guy delivering these won't let me down. My doubt about the TSG is the potential losses on the conducting plasma might be too big? At 24K still have a lot of extra power but here, every spared Joule counts...
On the other hand, the all-metal switch is probably transferring most of the energy through the plasma as well i guess...tough call.
Another thing that came to my mind, about these mixed parallel-series configurations is the balancing resistors. Do i need them actually?
Lets assume the 3x3 setup, so in theory each cap in series shoud get 1/3 of the total voltage. I assume that when i make a crossconnects between each individual cap in the series (so in fact it would look like a 3 sets of 3 caps connected in parallel, the capacitors should tend to balance themselves?
I think i will spend a week or two on the mechanical connections, as the caps have big, sturdy nuts (3/8 inch or so diameter) and i want to make busbars as short and as thick as possible. This will require a lot of drilling and machining...hoping to get some copper bars as i got some problems with Al bars before, because of oxydizing the surface.
EDIT: i must have been drinking too much last days. The nominal energy of a single cap is only around 30J so i need a lot of them to create a good effect. Nevertheless i will get as much of these as possible and post back about the results.
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pulse hv cap configuration
