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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Taking a break from the x-ray and starting a mini project. Essentially it's a big CW multiplier. I had all the parts so I figured why not.
The capacitors are USA made (really!) 3.3nF 20kV capacitors. They are the same size as the chineese 10nF ones so I figured they'd be very hefty and it turns out they are. According to the datasheet these caps are under-rated and can actually run at 30kV without ill effect, so they probably can withstand 40kV spikes (but I won't give them that). All 8 caps wiegh in at 13.5 ounces.
The diodes are strings of glass body SF4007's. They are avelanche rated, and I'll be putting 30 in series for each "superdiode" for a 30kV 1A diode that can handle 30A surges. The entire string is rather long, so the CW will be more of a square rather than a rectangle. :-/
Because I'm sick of oil this will be an open air multiplier tower, and as of right now I'm gluing the capacitors to wooden sticks with supposedly 5 minute epoxy. It's certainly not 5 minute though. In order to reduce corona to a minimum the tower will be coated with liquid electrical tape.
It'll be powered by an AC flyback, plastic insulated with the "paper windings" removed and replaced by an internal 4+4 center tapped primary.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Grenadier wrote ...
The capacitors are USA made (really!) 3.3nF 20kV capacitors. They are the same size as the chineese 10nF ones so I figured they'd be very hefty and it turns out they are. According to the datasheet these caps are under-rated and can actually run at 30kV without ill effect, so they probably can withstand 40kV spikes (but I won't give them that). All 8 caps wiegh in at 13.5 ounces.
I'm sure you'll recall that as frequencies increase, it is perfectly possible to exceed the continuous service current rating before the maximum working voltage is reached - something to be aware of in C&W design.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Is that a MATCH head I see ! ?
Also the Loctite and Devcon stuff is pretty good, I see you have the mixing tube one, the truth is they are 5 minute epoxy only in large quantities, these epoxies are exothermic , and thermally sensitive at the same time, so go figure.
I share your liking of the liquid electrical tape, it now comes in colors but Ive only seen red and black.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Nah that's just uncured epoxy. The cap-kabobs are now done, I just have to let the epoxy cure to a reasonable hardness now.
I tried covering a diode string with the liquid tape and it looked like crap, so I'm going to shrink tube the diodes and cover the rest with tape. But first I need to go to the craft store and get a wooden base for this.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Is the SF4007 the common diode peeps use to make high voltage diodes? With its Avalanche capability and simmilarities to the 1n40007, it seems ideal. Theyseem to cost about 15 cents each!
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Yeah they are the best ones for stringing together, but they aren't cheap. I managed to get a reel of 5000 though for $32. Should last me a few years if not more.
I got everything together, but I managed to blow up the driving mosfets. And then again... and again... and again... and again.. and again... and again. Now I need to place an order for some mosfets because I'm only left with those low voltage ones.
Project delayed until tuesday evening (I expect the mosfets to be here on that day.)
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Grenadier -
What's the purpose of the sticks and the large spacing between your ceramic caps?
Because the leads of adjacent caps are at the same potential, there's no need for any spacing at all between individual caps. The place any arc-over will occur is across the body of the cap, from lead-to-lead, and the sticks-and-spacing will have no effect on arc-over across the body of the cap.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
No the strings are just going to hang in the wind.
Well it's done now, and here is a picture of the contraption:
It sure isn't going to win any awards for style. I decided to take it outside to test it, but after a spark jumped to that metal wire, through the red plastic thing and into the deck, through the rubber sole of my shoe and jumped out of my finger to the battery terminal I decided that it was time for a less-conductive location.
Now here it is in action, ~85kV, .5s exposure, maybe 20 sparks.
This sonofab1tch is LOUD, almost like .22 gunfire. The diodes don't seem to mind no output resistor, and that's a good thing because this killed all of my output resistors. I decided to not coat it in liquid tape because the huge corona and ionic wind this thing makes is too awesome to get rid of.
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Mini project: Big CW multiplier
Should last me a few years if not more.
