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60KV 3.5 nF home made caps - complete protocol

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LAN_403

Mates

Mon Sept 24 2007, 09:17PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Hi guys,
Here is my complete protocol for making HV caps. It took me a while to reach this final design, so I hope you all appreciate it in your HV experiments. The caps are based on PP copolymer foils used in laser copiers and printers. Each sheet should stand 14KV and depending on how many sheets you use you can get different capacitance (two sheets 9nF - six sheets 3.5 nF). Picture is more than 100 words so have a look...Used alu-foil is the fat one (for grilling). Have fun... Mates

Total number of sheets 15 per cap (2 for covering each alu-sheet, 5 between the covered alu- sheets, 5 on top of the second covered alu-sheet and one more for final rolling)

Size of alu sheet 22x15.5 cm

BTW: I hope you all already have the hot-glue pistol!

According to the longevity:
The PP copolymer should be very resistant against ozone. I'm using these caps in my MARX generator and I have no fails after more than 100 shots. Also in my small TC these caps works without breakdown for quite a long time. But of course this is not professional product so I can not promise 1000h of continual work...

The required tools and supplies.

Cut the foil to size using a sharp knife

Use a smooth rolling pin (made of small fluorescent tube) on a smooth surface to get rid of as much air as possible. Air bubbles will cause corona and ozone to form above a certain voltage/frequency

Interleave the plastic sheets (5pc) between and above the already made doublets with aluminium foils between.

Use the rolling pin again and prepare the cables by removing 5-6cm of the isolation

Find the upper aluminium foil and place the firts cable on it. Let only the unisolated part of the cable in the contact with the aluminium foil.

Tricky part one: Roll the sheets slowly...

Find the most upper sheet (the lowest one before rolling)

Tricky part two: Unroll the most upper sheet until the second aluminium foil is exposed. Place the cable and roll it back. Hold the cap properly during this process otherwise it can unroll very quickly in your hand and you have to start again!

Use one or two more sheets to roll over to protect unrolling and it also makes the upper aluminium foil more than one sheet isolated.

Use the glue tape to protect unrolling.

Use the scissors and follow the upper cable with the tip and cut a track for it.

Put the cable through the track you have cut with the scissors. This is important for gluing the sides of the capacitor.

Cut the same track for the inner cable...

Place the inner cable inside the capacitor. You remove it back after the ends are glued.

Use the glue pistol to put some more easy-to-melt plastic at the ends and fix and isolate the cables with the glue properly!

Do the same on the other end.

Final appearance before gluing

Tricky part three: Push the end of the capacitor against preheated metal plate (use an electric cooker) to melt the PP foils and the hot-glue into a compact structure. This is very improtant part which needs a lot of focus. It will protect the cap before the side spark-overs. (Side spark-overs are the biigest risk for the final capacitor).

Remember to remove the inner cable. In case the whole is blinded after the previous step make a whole with a pencil while the plastic is still hot and soft.

Use more hot-glue to fix and isolate the cables. It is good idea to put additional PVC or silicon tubing around the cables and glue it to the main body of the capacitor.

Check the capacitance

Test setting for the capacitor. Use the spark gap distance to set the final volatage to approx. 50-60KV DC (5-6cm distance)

Put some earplugs and sunglasses and let the capacitor run for while. Remember discharge properly the capacitor after the test (and any other) run. It can store the charge for a long time and touching both ends of 3.5nF cap loaded to 50KV is shocking experience!

Sulaiman

Tue Sept 25 2007, 04:32AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Nice work but I have to ask;
how long have you run your capacitors with hv applied?

In the 80's I made various home-rolled capacitors and to cut a long story short -
if there was any air in the foil/dielectric they eventually failed
due to ozone created by internal corona 'eating' the dielectric.

Unless you can roll and seal them in a vacuum
or fill the capacitor with oil
I think yours will also fail.

Apply hv to your capacitor(s) and observe them in the dark (needs caution!)
ANY sign of bluish light means they WILL fail.

If you don't see any corona then WELL DONE !!

Simon

Tue Sept 25 2007, 11:20PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Do you have a wiki account? This is just the kind of thing we'd love to have there.

Tesladownunder

Mon Dec 24 2007, 01:04AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Sulaiman wrote ...

Unless you can roll and seal them in a vacuum
or fill the capacitor with oil
I think yours will also fail.
Apply hv to your capacitor(s) and observe them in the dark (needs caution!)
ANY sign of bluish light means they WILL fail.

Not sure if I agree 100%. Caps you made in the 80's may have failed in TC use. This is 100% reversal stuff, not just intermittent DC charging.
The prime example of a failure prone cap is in a TEA laser where you can have only a thin layer of dielectric and need a moderate voltage across it. Failure is common as 10kV plus is aplied continuously. However if you use a Marx to charge it in a few microseconds, a higher voltage can be applied and longevity is improved, presumably as the voltage is only applied momentarily. Less time for corona to do damage.
Of course at 60kV, everything will light up with corona.
TDU

Logan Kennedy

Mon Dec 24 2007, 05:10AM

Registered Member #1103 Joined: Mon Nov 05 2007, 06:02PM
Location: Houston
Posts: 80

I've made a few of these before, but I could never remove all the air from the conductor/dielectric layers. I never ran them for too long, so they still haven't failed. However I'm sure they would fail quite quickly if ran under extended duty.

I will have to use your method and see if I can produce a halfway decent DIY HV capacitor.

The best thing: they're cheap and fairly easy to make, and losing one or two during operation isn't too much of a loss.

cbfull

Sat Dec 29 2007, 02:05AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

When I made my first HV cap I used glass sheets and Al foil. I left it dry and the whole thing would light up blue with corona. I decided to fill it with oil and not only did the corona stop, the performance of it improved drastically.

Not a suggestion, just an individual observationa

Dr. Dark Current

Sat Dec 29 2007, 09:52AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Just a stupid suggestion, maybe you can "paint" the caps with oil before rolling them?

cbfull

Sat Dec 29 2007, 08:45PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

jmartis wrote ...

Just a stupid suggestion, maybe you can "paint" the caps with oil before rolling them?

That's not a stupid idea by any means! Ideally, just about any device handling high voltages would be pre-wetted or "painted" as you say with whatever insulating material is desired (oil, epoxy, etc.). Unfortunately in this hobby the equipment is not already built with our interests in mind.

What is typically done to extend the life of a HV device is to submerge the device (transformer, capacitor, rectifier) in the liquid insulating media and then pull as strong a vacuum as possible to remove as much air as possible from every nook and crannie. Pre-wetting during assembly could cut vacuum treatment times to a mere fraction.

Not sure if you are already aware of this but as an engineer I had to point out how practical and sensible that idea is.

Mates

Sun Dec 30 2007, 05:00PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

According to the oil suggestion...
My first rolled capacitors were combined with a silicon oil. Honestly, I had to leave the concept when I started to play with voltages over 40KV. Problem was, that the oil made impossible to glue the sheets on the sides and the caps started to spark on the sides... Moreover, once the spark started it made carbonized path and decreased the breaktrough voltage very rapidly and the cap was more or less destroyed...

I think, the corona problem is not so big like most of you suggest. The latest version of my caps are very robust and I have not destroyed a single one even by playing with voltages close to 60KV reaching hundreds of hard (no load) discharges. (Of course I mean DC...)

Cheers Mates

MOT_man

Tue Jan 01 2008, 09:27AM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

I just built a capacitor using your technique and poly sheeting from overhead enlargers. I tried 7 kV (AC) pulsed a couple of times through the capacitor. I saw some weak corona - but it wasn't bad. I also tried to vacuum seal the capacitor in a 2" PVC housing with end caps - poured in mineral oil and then vacuumed the whole lot. Seems to work fine... I'm sure 10 kV might be a bit much.... but I'll try it an see.

Just slide the hand rolled capacitor down the PVC tube and presto - I think my capacitors comes out to 8 nF due the number of layers I used. - It's a great idea! Thanks for the inspiration!

--- just tried 10kV ... no problems. 10 second burst...

--- just tried 12 kV NST.... no problems.... 10 second burst

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