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300KV Cockroft Walton multiplier

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Dave Marshall

Sat Feb 23 2008, 04:07AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Actually thermite, a Marx generator is very specifically a spark gap switched multiplier. A Cockroft Walton is a diode switched multiplier.

The CW multiplier is capable of continuous generation of high voltage (dependant of course on the load, and with a very high ripple) where the Marx can only generate very distinct pulses of high voltage with a comparably high off time.

Further discussion of this difference should be taken to another thread.

Dave

Mates

Sat Feb 23 2008, 09:49PM

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

I I performed some design changes. I placed the caps from the other side of the board to keep them far away from the diode connections. I also made a new HV resistor which holds.

I started checking the main corona losses. And I immediately found what was sparking inside the device last time. One cap had a failure and was side-sparking over (funny thing is that this was a completely new cap, which Iâ€™ve made recently. Old caps disassembled from my old Marx are still fine despite the corona stress).

I changed the cap and checked the corona once more. It is not so bad. You have to realize that the picture is taken at iso 800 and exposition is 10s.

Next I started with increasing the spark gap distance. At 25cm the spark was accidentally attracted by my metal cupboard. I store all my electronics over there so Iâ€™m curious whether I destroyed something

After I moved the table 25cm sparks are running without any problems. Main corona losses are detectable only at connections of the resistor.

However, I have another big problem. During the last run I noticed that something is sparking also in my main 220V plug-in. The sparking even continue 1-2 seconds after I stop the CW â€“ shit what should I do now. Where on earth is this HV sparking inside the plug-in coming from????

teslacoolguy

Sat Feb 23 2008, 09:56PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

very nice work, now what is the approximate repetition of those sparks?

Mates

Sat Feb 23 2008, 10:03PM

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

teslacoolguy wrote ...

very nice work, now what is the approximate repetition of those sparks?

I would say around 10-15 per second. But, I can easily increase the power of the ignition coil driver...

Mates

Mon Feb 25 2008, 11:50PM

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

I increased the power of the iggy driver by 30%. I played with increasing the distance of the spark gap. I reached 30cm and two of my caps started side spark over

. I'll have to replace them a probably play more with the design of gluing the sides together. The side spark-over is a stupid problem - the weakest point which limits the rating of the caps.

Here are the latest picturesâ€¦

30 cm sparks front view

The back view and the side sparking cap...

Tesladownunder

Tue Feb 26 2008, 02:54PM

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

Good stuff.
When designing HV you need to account for spacing things 3 times as far apart if there is a surface connecting eg like a cap or diode string. Surface tracking is why this ignition coil sparking 3 inches, has a 9 inch tracking spark.

TDU

1204037693 10 FT39162 Ignitioncoilsinglesurfacetracking

Mates

Tue Feb 26 2008, 04:38PM

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

Tesladownunder wrote ...

When designing HV you need to account for spacing things 3 times as far apart if there is a surface connecting eg like a cap or diode string. Surface tracking is why this ignition coil sparking 3 inches, has a 9 inch tracking spark.

Isnâ€™t it the surface tracking problem mainly of the AC? I never experienced bigger problems with DC discharges (usually 20% larger distance than free air is enough).

The cap side sparking which I referred about is not due the surface tracking problem. My cap was not capable to stand the electric stress and it breaked down around the sides (the dielectric layer there was not very good because is based on amateur gluing with high temperature).

BTW: In case you want to get rid of the surface tracking of your coil just dip it inside a cooking oil (like I did). It is a cheap non-toxic solution which works.

ramses

Tue Feb 26 2008, 06:35PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

he might be referring to the edge of the foil, which is very sharp, even though you can't cut yourself.

Mates

Sat Mar 01 2008, 09:50PM

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

OK, I hope it is not starting to be boring. I redesigned the caps, so finally they do not spark over. The trick is in gluing the sides really properly. (I also tried oil and wax - both failed). Now I run easilly 35cm sparks (still litle bit afraid to go for longer distance of the spark gap). Moreover, sometimes happens like you can see on the second picture that the electricity finds a way around the resistor. It always makes a laud bang and the total length of the track is over 50cm!. Luckily the diodes do not complain

I'll add other stages soon...

So enjoy the last pictures

Corona analysis...

Here is the case when the electricity finded the way around the 1Mohm resistor

and litle bit of art

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