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Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Hi guys, I completely redesigned my CW multiplier. I experienced problems with caps failures in my last CW project so this time I decided for overkill. The guillotine like construction you can see on the pictures is 3stage multiplier. I decided to use multiple caps in series. This is little bit more work but HV stress on the dielectric is much lower in this setup and I don’t need to glue the sides of the caps (which is dirty smelly job). First stage is made as 2x5caps in series; each can stand 25KV at 9nF so the total rating of is 125KV. They will never see more than 100KV (rather much less). The next stage is made of 2x6caps in series. Third stage is planned as 2x7caps but I run out of transparencies so I had to use my old 60KV caps (always two in series) from my previous CW.
The CW is a monster. When it runs and I came closer I can feel a real wind coming to me (it is almost scary). Also the lamp in my room is shinning despite is 1,5m away (you can see on the second picture). Tomorrow I'll start to play with maximum spark length (today I managed 30cm sparks and I had to stop, because is too noisy). I'm optimistic I plan at least 50cm sparks, maybe more....
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Nice! Maybe if you leave more clearance on the sides of the caps (this means bigger dielectric sheets or smaller alu foil), you will experience less side spark-over problems.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
jmartis wrote ...
Nice! Maybe if you leave more clearance on the sides of the caps (this means bigger dielectric sheets or smaller alu foil), you will experience less side spark-over problems.
Yes you right, that's why I switched to the series caps design. Now every cap has a side air distance between the alu foils 2.5cm. The lowest stage employs 2x5caps in series so the total side distance is 12.5cm. I do not expect a sparks-over of such lenght in the lowest stage. Next stages will have always 2.5cm more, so I think the side-sparking problem is solved once forever! Now the big question is how well the dielectric (PP-CP) will deal with the ozone which seems to be the only limitation of the longevity...
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Mates wrote ...
Yes you right, that's why I switched to the series caps design. Now every cap has a side air distance between the alu foils 2.5cm. The lowest stage employs 2x5caps in series so the total side distance is 12.5cm. I do not expect a sparks-over of such lenght in the lowest stage. Next stages will have always 2.5cm more, so I think the side-sparking problem is solved once forever! Now the big question is how well the dielectric (PP-CP) will deal with the ozone which seems to be the only limitation of the longevity...
You do not need more caps for higher stages, actually they will probably see a bit less voltage than the first one.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
jmartis wrote ...
You do not need more caps for higher stages, actually they will probably see a bit less voltage than the first one.
You are not completely right.
First and main reason why I use more caps in the upper stages is to lower the capacitance (this makes the CW to work better)
But, the highest voltage stress is not in the lowest stage as you (and the logic) suggest. First of all it is my practical experience (if there was a cap failure in my old CW than always one of the top caps). The reason for that could be that there is always significant flow of electricity away from the first stage (stages), mainly due to corona loses. In other words, the lowest stage is continuously discharged into the upper stages - the upper stages mimic the load, even if there is no real load connected to the CW output...That's why only the top caps are so stressed if I left the CW running without discharges. It is an effect of the fact that the device is so imperfectly made
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