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LAN_403

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Mon Jul 31 2006, 02:43AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I have a question about Cockcroft Walton or Villard Cascade voltage multipliers....

In theory the caps only see a dc voltage... But as you draw current you get ac through them...

So I was wondering if electrolytics can be used? I want to use these 200v/820uf caps I have laying arround to make a 2 stage multiplier to take the 120v mains (160v p-p) up to 600v+ or so for an off line vttc. I calculate that with 800ufd I can give 500ma at 600v... But I worry that my caps will be looking at as much as 320vac, which they most definantly will not like.

So, how should I go about solving the problem? would it be 'safe' to put the 2 caps back to back to make a bipolar electrolytic? perhapse I need a diode in antiparallel with the caps? If I do that I will only have 400uf, but that should still be enough (I only need 200ma)

But then I am still running the caps at more than their rated voltage... But then I read somewhere that as you draw current off the multiplier you reduce the amount of voltage on the caps, so I think they might just be ok... So do I need to use 8 caps for each of the 4 caps in the multiplier for a total of 32 caps (a wooping 500J) That would be with the caps in a 2x2 arrangment for a 820uf/400v cap, then in antiparallel for a 400uf/400vac then the 4 caps for the multiplier.

Thanks!

ragnar

Mon Jul 31 2006, 03:17AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Yup, if you want to make a bipolar electrolytic capacitor, connect the negatives together and place a diode across each capacitor with the cathode/stripe to the cap's positive.

I've been running a 10,000uF 63V electrolytic at 60V for a while now and he seems OK -- he's probably a quality cap though, and I suspect even if you go marginally over the ratings you'll get short-lived caps.

Personally I wouldn't rely on the load to keep the caps under their ratings... but hey, that's what dummy loads are for. =)

dan

Mon Jul 31 2006, 03:17AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

Yeah that voltage rating seems a bit low. I have used electrolitics with pretty good results in a Villard cascade. I use it for charging. The caps don't seem to mind it but charging only takes a second or two. I would try to stay away from electrolitics for your application. I would probably look for some motor run capacitors but anything above 15-30uf is going to cost you if you buy them new. Never tired to make bipolar electrolitic with diodes before but it sounds like it would work. Since the capacitors really aren't in series(physically yes but electrically no) the new pseudo-bipolar capacitor should retian the same 800uf capacitance.

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Mon Jul 31 2006, 03:33AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

really I need electrolytics, since I need at lest 300ufd to get the necessary output power... A motor start cap of 300ufd would be huge... I am really tired of lugging that mot around (thus the offline coil-I was hoping that 4 little caps would work...)

I am thinking of using a little autotransformer to give me 240v, then use a full wave voltage doubler to give 700ish volts. Seems like a much more elegant solution requireing only 1/10 the caps

The coil would never have been truly offline since I needed the filament transformer...

dan

Mon Jul 31 2006, 03:40AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

If you really want to make it run off-line you could use a voltage divider to power the filiment off mains. But a small filiment transformer isn't that heavy anyway..

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Mon Jul 31 2006, 03:49AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I need about .5a for the filament, not something easy to take off the line directly...

I am going to build it arround the autotransformer for now and see where I get. I might rethink it later...

Sulaiman

Mon Jul 31 2006, 06:19AM

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

600V @ 200mA is 120W, say 150 VA transformer .. not too heavy.

Electroholic

Mon Jul 31 2006, 09:24AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

lytics will be fine in a CW.
i have been using the same one as a charger for my CG for almost two years now.
no need to pair them with diodes to make "bipolar" lytics.
but increasing the voltage rating won't hurt.
say 330V?

Tesladownunder

Mon Jul 31 2006, 12:40PM

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

100VAC in and 2000VDC out using electrolytics.

Peter

Steve Conner

Mon Jul 31 2006, 01:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

10kV @ 100mA from a MOT using series strings of cheap 22uF 450v electrolytics

I can't see how you arrived at a ripple voltage of 320V from 0.5A and 800uF.

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