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doubler capacitors.

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dmg

Wed Feb 03 2010, 05:35AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

I would like to take 6KVAC and via a voltage doubler, turn it into 12KVDC.
I wondered If I can use electrolytic capacitors with the appropriate voltage ratings for this type of doubler, as I am unsure whether electrolytic capacitors will explode.

Myke

Wed Feb 03 2010, 06:13AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Electrolytic capacitors aren't made with that high of a voltage rating. The highest I've seen is only 450V. You probably want to use an oil capacitor since those can go into the 100s of uF and into the 10s of kV.
If you need to series caps, remember to put some balancing resistors across them so that one doesn't see a lot higher voltage than the other due to imperfections.
You'll need bleeder resistors across the caps so that they don't hold their charge for a long time. Let's say you had a 10uF cap with 6kV across its plates. It would hold 180J which doesn't feel very nice going through your body. I would chose a resistor that would discharge the caps in a reasonable time (5RC for ~99% of the cap to discharge) but not be such a low resistance that they hog a lot of power.

dmg

Wed Feb 03 2010, 06:19AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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yes, as a general rule of thumb more capacitance = more current I can pass through the doubler (in case you are wondering this is for DC resonant charging for tesla coil.) but that was my original plan, either use a bank of electrolytics or microwave capacitors, while I knew that the microwave caps can take it, I wasn't too confident that the electrolytics would survive, the highest voltage rating I have for electrolytics is 400 WVDC,

so If I understood you correctly, I can build a bank (for sake of argument lets say each cap is 400wvdc at 200uF) and If I place a 10 meg ohm ballancing/bleed resistor across each capacitor, this setup could work?
I kind of want to use electrolytics because of thier high capacitance.
also, does the rating of the capacitor has to be just the same as the input voltage? (6kv) or has to withstand the doubler's output? (12KV)
I dont want to be covered head to toe with electrolytic juice...

Mathias

Wed Feb 03 2010, 06:34AM

Registered Member #1381 Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74

You can use polarized capacitors (like electrolytic) for this kind of doubler arrangement. But if you apply 6kVac (rms value) than the rectified voltage will be the ac peak voltage (Vrms*âˆš2 - the voltage drop of the diode(s)) times 2.
In this case 17kVdc . So the diodes and the capacitor should have high enough V ratings. (20-30Kv for the diodes (diode chain) , 18-20KV for the capacitor chain (be sure to use balancing/bleeder resistors (couple of Mohms) , that have a high enough working voltage)

If you decide to build this, be very carefull around the charged capacitors , each could deliver a lethal shock!

Proud Mary

Wed Feb 03 2010, 10:21AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The ESR of these long strings of electrolytics will all add together in an unhelpful way, increasing the output impedance of the doubler, generating heat, and causing voltage 'sag' when current is drawn.

Steve Conner

Wed Feb 03 2010, 11:06AM

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

I built this years ago:

and it worked remarkably well. More details here

dmg

Wed Feb 03 2010, 02:18PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

I'll use that arrangment then, for my diodes, I like to use strings of microwave oven diodes, they served me well In the past. I can put a filter and a smoothening bank on the output.

radiotech

Wed Feb 03 2010, 06:19PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

10 meg is high for cap voltage balancing; you are dealing with differences of capacitance of the series elements as well as leakage, especially if you are stringing dissimilar caps together.

I would start from the load you are driving and work back to what ripple you are designing for.

Little caps do go off like firecrackers, but big , heavy, or metal can types tend to bulge or pop their top seals if overheated. I've got a whole casefull of failed, some bulging, SCR duty caps from the Calcoil induction heaters we repaired, as well as some big electrolytics
from a VFD, 700 HP wastewater lift pump. These popped the top and messed up the bussbars they were bolted to when the scr module shorted. A mot is only good for an amp or so bolted secondary.

I have a stash of M-ovens collected 15 years ago, big old, heavy, mostly with soft magnetrons,

Arcstarter

Wed Feb 03 2010, 07:49PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I used electrolytic capacitors for my quadrupler for a DC resonant charging coil. Worked great. I would not worry about the ESR, you will not likely get any more than 150ma out, or something. Probably more like 75-100ma. I use i think 300Kohm bleeder/balancer resistors. Got a tiny bit warm, nothing bad at all.

If you use Steve's schematic, you do not need a smoothing cap. Make sure you use a de-queing diode made for a few times the voltage output of the multiplier, best with balancing resistors in parallel with each diode in the string. The charging choke needs to withstand a very high voltage, it is a pain in the ass.

Good luck.

HV Enthusiast

Wed Feb 03 2010, 08:51PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

First thing you want to do is get some requirements for your design.

How much output current do you want?
How much voltage droop (sag) can you tolerate per your load?

From here, there are some simple equations available (Search google for C-W Multipliers) to determine what size C you need.

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