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High voltage DC power supply construction

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EHT

Thu Jun 26 2008, 10:51PM

Registered Member #1564 Joined: Wed Jun 25 2008, 05:09PM
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Posts: 11

When I put my first CW together it was more experimental and to be honest I did not put much thought into component values. Shaun, you mentioned earlier about a calculation to see what capacitance would best work with 50khz, should I be looking for what value capacantance will resonate at 50khz?

Steve Ward,

That is a very nice CW there, is it yours? Im essentially looking to make something like that, do you think my driver is suitable? I viewed the video clip, that is an impressive arc

Firefox

Thu Jun 26 2008, 11:46PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

There is no ideal capacitance to go with a frequency, higher is generally better. However, you can match the impedance of the capacitors in the first stage with an inductor to increase efficiency. Here, Blaze Labs has two very nice articles on the subject of both the multiplier itself, and putting an inductor in the circuit for impedance matching. The second article shows how close to an ideal multiplier the impedance matching gets you (97% efficiency according to the article). Keep in mind that the impedance matching inductor is not easy to find, but Blaze Labs has done most of the work for you in the second article.

Shaun

Fri Jun 27 2008, 12:19AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

No, resonance is not involved here. But what i meant about finding the "right capacitance" was this:

Your power supply, according to the datasheet, appears to have a maximum input power of 120W, so we can assume max output power is close to that.

So say you were to make your multiplier stages using tiny capacitors in the 10s of picofarads. Yes, you would get 150kV out. However, the capacitors are so small you may only be able to get a few watts of power out of it, even at 50kHz.

Conversely, say you used massive energy storage capacitors in the 10s of microfarads. Again, 150kV output, and full use of available power, but now ripple is huge as capacitors recharge. Plus now you have to spend more money on larger capacitors.

There IS a minimum capacitance that will allow you get the maximum power output that your source can provide. Any smaller, and you get less output power. Any larger, and you increase ripple and waste money.

I wish I could tell you how to calculate that value, but I don't know the math.

EHT

Fri Jun 27 2008, 11:00PM

Registered Member #1564 Joined: Wed Jun 25 2008, 05:09PM
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Firefox: thanks for the link to those pages, they are very informative.

Shaun: do you know the name of the equation so I can google search it?

Perhaps I should follow similar values for my multiplier as in Steve Wards CW device.

Steve Conner

Sat Jun 28 2008, 12:20PM

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

That's just plain wrong. Ripple doesn't increase with the size of the capacitors. It gets monotonically smaller as the caps get bigger. The reason is that they don't fully charge and discharge at each cycle: they only have to fully charge once when you turn the power on, and from then on, the charge only changes by a little.

The only reason not to use infinitely large caps in a C-W stack is the infinite size and cost of them. And I guess having to wait until the end of time before they charge up at power-on

jpsmith123

Mon Jun 30 2008, 03:15PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

@EHT: For some reason I assumed your output winding is center tapped? Is it, or is it single-ended?

Also, with regard to diodes, I bought some 2CL2FM diodes on ebay that I plan on using in a multiplier. They're rated @20 kv, 100 mA. IIRC, depending on which specs you look at, they can supposedly handle a 10 amp surge. But to protect against a high current arc destroying the diodes, I'm planning to put some resistors in series with them...maybe 220 ohms or so.

BTW here are some images exported from CircuitMaker 2000.

1214838889 1321 FT48322 Fw Multiplier Circuit

Mates

Mon Jun 30 2008, 05:04PM

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

I'm only reminding my half-forgotten CW project

It could be helpfull regarding the diodes...

BTW: Why do you need 150KV DC? Part of some larger project?

EHT

Mon Jun 30 2008, 06:20PM

Registered Member #1564 Joined: Wed Jun 25 2008, 05:09PM
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Posts: 11

Jpsmith123,

The output transformer is of the hv flyback type, its secondary is not centre tapped.

The diodes you got on Ebay I believe are the same as the ones I previously used, they all got fried after a short time. This time I think I will make my own diodes by soldering a chain of 1000v + @ 1A+ fast recovery diodes in series to obtain the desired value.

Its the capacitor value I'm not sure about

Mates,

Thanks for the link, I want the 100-150kv for two reasons, fun and also yes I do need a high voltage dc source for a larger project.

EHT

Mon Jun 30 2008, 07:07PM

Registered Member #1564 Joined: Wed Jun 25 2008, 05:09PM
Location:
Posts: 11

Some photos of the original stage, it was placed in a pvc tube and potted in paraffin wax. I think paraffin wax is not the best option, better to use hv transformer oil.

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