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different capacitience in a CW?

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803

Thu May 13 2010, 10:07PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
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Posts: 191

Can you use different values in a voltage multiplier? How will that affect ops.?

Thanks,

Paul

Proud Mary

Fri May 14 2010, 12:11AM

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

It is actually advantageous to use different values in a C&W multiplier as follows:

In a 4-stage C&W, you should have 4C in the first stage, 3C in the second, 2C in the third, and C in the final stage.

A C&W designed in this way will have considerably less ripple and output sag, than one where the same value of C is used in every stage.

As for using some random values of C, just remember that the spark gaps are no more than switches which connect all the capacitors together in series. As you know, the formula for capacitors in series is 1/C = 1/C1 + 1/C2 + 1/C3 etc etc. What this means is that the total capacitance will always be less than the smallest of the capacitors in the chain. If you join a series of monster capacitors together and have just one tiny cap of 1pF in that chain, the combined total will be less than 1pF, be the other capacitors never so large.

803

Fri May 14 2010, 06:59PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
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Posts: 191

thanks agaain, Mary

One thing, don't you mean diodes.

From,

Paul (sory for offending you, didn't know)

Proud Mary

Fri May 14 2010, 08:17PM

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

803 wrote ...

thanks agaain, Mary
One thing, don't you mean diodes.

No, Paul, I mean capacitors - first stage four caps in parallel, or one cap four times the value - 2nd stage, three caps in parallel, or one cap three times the value, and so on.

Check it out on the C&W multiplier javascript calculator at cosmicrays.org here:

In any case, it's a good site to have a look at, with some simple but inventive circuitry for cosmic ray detectors of different sorts.

803

Fri May 14 2010, 08:46PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
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Posts: 191

No you said that the spark gaps work as switchs, that's what I ment

IntraWinding

Fri May 14 2010, 10:11PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I'm pretty sure the confusion here is that 803 is talking about a Cockroft Walton Voltage Multiplier and Proud Mary is talking about a Marx Generator.

I've personally seen the ....4C, 3C, 2C, C system used in the former.

Alan

Proud Mary

Fri May 14 2010, 11:40PM

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

IntraWinding wrote ...

I'm pretty sure the confusion here is that 803 is talking about a Cockroft Walton Voltage Multiplier and Proud Mary is talking about a Marx Generator.

I've personally seen the ....4C, 3C, 2C, C system used in the former.

No... I am indeed referring to the Cockcroft and Walton voltage multiplier. And I must apologise for having introduced confusion into this thread. "It's only me, it's not my mind, That is confusing things" as the Beatles had it.

Optimal design of a Half Wave Cockroft-Walton Voltage Multiplier with different capacitances per stage

Kobougias, I.C. Tatakis, E.C.

Power Electronics and Motion Control Conference, Poznan, 2008

ABSTRACT

Even though the half-wave Cockroft-Walton voltage multiplier (H-W C-W VM) is one of the most common AC-DC step-up topologies, most of the VM designers persist in using equal capacitances in every stage, a fact that leads to a non optimal design. The aim of this paper is to introduce a new designing method of H-W C-W VM that lays both on the choice of the adequate capacitance values to minimize the output voltage drop and ripple and the calculation of the optimal number of stages that is necessary to produce the desired output voltage with the minimum base capacitance value. In this way the voltage gain is maximized and the required capacitance value per stage is minimized. The theoretical analysis is validated by PSPICE simulations and experimental results, accomplished on laboratory prototypes.

IntraWinding

Sat May 15 2010, 06:00AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I don't suppose Optimal design of a Half Wave Cockroft-Walton Voltage Multiplier with different capacitances per stage available to read anywhere without a membership of some sort?

Proud Mary

Sat May 15 2010, 10:22AM

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

IntraWinding wrote ...

I don't suppose Optimal design of a Half Wave Cockroft-Walton Voltage Multiplier with different capacitances per stage available to read anywhere without a membership of some sort?

You'll have to find someone with institutional access; a student perhaps, or you could pay twenty squid for peepshow access.

There's even a javascript calculator on cosmicrays.org which calculates ripple reduction for this topology, so it's hardly obscure knowledge:

IntraWinding

Sat May 15 2010, 12:58PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Yes, you're right. Inaccessibility of academic papers is something that really annoys me. I really hope it changes. I can ask a friend who's a researcher in the US, but I don't want to keep hassling her every five minutes. It was easier when I was doing an OU computing degree a while back as I got access to all papers. Hey ho! (annoyed)

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