Cockroft-Walton question.

For what purpose?

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

With that damn transformer, of course. ;-D

I agree completely. Fortunately, with sufficient skill, you can run a small, light resonant transformer at a high frequency and do far better than any C-W circuit. I suggest you look into that approach.

For how long a time period do you need to continuously supply 100s of watts of these kVs, and with what duty cycle? More detail, please.

Marx generators usually use spark gaps, but they're still additive per stage. There's a neat megavolt Marx out at Livermore that uses laser-triggered spark gaps and water-filled clear plastic tubes as the charging resistors.

John

Your mention of the ripple brings to mind another question. The ripple means there would be a more or less sine wave pattern about an average DC voltage on the output. Is there any way to shift down this sine wave so that the voltage will be AC? I was thinking since we already have this varying voltage in the output it would be nice if we could use it to create the AC we need to feed into the next CW. I know we can just put another DC to AC inverter on the output, but I was wondering if there might be an easier way given the output is already varying.

Bob Clark

The circuit that I described as a theoretical possibility that isn't used, only to discover that someone was using it, actuall does multiply.

Consider a bog-standard Cockroft-Walton voltage multiplier that has a DC-to-AC inverter on the front end, and enough levels to get a nice

Now take that 10X DC out and use it to feed another bog-standard Cockroft-Walton voltage multiplier that has a DC-to-AC inverter on the front end, and enough levels to get a nice 10X increase from the DC in to the DC out.

The downside is that doing this throws away the main advantage of the Cockroft-Walton configuration; that no component is subjected to more than a small fraction of the output voltage. A regular Cockroft-Walton can go as high as you want.

(I am still amazed that anyone would do it that way given the many alternative topologies that seem, on the face of things, to be better choices.)

You are off by a factor of two. The nominal output voltage (no load) assuming perfect parts is *twice* the peak input voltage multiplied by the number of stages.

I have never seen the topology I described above either, but Winfield says that he uses it, so obviously my experience isn't broad enough to have ran into it in practice or read any literature on it. It does indeed multiply, so from a voltage standpoint you can use three copies of a Cockroft-Walton voltage multiplier with a DC-to-AC inverter on the front end, and enough stages to get a nice 10X increase from each copy. That's 30 to 40 capacitors and associated diodes, plus whatever parts you need to make the 3 DC-AC inverters.

That "hundreds of watts" specification concerns me. Cockroft-Walton voltage multipliers aren't all that good at putting out current. Here is how to figure your currentL

Let's look at a single Cockroft-Walton voltage multiplier that with a DC-to-AC inverter on the front end.

Start with the oversimplified model of the nominal output voltage being twice the peak input voltage multiplied by the number of stages,

Now reduce it by the number of diode drops you have.

Now figure the voltage drop;

Edrop = I1oad /(f*C)*(2/3*n^3+n^2/2-n/6)

Edrop = voltage drop Iload = load current f = frequency C = capacitance n = number of stages.

Now figure the ripple; Assume all capacitances are equal.

Eripple = Iload/(f*C)*n*(n+1)/2

You can reduce the ripple by making the capacitors smaller at the top and larger at the bottom: C1,C2=10*C C3,C4=9C, etc. This gives you;

Eripple = Iload/(f*C)

As you can see, you want to keep that frequency as high as you can before the switching losses start kicking in. I have had good results with starting at a low frequency and ramping the frequency up - sort of a poor man's soft start.

(Disclaimer: I am away from my reference books and I don't have the above equations memorized, so I snagged them off the web. Make sure they are right before actually building anything.)

What about a spark gap?

Thomas

Make DC into AC, or more accurately, into pulsed DC. But I have a feeling at least half the voltage gets lost doing this. And will spark gaps scale indefinitely in voltage? Probably not.

Thomas

...

...

Yes, no argument there, but the problem is, you misunderstood what Winfield Hill said. In the post you refer to (

-jiw

. WH replied (more or less) "I've used that" meaning multistage CW, not multiple ladders/inverters.

The purpose is make a self-contained power unit for a lifter:

Experiment 14 (21/03/03)- 100g payload lifter.

The power units used so far are heavy and have to be left on the ground, the power being delivered to the craft through wires. The best ratio of weight lifted to power required so far has been about 1 gm lifted to 1 watt of power, as in that example by Blaze Labs. In this example the payload weight is 100 gm, with total weight about

Thunder Power Li-Poly Batteries.

When you calculate the watt-hours they can put out by multiplying the mAh given (milliamp-hours) by the voltage and compare that to the given weight, you see the weight in grams is about 6 times the watt-hours. Then assuming the best lifter takes about 1 watt to lift 1 gram, you see lifting the battery alone without considering the weight of the power supply or the lifter body, could only work for 1/6th of an hour,

Bob Clark

You didn't read very carefully. I specifically characterized as "silly" your assertion that multistage Cockroft-Walton voltage multipliers are not useful, because one should simply start with higher-voltage transformers. "The reason you don't even see a two stage system in practice is that there is a much better way to do it; use a step-up transformer to drive your first stage with a higher AC voltage."

If you will recall, I went into some detail in several posts as to why I thought this was wrong. "Frequently it's inconvenient to get a higher-voltage transformer. For example, check the meager HV offerings from Signal Transformer. ..." As it happens, the poor availabilty of small off-the-shelf ac-line transformers in the 250V to 2kV region is one of my pet peeves, and the subject of a continuing search. Suggestions by some that I use neon-sign transformers doesn't help me. Don Klipstein's suggestion of an oil-burner ignition transformer may be helpful for some apps, at least they're small.

Yep, here we go, another wrong characterization.

...Jim Thompson

That doesn't make sense. I desribed multiple ladders/inverters and said that you don't don't see them. I was quite specific and went into detail about how using a lot of ladders requires inverters that can take the HV, and that even a two-ladder design isn't as good as a design with a transformer in it. Winfield went out of his way to call me silly and wrong in a direct reply to that specific claim, and he did it twice. Why would he do such a thing if what I described really wasn't used in practice and he was talking about multistage CW? That makes no sense. I can only conclude that I was wrong about multiple ladders/inverters not being used in practice. Winfield's comments make no sense in any other context.

Not hardly. It has a "stepping" waveform.

...Jim Thompson

It sounds like he wants to take an "Ionic Breeze" and operate it sideways. ;-)

Cheers! Rich

Come on, Guy, get over yourself. Personally, I'd be honored and star-struck if the likes of Win Hill thought enough of something I said to bother to call it silly. Albeit, I do sometimes aspire to silliness. ;-)

Cheers! Rich

Ozone doesn't turn water into nitric acid, does it? You're probably thinking of oxides of nitrogen, which, admittedly, probably also get generated by the arc - lightning supposedly "fixes" nitrogen, which is good for the plants, or so I've been told.

But I would have no qualms whatever about bubbling plain ozone through water - it dissolves about as well as O2 does, AFAIK, and sterilizes better than chlorine, and doesn't hang around the way chlorine does.

Cheers! Rich

I shouldn't have to read carefully to pick out the places where you mis-characterize my words and then call the straw man you have set up "silly." Having never in my entire life ever even entertained the rather silly idea that that multistage Cockroft-Walton voltage multipliers are not useful, I naturally assumed that you were replying to the actual scheme that I described in the post that you replied to.

I fail to understand why you would want to put words in my mouth; I make plenty of *real* errors that you can flame if that's what you wish to do with your life.

Indeed we do. Maybe you should try to avoid doing that in the future. Before you fire up your flamethrower, I suggest that you attempt to find a quote where I claimed that "multistage Cockroft-Walton voltage multipliers are not useful." My actual claims were threefold:

(I also floated the idea that there might be a DC-AC Inverter that doesn't require any components to withstand more than a small fraction of the H.V. output; I have never seen such a beast, but that doesn't mean it doesn't exist)

--> Cockroft-Walton --> H.V. output) is not seen in actual practice.

If you want to twist the above claims into "multistage Cockroft-Walton voltage multipliers are not useful" and then engage in childish name- calling based on the words that you have put in my mouth, I think it reasonable to request that you produce a quote where I claimed that "multistage Cockroft-Walton voltage multipliers are not useful" instead of what I actually claimed (See [1-3] above).

Or - and I realize that this is a huge departure from the standard practice here - you could try asking what someone means rather than immediately firing up the flamethrower. A simple "are you saying that multistage Cockroft-Walton voltage multipliers are not useful?" would, in the case where I actually believe such a thing, give you more ammo for the flamefest to come. In the case where I *don't* believe such a thing, asking for clarification would identify the place where I was unclear and gave the wrong impression. Then you could find something else to call me names over; being imperfect, I make many

"A little rudeness and disrespect can elevate a meaningless interaction into a battle of wills and add drama to an otherwise dull day." -Calvin discovers Usenet