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Nickel

Mon Mar 19 2018, 10:54AM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

Since seeing Nickademus's VDG many moons ago, I've gone rash and built a pair of them for my own amusement. I've taken dozens of pics of parts in the making and will be happy to answer any questions.
It is also my first attempt at taking video using my phone. (Galaxy S2) A bit shaky at first, until I put the phone on top of my lathe head-stock, being the nearest safe place for it.
Have a look, see what you reckon!

Nickel

Thu Mar 22 2018, 09:36PM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

I have now tried the pair of them together, but not getting the result I'd hoped for. With the domes 12" apart, they just sit there sizzling like two angry demons, occasionally 'popping'. With the domes 18" apart, they just sit there sizzling, occasionally 'popping'.
Am I expecting too much? I know they are both generating the max for 12" dome, ie +450kV, but no nice big fat snapping sparks.

After a bit of thinking perhaps this is the trouble:
The terminal capacitance for each unit is ~17pF.
I am trying to put two of them in series, so total capacitance is 8.5pF
With a potential difference of 900kV, as soon as the air-gap starts to break-down, there is insufficient capacitance to sustain the plasma, and the voltage drops sharply, so no "big bang".
I've left them alone in the Wshop for a while, because the radio was getting nervous and upset and kept changing channels; also my RS485 link controlling the solar MPPT/PV kept dropping off, and upsetting the PC indoors.
If anyone has any bright ideas (Finn?), please let me know, otherwise they go in a box until my enthusiasm peaks again!

1521754565 4367 FT181790 20180320 145706 Reduced 1

Sulaiman

Fri Mar 23 2018, 09:28AM

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

Does one vdg produce a positive voltage and the other negative ?

If not then I would not expect inter-vdg arcing :)

Nickel

Fri Mar 23 2018, 11:16AM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

Yes; one VDG positive terminal, the other negative terminal. The bottom charge-combs are connected together and to ground. This gives +450kV on the positive terminal with respect to ground; ie electron starved.
The negative is at -450kV with respect to ground, ie electron surplus.
The potential difference between the two terminals should therefore be 900kV with respect to each other.
I have noticed that when a spark occurs, it does not go the whole distance between the globes, but appears to spark to an invisible sphere surrounding the electron-surplus globe. This happens with both of the units; the electro-negative sphere develops it's outer charge-sphere that the electro-positve sphere sparks to.
Where the spark meets the charge 'sphere' it stops and becomes a mauve cloud of what I assume to be corona discharge.

Next job is to build a pair of 5 gallon Leiden/Leyden jars to up the capacitance of the system.

Sulaiman

Fri Mar 23 2018, 11:23AM

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

I imagine that you are producing some kind of continuous discharge between the two vdg. that reduces the available voltages.
Try larger and smaller separation ?

Conundrum

Fri Mar 23 2018, 11:39AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Maybe use a properly shielded camera? S2 isn't bad but I've had problems with EMI on these.
I came up with a variant of this using a camera such as the u1010 which has a metal shield so in theory should be far less susceptible to EM related problems.

Nickel

Fri Mar 23 2018, 12:42PM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

Sulaiman, I think you are probably right about the possible continuous discharge. At the moment I try to keep my workshop at 40% RH or below, to protect my tools and machines.
I'll try moving them further apart, to see what happens. It seems that the electron-cloud that surrounds the negative terminal is big enough to bleed voltage down to ground and surrounding earthed machinery, as well as any objects nearby. If I stand within 3 feet of either terminal when they are running individually, I get charged up either way, + to the point that sparks sneak from my feet, through the sides of my boots to the workshop floor. Amusing and tickling, but not what I set out to do.
The next thing to try is corona-reduction at the entrance holes to the terminals. I have experimented with 22mm copper central-heating pipe, but that is still work in progress. I've wasted a lot trying to achieve a re-entrant curve, so there is only cable-sheathing split to go over the edges of the hole.
Until I do a bit more shielding of electronic stuff, I have put a hold on operations. I have a roll of 2" chicken-wire fence to make temporary earthed cages around the more expensive stuff.
In the meantime, here's a lower spray comb:

1521808933 4367 FT181790 20180217 161820 Resized

Nickel

Wed Mar 28 2018, 08:47AM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

I had a play outdoors last night with the positive unit in the semi-dark. This has shown up where my losses are going. As many have found before me, the entrance to the top sphere needs to be a re-entrant curve to stop corona losses.
Outside in the dark, the base of the dome shows a continuous weak 'spark' pointing towards the ground. After 1 inch, this becomes a mauve 'cloud' about 2-3" diameter, 6-8" long pointing ground-wards.
My tube-bending skills are not up to crease-free bends for tight radii, so any ideas or sources for a corona-prevention torus would be appreciated.

Nickel

Sat Nov 17 2018, 04:35PM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

I've been doing a lot of thunking recently, and intend to resurrect my project. Research on electrostatics shows there will be a field-concentration where the column enters the top sphere. The cure is to form a re-entrant curve to the dome at the column entry point.
After many failed attempts to make this using assorted bits of pipe, The answer leapt out at me from the window of a catering supply shop of all places!
It's called a 'Bundt pan/tin', 'Savarin mould', or 'Kugelhopf pan'.
A lot of them are fluted, which is no good at all.
The one to look for is a plain one, available in all sorts of sizes, with one for everyone's application!
A simple hemispherical bowl inverted on top of it will provide exactly what is needed:
Google

Nickel

Tue Nov 20 2018, 09:37PM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

I've just found this lovely machine on the youtube:

Well worth a look if you are a crackle-addict!!

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