Decided to make a Van De Graaff generator

[ck8080]

Hi,
I recently decided to make a Van De Graaff generator and I have a question regarding the top load sphere.

I managed to get myself a very nice 60cm-diameter stainless steel polished sphere.
Before I start cutting up the bottom part so to make room for the top roller with the collection comb to go inside the sphere, do you guys know if there's any reason the generator would not work if I keep the electrode outside the sphere so as I don't make a hole on it? Is there a reason why the top electrode has to be inside the metal sphere? Can I just keep the whole sphere intact and suspend it on a non-conductive frame above the top roller and charge collecting comb and just attach the electrode on the outer side of it?

 

[Keith_McClary]

keep the electrode outside the sphere so as I don't make a hole on it?

This image from Britannica shows a cylindrical housing for the charge remover attached to the bottom of the sphere (the inner circle in the diagram). Something like a steel mixing bowl (without sharp edges) might be better than a cylinder.

 

[anorlunda]

That sounds like an exciting project. I moved it to the DIY forum because you might get better answers there.

@Keith_McClary that image you posted is very difficult to see. Can you post a better one?

 

[Keith_McClary]

@Keith_McClary that image you posted is very difficult to see. Can you post a better one?

The image looks to me the same as on the linked Britannica site.

I should add that the reason for using a sphere is that any projecting bumps or edges will have a stronger electric field, leading to discharge. Having the charge collector brushes outside the sphere (but connected to it) spoils this.
That is why I suggested a conducting bowl, but adding this bump would still result in a less than optimal shape.

 

[gleem]

Can I just keep the whole sphere intact and suspend it on a non-conductive frame above the top roller and charge collecting comb and just attach the electrode on the outer side of it

While an intact sphere will hold the charge a little better the upper comb assy has sharp edges exposed to the atmosphere which will produce coronas resulting in charge loss, charge that will never reach the sphere. You must minimize all sharp metal surfaces at the top of the device. Commercial machines fold the surface of the dome back into the hole. One way around the problem of the hole's sharp edge is to place another smooth metallic surface with no sharp edges in contact with the sphere around the hole .e.g., a ring made of a tube or rod of sufficient large radius of curvature.

 

[Tom.G]

Can I just keep the whole sphere intact and suspend it on a non-conductive frame above the top roller and charge collecting comb and just attach the electrode on the outer side of it?

Yeah, it worked when my father and I built one about 60 years ago! But as @gleem said, it will probably work a bit better with the upper comb inside the sphere. The drawback of having the upper pulley and comb inside the sphere is it's difficult to adjust either of them.

Oh, and both the ion source and the spray voltage are optional, we just connected the lower comb to ground.

We also had a bit of a bootstrap circuit. The lower comb was slightly above the pulley. Another comb was facing the downward-traveling belt at about the same relative position, and this new comb was connected to a small plate near the inside surface of the upward-traveling belt, in line with the grounded comb. I don't recall for sure if it helped or not.

    | ----belt---- |
    |              |
--> | !---------+  | <--+ comb (bootstrap)
    |  \plate   |_______|
    |              |
    |              |
    |   pulley     |
    \     here    /
      \ ________/

Cheers,
Tom

p.s. Have Fun!

 

[Rive]

Is there a reason why the top electrode has to be inside the metal sphere?

Yes: the excess charge will be 'deposited' on the outer surface of the sphere, leaving the inside free of E field. That's what makes it possible to sweep further charge from the belt, based on the E field of the belt only.
So, to make it work you have to place the upper comb/assembly into some kind of 'inside'. That cylindrical extension shown on the picture in #2 has the same function.

If you don't want to cut up that sphere you might try to build something similar instead.

 

[Keith_McClary]

Of course there numerous DIY projects on the web (also image search). This youtube uses a bowl to avoid cutting the sphere.
This page has a (somewhat discouraging) how-not-to experience, emphasising the importance of the shape and smoothness of the ball.

 

[lesaid]

Hi,
I recently decided to make a Van De Graaff generator and I have a question regarding the top load sphere.

I managed to get myself a very nice 60cm-diameter stainless steel polished sphere.

Just to sound a note of caution. A 60 cm sphere, by my calculations, if at a guess, it achieves say 500kV, will store over 4 joules (capacitance 33 pF). If it achieved 800 kV, that would be around 10 joules.

If my quick calculation is right, this is verging on what I believe to be dangerous, potentially lethal territory. Perhaps a real expert might like to chip in, but I'd be wary of a van-de-graaf machine with a 60 cm diameter sphere.

I find that a small, 10 cm diameter torus will easily accommodate over 60-70 kV without significant losses. A 60 cm sphere has much less curvature and will achieve a very much higher voltage. I don't know quite how much! And energy goes up with the square of the voltage.

I suspect that great care might be needed!