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Registered Member #3026
Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54
Hello everyone,
I am building a Van De Graaff generator with a 2 inch wide rubber belt (exercise material) a plastic bottom roller and a steel top roller. My two brushes are copper screen material. Does anyone know the optimal positions for the brushes to align with the top and bottom rollers? Should they be on the ascending side of the belt or on the descending side? There are many diagrams and plans on line but none of them seem to recommend one placement over the other.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
radioman -
Spend some time at Antonio's electrostatic site, probably the most comprehensive collection of info on electrostatic machines on the planet:
See the info on his bipolar Van de Graaff machine at: , which also contains links to much additional info on design and construction of Van de Graaff machines.
Registered Member #3490
Joined: Wed Dec 08 2010, 11:55PM
Location: The Granite State
Posts: 34
Put the brush on the bottom with the plastic roller.Its easier to adjust brush placement this way.I've seen many suggestions for belt material,but I bought a roll of scrap neoprene rubber and made a belt from that using my wifes pinking shears to make the joint.Super glue and clamp the joint and you'll be all set.If I remember correctly, the more belt surface you can expose to the brush in a given time,the greater the output.With a 3"x30" belt and 12" inch gazing ball terminal,my generator makes 2 foot evilly painful arcs.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
radionam -
I've seen designs with the spray and collector combs places either at the 6:00 and 12:00 positions (directly below and directly above the centers of the rollers), as well as at the 9:00 position on the "rising" side of the belt.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
There is an optimum position, but it depends on the geometry of the rollers and combs and is difficult to predict. With the bottom comb exactly below the bottom roller, too much charge may be attracted to the belt. When the belt leaves the bottom roller, part of this charge leaks away, because the surface charge density on the belt exceeds the maximum allowed in air. That this is happening can be seen by observing the machine in the dark. Sparking is seen in the belt as it leaves the bottom roller. This causes more power to be used to move the belt and may reduce the duration of the belt. A comb positioned just where the belt leaves the roller may be better. The ideal would be to mount the comb so it can be positioned at any angle around the roller, so the position that maximizes the output current without producing sparking where the belt leaves the roller can be determined. The same is valid for the upper roller. But the difference is probably small and the exact positioning not critical.
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Van De Graaff generator question
