Van De Graaff let – A miniature VDG

VDG 'Let

This is all Ikea's fault !!..

My wife spotted their new Blanda Blank Bowls, each one is an almost perfect hemisphere 50mm in diameter and they come in packs of 4.

They NEED an application...

(I've since read Ikea's description and the packaging they come in and I still have no idea what they are supposed to be used for ?, eggs? ,spice?, HV Experements?)

After some thought of what they would be good for, the idea of a small Van De Graff came to mind.

The VDG'let was born..

The top Sphere of the VDG'let was made from two bowls (as above) .

The lower half  was drilled with a 20mm hole to allow mounting to black 20mm electrical trunking which forms the supporting column.

The top of the trunking was cut in a V to allow a 2mm brass rod to sit across the center of the tube. This rod carries a 2.5mm id aluminium tube that is freely spinning. As the bottom roller is also made of aluminium this roller is coated with PVC tape to ensure it will take a different polarity charge than the lower end. See the Tribo electric Series

The trunking was attached to the bowl with a lump of epoxy putty and the roller secured in place with a 10mm long sawn off peice of PVC trunking connector.

Be careful, drilling large holes in small pieces of stainless steel is not for the faint harted..

The top hemisphere (bowl) contains the pickup comb. This is a peice of stranded wire bent to a fan shape and stuck into the bowl. This is arranged so when the two halves of the sphere are together the wires just miss the roller beneath.

The belt is made from a peice of latex rubber cut from the wrist of a medium latex glove.

The length of the supporting column has been arranged so this belt fits nicely from the bottom to top rollers so there dosn't have to be a join.

The best way of cutting this strip is to insert a business card into the wrist part of the glove, and cut an 8mm strip through glove and card. This is not as easy as it sounds, I trashed a number of gloves before getting this right.

The Bottom box contains a 12v DC motor crudely attached so its position can easily be changed to keep the belt in the centre of the trunking.

The motors shaft has been extended with an aluminium tube, this is in direct contact with the belt.

Another earthed comb made from stranded wire is placed near the belt.

Another hemisphere is used here to shape the electrical field (ok, so it looks good and you do get 4 to a packet) Idealy the motor should be within this hemisphere, but I couldn't get a motor small enough. and mounting a motor to a hemisphere is much more dfficualt.

Picture of it working.
Use your imagination


The the VDG fired up first time ...WOW...

I can get about 22mm sparks from the spheres in good dry air (difficult to find in the uk !) Charging current is an estimated 0.5-1uA

I have tried using other materials for the rollers, but PVC-Latex-Aluminum appears to work as the best combination (so far). I'm looking for some sort of nylon roller to replace the aluminium one if possible.

In really dry air I can get the sparks to track all the way from top to bottom via the PVC column, but this is rare so I'm not going to try to insulate the column any futher.