The Van De Graaff Generator invented in 1929 by Robert J Van De Graaff uses a couple of rollers, combs, and a rubber (or nylon) belt to create a charge of a few thousand volts. This charge is drawn off into the center of a hollow metal sphere, this last process takes advantage of the Faraday Ice Pail effect to multiply this charge to tens or hundreds times the input voltage. The charge production occurs in three main areas: the rollers/belt, the combs, and the output terminal.
There are two main types of Air Insulated VDG's, externally excited and self exciting. This instructable will focus on the latter as any self excited VDG can be easily converted to an externally excited unit by attaching a high voltage DC source to the lower comb and removing the comb from ground.
The rollers in this unit must be selected so that they are as far apart on the triboelectric series as is possible. Below is a simple chart for reference:
Human Hands (if very dry) +++ Positive +++
Polyethylene (scotch tape)
Polypropylene Vinyl (PVC)
Teflon --- Negative ---
For this unit I used a PVC(vinyl) roller for the top and a Nylon Roller for the bottom, this will give a unit that has a negative output voltage. To create a unit with a Positive output voltage simply reverse the roller arrangement.
Instead of the standard spherical output terminal I recycled a helium tank as the output terminal.
This is not going to be a specific article on how to build this particular VDG (Van De Graaff [saves typing]) but rather what you must do to build a VDG. I will however try to explain the processes involved for each of the components necessary so that you will have a concept of what it is supposed to do which will aid you in your own construction and troubleshooting.
Whether you decide to build a large 700KV model or a small table top unit the components are exactly the same only the size is different. Also when it comes to VDGs, size does matter, as the output voltage is determined by the size and shape of the output terminal, the size of the rollers, the speed which the charge is delivered to the upper roller, and the distance between the rollers.
So while this unit which is just about 4 feet tall and delivers approximately 500,000 Volts I have a table top version that is 22 inches and only delivers about 80,000-90,000 volts. So if you don't want to commit to a large project then you can build a small unit and then scale it up once you are familiar with construction techniques.
The last photos are of my first VDG project and you will see the components are the same as in the large one. This was created from an online article here:
* Furniture rollers (see step 3) * ~2 inch OD PVC Pipe (1 1/2 inch ID) * 4 inch ID PVC pipe - 2-3 feet long * 4 inch mounting flange * 4 inch connectors * Steel rod for axle * 'Skateboard' Bearings, or other sealed bearing * Electronics perfboard or circuit boards * Stick Pins * Lamp Dimmer Switch * 120V AC Toggle switch (may be optional, see step 2) * Fuse holder * AC Electric sewing machine engine or other 1/8 to 1/2 HP AC engine * Output Terminal (Stainless Steel Gazing Globe, Helium Tank or other metallized globes * Nuts, Bolts, Screws, etc.