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:
Triboelectric series
- Human Hands (if very dry) +++ Positive +++
- Leather
- Rabbit Fur
- Glass
- Human Hair
- Nylon
- Wool
- Fur
- Lead
- Silk
- Aluminum
- Paper
- Cotton
- Steel (neutral)
- Wood
- Amber
- Hard Rubber
- Nickel, Copper
- Brass, Silver
- Gold, Platinum
- Polyester
- Styrene (Styrofoam)
- Saran Wrap
- Polyurethane
- Polyethylene (scotch tape)
- Polypropylene Vinyl (PVC)
- Silicon
- 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:
scitoys.com/scitoys/scitoys/electro/electro6.html
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Signing UpStep 1: Parts List
* 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.
* ~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.
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May I hazard a guess that you are/were a teacher? Your enthusiasm and style remind me of a couple of my teachers (that's a good thing just in case that wasn't clear ?).
Thank you so very much for your kind comments, I really needed it right now!!
The very first VDG I constructed was from an online article at this URL:
http://sci-toys.com/scitoys/scitoys/electro/electro6.html
It was relatively easy to build (and modify) and after seeing the voltages it developed I was hooked. My next build was full size and much more difficult but I joined the Van De Graaff generator group on Yahoo. Those folks are quite knowledgeable and happy to share what they know as they love to build them as much as I do. (I recommend signing up with them)
Good luck with your build, should you run into any snags, post a comment and I will see if I can help.
As to be a teacher, no, just a geek who loves to build High Voltage stuff.
Thanks again,
Todd
I really liked what you made and even im trying to make one. But the problem is that i have just about 10-15 days for my school science fair to go, and i need to construct one for the fair. Can you please suggest me some easy and simple ways to construct this in such a short time?
Pleas help me. You can contact me through email- raghu.jjj@gmail.com
yes the current is very low and it is only slightly more painful than a shock from the carpet.
Of course you should never discharge a capacitor or leyden jar charged on a Van De Graff Generator through your body! That could be fatal!!!!
Thanks for looking and feel free to ask any other questions.
Haven't finished it yet, though I have confirmed that there is enough contact between the pieces for it to have electrical continuity.
I didn't cover one of the hatches, I've cut a hole in it and glued it to a PVC coupler to more easily attach it to the support column.
Before I discovered that the xmas globes will work, I covered one in aluminum duct tape. I used it as a ground terminal on my first and later as an output terminal on a mini that I built. I works but even with smoothing it there will be some corona leakage. I have attached a pic for reference. The small globe in the pic is a plastic ornament. These come in a variety of sizes and can be had for little money at the end of the xmas season. They work well on small ones but the metal paint vaporizes on larger units.
Sorry for taking so long to reply, haven't been on the computer in days.
Rubber tourniquet should work, though it won't be that wide. You don't say how large your Van De Graaff is so I don't know if the width would be an issue. There are many other materials that can be used. Google "Van De Graaff Generator Belt Material" and you should find some good links.
The charge is developed first at the separation of the lower belt, so make sure that the comb is at or close to this point. If the comb is on the wrong side little or no charge will develop. The same goes for the top of the belt, the comb should be at the separation side. Some people have reported good results with the comb directly below the bottom roller and directly above the top roller. Finally the comb should be as close to the belt as possible without making contact with the belt.
The combs can be as simple as some strands of wire fanned out, the main idea is to have pointed or small diameter wire to concentrate the charge. The lower comb assembly must have a very good ground, preferably an earth ground.
If you are creating a self excited VDG then the roller materials should be as far apart on the Tribo-Electric scale from the top of the page.
The two biggest 'killers' of VDGs are Dirt and humidity. Both of these serve to dissipate the charge before it can be created. Make sure it is clean!! If the relative humidity is above 30-35% then consider a 'dryer'; this can be as simple as a low wattage light in the base, this will help dry-out the components and lower the humidity inside the unit. Also, make sure to cover the lower part so that it cannot pull dust and dirt from the the floor;ground. I recommend using alcohol to clean the components, including the belt. I have used soapy water and then a hair dryer to dry the belt, If you dry the belt with a cloth it should lintless so that you are not transferring lint to the belt while drying.
Well that's about all there is to advise on, let me know if this helps or if you are still having problems
Todd
The maximum voltage the output terminal can charge to is in part determined by the size of the terminal. The larger the terminal that larger the output. check out step 8 for tips on alternate ways to create the output terminal. The terminal in this project is about 33cm by 38cm and can easily produce sparks 15cm to 22cm. the small units pictured in the article have an output terminal 20cm in diameter and only produce sparks 5cm to 7cm.
Fast Motor speed is not necessary to create a large charge, it is the output terminal. A fast motor will help maintain the charge which is constantly drawn off by discharges a the entry hole and the terminal itself.
Belt width, belt material, roller material, diameter of the rollers and the distance between the rollers all play a part in 'pumping' up the charge.
Think of charging like blowing up a balloon, there is constant pressure outside the balloon to remove the air inside, if you pump more air into the balloon than it can handle it will leak out where the air is being pumped in or it will break. Obviously the output terminal does not break,it is the air around the output terminal that breaks down and allows the terminal to discharge through the entry hole or to a nearby grounded object.
Good luck with your project and thanks for looking and commenting
Toddjwood
Thanks again,
Toddjwood
Thanks for taking the time to look at my Instructable!
Yes they are both absorbent to moisture and both are not particularly good insulators. you need to use a material that will not allow the charge to pass through the belt to the rollers. Most of the charge is developed by the separation of the belt from the roller. Triboelectric actually means electricity by friction, but the charge is developed because the two materials have a tendency to either lose or gain electrons when the dissimilar materials separate. The rollers will become charged with opposite polarities and the combs are used to pull charge off of the belt or spray electrons onto the belt. With a good insulator the charge is held long enough to reach the top of the VDG and can be collected inside the output terminal. This is why there are different materials used for the upper and lower rollers. The belt is essentially neutral and will hold either a negative or positive charge.
Having said all that, people have used both materials for belts and if woven tight enough and kept dry they will work. but they both also produce lint which will dissipate the charge and require more frequent cleaning.
Yes, I know I talk a lot. sorry. I hope this answers your question.
toddjwood
Thanks for looking and the comment!
The purpose of the 'pointy' ends is to increase the amount of corona discharge. It is the ionized air that actually charges/discharges the belt. The sharp ends help raise the output voltage to the breakdown voltage of air allowing the charge to travel through the air and charge the belt or pull the charge from the belt. Having multiple ends increases the number of regions that are charged. The surface of a plate would not effectively transfer the charge, it would have to be much closer (thousands of an inch) and the voltage would not be as high. Think of it as pumping water through a large pipe into a very small pipe, the pressure increases as the same volume of water tries to travel through the smaller diameter pipe. As voltage is electrical 'pressure' you can use the same analogy.
Hope this explanation helps.
toddjwood