Thursday, January 21, 2010

Build your own Van de Graaff generator!

Last year I set out to build a van de graff generator, and after a year of tweaking, I have something that works (most of the time).

The first step in building these is understanding the principles of how it works. I will attempt an explaination here, but please check out the wikipedia here.

The charges for this machine accumulate on smooth conductive surfaces, which is the reason for the large metal globe at the top. Generally, the larger the globe, the higher the voltage. Inside the tube, there is a belt with 2 rollers. The belt is made from some electrically neutral material that is also an insulator (latex). The rollers are made from materials that have opposite polarizations in the triboelectric series. I chose aluminum(+) and silicone rubber (-).

When the belt comes in contact with one of these materials, electrons are either pushed to one side of the belt, or pulled from one side. In the case that electrons are pulled from the other side, the metal brush is there to provide replacement electrons. (the reverse is collecting the extra electrons). The belt moves along over the roller, and eventually seperates from the roller. When the belt is seperated from the roller, it will then have a charge on the outside of the belt. (for those concerned about the conservation of energy, dont worry. The work of seperating the clingy-belt from the roller equals the charge on the belt.)
A couple of mechanical challenges when building this:
  • Attaching the bearings to the pvc pipe so they do not move.
  • keeping the belt on the rollers
  • keeping the brushes in place
  • attaching the bottom roller to the motor
These are not really challenges for those that are mechanically inclined, but I found mighty-putty to of use in a couple of places.
Lets start with the metal globe at the top. This is essentially 2 14" steel salad bowls from ikea. a hole is cut in one of them for the pvc pipe to fit into, and they are taped together inside and out with aluminum tape. after you cut the hole, use a file to take off any sharp edges with a file.

The globe will be detachable, and connected to the brush by alligator clip. The wire must be inside the globe; The reason being, that the globe becomes the center of an electric field. If it has conductive points on the outside of the globe that are not smooth, they will spew electrons in a process called corona discharge. What you really want is electrical breakdown, which is more impressive.

Get a PVC pipe, and cut to about 4 or 5 feet. (if you have high ceilings, use a 6 foot pipe)

Cut a U shape into each end to mount the bearings on. Make sure the u-shaped cuts are lined up, which will line up the belt rollers for less friction. The bearings should fit snugly, so they do not vibrate.
Keep in mind you will be putting the metal globe over the top roller. For that reason, I used mighty-putty on the top roller to attach the axle to the bearing:
To make the roller, I used a 5/16" x 6" carraige bolt as the axle. For the top roller, I cut off the head. The bearings are held in place by bolts. (btw, axeman is a good place to get cheap bearings) The roller was made from a rubber sanding drum drill attachment, which was cut to size, and then screwed onto the bolt. The drum is then wrapped with electrical tape (or any tape you like) to give it the right shape. You want a "crown" in the middle of the roller, as the belt will naturally tend towards the center of the crown. (this is how you keep the belt on the roller)
The bottom roller (above) has an extra rubber roller with a notch to hold the drive-belt.
Aluminum Tape, usually used for ventilation ducts. Used to cover the bottom roller, and seal the globe halves together.
Silicone tape, used for plumbing, used to cover the top roller.

A toilet flange holds the pvc pipe upright. This is bolted to a wooden base, purchased from the IKEA damaged goods section. The brushes are made from braided copper grounding wire. Aluminum tape holds the brush in shape, and a screw/washer attaches it to the pipe, allowing you to align the brush before assembly.
The bearings for the bottom roller are held in place by plastic inserts.

The latex belt is made from one of those pilates yoga straps, but get the heavy-duty-tension version, for more durable, thicker latex. After the rollers and the bearings can be mounted, you will have an idea of how long to cut the belt. Make the belt 10% shorter than it needs to be, so that it will stretch and have some tension when it is on the rollers.
Use rubber cement to glue one end to the other. Cut on the diagonal for a stronger bond and less of a chance it will catch on a brush. When you put the belt on, make sure it is facing in the direction in which the seam will not catch on the brush.

Next, the electronics
Go to axeman or other surplus/hobby store to get a 120V AC 2-pole motor. Most hardware stores will have dimmer switches. You can buy 3-prong plugs and wire, but most likely you have this already. Be sure to look up which wire is hot, neutral, and ground. The hot should go to the switch first.
The wire on the left leads to the bottom brush. The right side attaches to the grounding leads on the other components.
The aluminum plate keeps he motor in place, acts as a heat-sink, and is conductive for grounding.
I attached the drive-belt-pulley with mighty putty.

You can use smaller serving bowls to make an electrode. drill a hole for a conductive handle, and attach handle to grounding wire.

With some aluminum tape and a jar, you have a Leyden jar. This will make the sparks 50% more painful.

Well, once you are finished, show it to your friends after they have had a couple, and they will be more likely to volunteer for shock therapy.

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