Van de Graaff generator, final post


I recently disassembled my Van de Graaff generator, it now lives in several pieces in a corner of my room. I'll hopefully rebuild it some day, but for now I don't have space. There's a bunch of things I'd do differently now that I'm much better at building things and have access to far more powerful tools. My original VDG was almost entirely made with hand tools, even cutting the hole in the bottom of the mixing bowls. Now I have access to several machine shops and have the tools to design things on my computer before making them.

But for now to get a little bit more closure, I'm copying my old write up I did after finishing the project. I believe I did this sometime in the first half of 2009, I've only edited the formatting and some of the links. The content is the same as was on my old site.

vdg-bottomcomb.png vdg-topcomb.png
Current design

Van de Graaff generator, designed and built from scratch by me, with minor help from my parents.

The top is made of two steel mixing bowls. The bowls are connected together with aluminum tape, which is set up to contacts both bowls (the glue side of the tape is non-conductive).

The lower bowl has a hole cut in it's base, where the top of the support column enters with the top comb (as seen above). The comb is made of aluminum screen, which contacts the inside of the bowl. The screen is as close as possible to the belt, without touching. If the comb touches, the the Van de Graaff generator doesn't work (and it scratches the belt).

The top pulley is made of three needle rollers wrapped in aluminum, with a gradual buildup in the center. Pulleys larger in the center cause belts to automatically center themselves, since the belt is being pulled in both directions as the same time. The needle rollers roll along a PVC pipe, reinforced by a metal rod. (Note: I originally designed the top roller as a PVC pipe spinning on another pipe. It wound up causing too much friction, and the pipes actually melted together. The problem was completely solved by adding the rollers).

The belt is fairly thick neoprene, which I cut to make sure it was perfectly even. Superglue gel holds the joints together. It's easily strong enough to hold the belt together, even when sitting around for months or spinning at full speed. The belt travels up and down a tall PVC pipe, which supports the bowls at the top and is supported by a large wooden base.

The lower roller is PVC, reinforced by a metal rod inside it. It is mounted in ball bearings, one on each side. The bearings are jammed in the wood, held in place by the wood. On the outside of the bearings are short segments of copper tubing, glued on to prevent the roller from moving side to side. The metal rod sticks out of the PVC far enough to clamp an electric drill on, to power the entire belt.

Old design

When I first built my Van de Graaff generator, I didn't want to cut a hole in the mixing bowl I was using for the sphere, since I didn't know if it'd work yet. While it worked, it could only create tiny sparks and barely lift tissue paper.

When MIT ESP Junction application opened, I decided to write up how a Van de Graaff generator worked for my second choice class's application. To learn more about it, I started watching the 8.02 OCW classes related to Van de Graaff generators. Lecture 8 had a very dramatic demonstration of why the comb is inside the sphere. When you charge the sphere from the outside, you can only reach to the same voltage as the power supply your charging it with. Once the sphere reaches that amount of charge, the potential difference between the power supply and sphere is zero. However, if you charge the sphere from the inside, the charge stays on the outside of the sphere and there is no electric field on the inside. This lets you charge the sphere far beyond the power supply, only limited by the charge leaking off the outside into the air.

The story

I first decided I wanted to make a Van de Graaff generator while I was in an MIT HSSP class about Maxwell's Equations. After the class, I started reading about them in Homemade Lightning. It took a couple weeks to design & build it, and even worked first time I switched it on.

Van de Graaff generators are a lot of fun to make, and pretty simple once you learn the theory. I highly advise anyone interested in having their own lightning, playing with static, or just having fun zapping their friends to make a Van de Graaff generator.

Here are some blog posts I made while I was building my Van de Graaff generator

03/22/09 (Van de Graaff theory)
03/23/09 (Parts overview)
03/23/09 (More detail on parts)
03/24/09 (Revised & more detailed parts list)
03/31/09 (Progress update)
04/14/09 (Assembly overview for original version)

Since those posts, I've revised the design a little. The first problem I hit was that the PVC rollers were melting due to spinning so fast. I bought a set of needle rollers, and taped them together to create a metal roller. I discovered that when I built up the center of the roller to make a gradual curve, it automatically centered the belt (the belt gets evenly pulled in both directions trying to shorten itself, and winds up going nowhere). The other problem was the bottom roller was getting bent upwards by the belt, so I superglued a metal rod in it to reinforce it. The PVC on the outside of the roller (as seen on the left hand image at top of page) was enough so no electrons jumped to that rod.

After fixing the rollers, I started working on the upper terminal. The original design had the comb under the upper terminal, where most Van de Graaff generators have the comb inside the upper terminal. I'm not quite sure about the physics, but I'm pretty sure that when the comb is inside the upper terminal, no matter where the electrons go they wind up on the terminal. However, if the comb is outside, they can jump anywhere in range. After I moved the comb inside, I got MUCH more impressive sparks, including a cone of extremely dim violet lightning, when set up correctly. Sadly, the most interesting ones were too dim for my camera to see.

My next project involving my Van de Graaff generator is to figure out how to make it work when it's humid out. As is, I can only turn it on during winter, and if I'm lucky late fall and early spring. It just doesn't build up charge unless it's dry enough. So far I have no idea, other than building it a box that I could pump the humidity out of.

Update on 04/20/2010: I have done some experimenting, I'm convinced it's the humidity causing charge to leak off too quickly. It's possibly to hear the charge jumping off (crackling) when much farther away from the VDG than in the winter. Unfortunately I haven't come up with any ways to make it work well while humid.