Van de Graaff Generator
March 21st, 2009 Mark Rehorst
I was a kid I had a very small Van de Graaff generator (VDG) and it was
a lot of fun. I always wanted a larger one but never got around
to making one until Alex told me about someone bringing one to school
and demonstrating it in his class. Then I was at an Ikea store
and saw those bowls and the decision was made for me - it was time to
Click small images to see larger, more detailed images.
I originally built the generator
with a positively charged sphere, as shown in the photos and plans on this page.
I have since swapped the rollers so the sphere is negatively charged
and find it gives longer, more impressive sparks.
Now the VDG throws off painful, blue, lightning-like 10" sparks
to my fingers within a few seconds of plugging it in. It produces such
sparks every second or so. If you want that sort of performance I
recommend that you build your generator with the teflon roller up in
How VDGs work:
VDG works on the principle of triboelectricity. Triboelectricity
is what you experience when you rub a balloon on your head and your
hair stands up. When two different materials come into contact
and are then separated, some electrons jump off one material and are
acquired by the other. That puts a positive charge on the first
material and a negative charge on the other. A VDG merely
automates this process.
VDG has two rollers and a belt with a motor to drive it all.
The rollers are typically made from plastic and the belt from
rubber. As the rollers turn the belt is moved along. As it
moves away from contact with a roller, the belt acquires one charge and
the roller the opposite charge. Brushes are used to concentrate
the charge on the belt at one end and to pick it off at the other end,
inside a metal sphere. There are a lot of web sites that will
tell you all about how VDGs work, in more detail than I want to get
into. Check any of these sites:
More info than you can digest in a lifetime:
Lots of practical, DIY info and help for the asking here:
Here's a brief biography of the inventor:
Here is Van de Graaff's original patent on the machine:
Van de Graaff patent 1991236
Building a VDG:
are really only two tricky parts when it comes to making a VDG.
You need a metal sphere for the top electrode and a rubber belt.
You can use a motor or a hand crank to turn rollers/belt.
The sphere came to me in the form of a pair of steel salad bowls
from Ikea for about $6 each. The rubber belt is made from a
"resistance band" from a Sports Authority store.
I used 4" PVC pipe for the main structural tube and a flange to
hold it to the wood base.
Clicking the image
below will get you a .pdf file that has sort-of detailed construction
drawings in case you're interested in trying to make a similar device. Right click to save the file to your computer.
rollers are made of 1 1/2" PVC pipe with end caps. I drilled
holes in the end caps then put a threaded rod through the roller.
I used nuts to center the roller on the rod. Skate wheel
bearings allow the rollers to turn smoothly. I soaked the
bearings in degreaser and flushed them out before using them- a little
grease in the wrong place will kill the output from a VDG. The
bearings are retained on the threaded rod with more nuts and then
they are retained in the side walls of the pipe with sheet metal
screws. The generator turns smoothly and quietly. OK, well,
the word "quietly" is stretching the truth a bit, but it isn't nearly as loud
or unpleasant as a vacuum cleaner.
in the center of the rollers prevents the belt from slipping off.
That is done by wrapping each roller with duct or masking tape,
letting the tape pile up thickest at the centers of the rollers.
materials are chosen from a triboelectric series
- Teflon for one and aluminum for the other. Don't worry, the
rollers don't have to be made of those materials, only a thin layer on
of the rollers. I used plumber's Teflon tape for the bottom
roller and aluminum tape for the top roller. (NOTE- this makes
the sphere acquire positive charge- swap the rollers for negative
charge) The belt rubber
(latex) is between aluminum and Teflon on the triboelectric series so I
get extra fast charging of the VDG. You can use any materials you
want to make your VDG. Using the triboelectric series table will
help you select those that provide the highest output current.
Teflon is negative relative to the rubber belt
in the triboelectric series so the teflon roller becomes negatively
charged and the belt carries positive charge (by becoming electron
deficient) up to the sphere. Aluminum is positive relative to the
belt so the belt carries negative charge (electrons) downward from
the sphere. If the roller locations were switched the
generator polarity would be reversed. The positively
charged, upward moving part of the belt passes close to the negatively
charged, downward moving part of the belt. If your belt tension
isn't high enough the two parts of the belt may touch each other due to
the electrostatic attraction. That would cause all sorts of
problems for your generator. This sort of problem can be overcome
by increasing the belt tension, using bigger rollers to move the two
side of the belt farther apart, or using more rollers to separate the
sides of the belt further. I think you could also fit a piece of
plastic inside the pipe to isolate the two sides of the belt.
a bit of work to mess with all the skate bearings. I think it
would be easier to use a couple bicycle wheel hubs for the rollers.
Then the axle wouldn't have to rotate with the roller. You
could use a pair of small pulleys with an O-ring as a belt to couple
the motor to the bottom roller.
size of the sphere (and its smoothness) ultimately determine the
maximum voltage you can get from a VDG. If you want super high
voltage you need a really big sphere. Ikea
sells hemispherical steel salad bowls in different sizes for very
low cost. A pair of these bowls
makes a nearly perfect sphere. I used a fiber reinforced cut-off
wheel on a Dremel tool to cut a hole in the bottom of one bowl so it
would fit over the 4" PVC pipe. The hole is a little over-sized
so that a piece of slit urethane tubing would fit on the sharp
edge and hold the bowl in position on the pipe. It is important
to keep the sphere as smooth as possible to maximize the charge that
can accumulate on it and thus the voltage - covering the sharp edge reduces charge leakage
into the air.
the generator was working properly it was time to install the top half
of the sphere, an unmodified bowl. I found that the seam where
the two bowls meet was the place where the most corona discharge
occured and that by covering the entire seam with duct tape the output
voltage of the generator increased dramatically.
went against conventional wisdom and tried using a bicycle tire
tube for the belt. This time the conventional wisdom was right- I
got no output from the generator at all. That set me off on a web
search to find out what others were using in their VDGs. I found my answer in a Yahoo discussion group devoted to VDGs. I ran out to the
local Sports Authority
store and bought a package of resistance bands and used it to make the
belt for the generator in less than an hour. When I
turned on the power the generator immediately started
throwing off sparks!
resistance bands are 6" wide and about 48" long. I cut one down
to about 1 1/2" wide with a pair of scissors. A little contact
cement closed the ends to form a loop. My rollers are about 38"
apart so I had to stretch the belt to get it over both rollers.
Stretching the belt made it narrow, like you see in the pictures,
so I put in a fresh one that I cut to 2 1/2" wide. After
stretching it was about 2" wide and covered the rollers almost
completely. The wider belt charges the generator faster.
motor isn't critical. Some people use old electric drills or
motors taken from electric fans. It doesn't take a very
powerful motor to turn the belt/rollers when you use ball bearings on
the roller shafts. You could probably get away with a battery
operated motor. I didn't have any junk motors laying around so I
bought a surplus 117VAC motor from Herbach and Rademan
for $8. It runs at about 1800 rpm. It is mounted to the
base board of with a pair of 4" angle irons. The motor is coupled
to the bottom roller shaft with a small piece of urethane air tubing
that has been slit length-wise - the same stuff I used to cover the
sharp edge where the bowl was cut for the bottom half of the
sphere. The air tubing can flex a little
so alignment of the shafts is not critical. Nylon
tie-wraps hold the tubing to the the motor and roller
I were doing it all over again I think I'd bolt the motor right to the
pipe and eliminate the box. The whole thing could be operated on
a table top in a horizontal position, assuming the belt would be stable
in that position. I'm not sure the great length is really
necessary either. It might be OK to go half as long as I did.
Many people have emailed me
about this project and many have duplicated it. One thing I am
often asked is how to wire the motor - apparently Herbach & Rademan
doesn't always supply a wiring diagram. I have prepared a diagram
for you to download here:
Herbach & Rademann capacitor start motor wiring diagram
are not critical at all. I drilled holes in the pipe at the same
level as the rollers then threaded some wire through them.
At the top of the VDG the bare wire is aluminum taped to the
metal bowl (the photo just shows it hanging and touching the bowl- I
later added a piece of tape). At the bottom
roller the end of the brush wire connects to the frame of the motor
which is grounded to the AC power grid (use a 3-wire line cord!).
The brushes are made from metal tape- I used aluminum at one end
and copper at the other but the material doesn't matter as long as it
is metal. Just fold the tape over the wire next to the roller,
then snip away at the tape with a pair of scissors to make a bunch of
pointy corners. Make sure that the brushes don't touch the belt.
it all together, make sure the belt and rollers turn freely. Get
all your valuable electronics out of the room. Plug it in and see
what happens. If it starts charging you'll know it when the hairs
on your arm stand up even though you're standing a foot or two away
from the VDG. Now you're ready to put the top bowl in place.
Set it over the other bowl and wrap duct tape around the entire length of the seam where the bowls meet.
How does it perform?
originally built the generator with a positively charged sphere
(aluminum roller at the top of the column). When I first turned
it on the generator immediately threw 2" sparks
every 1/2 second. One thing I discovered after looking at the VDG
operating in the dark was that there was one point on the sphere that
was discharging into the air continuously. That point was along
the seam where the two bowls meet. I covered the seam with duct
tape and suddenly I was getting 8" sparks! I have since swapped
the rollers so the sphere acquires a negative charge and I get 10"
sparks that look a lot more like lightning. Another thing I
that putting a plastic bag from the grocery store over the sphere also
increased the length of sparks. I think the bag prevents
some of the more "casual" corona discharge and allows the voltage on
the sphere to build up even higher. When you get zapped with
plastic covering the sphere you really get zapped!
Please note: performance of any VDG is critically dependent on the
humidity of the air in which you try to operate it. I tried to
operate my VDG on a rare rainy day in Phoenix once and got no sparks
from it at all. If you like in a humid place or it is raining
outside you may not get much or any output. I have seen patents
from the 1930s for VDGs with heaters built in to dry the air inside the
machine to improve performance in humid environments. If you live
in a humid place try blowing warm air from a hair dryer into the machine to see if it works better.
Need higher voltages? Look at this: http://www.pelletron.com/charging.htm
Here's the original Pelletron patent.
Maybe my next project...
You can email me here:
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