Re: Van de Graaff problems

[William Beaty <billb@eskimo.com>]

On Sat, 8 Jun 1996, Pete Lohstreter wrote:

> I was contacted by the local science museum recently.  It seems their VDG 
> has stopped working.  I thought it was just in need of a good cleaning 
> and adjustment, so I went by the museum today and here's the story...
>
> Their VDG was made by a graduate student at the U of Texas out of spare 
> parts.  The base is made of cast aluminum with a small induction motor 
> connedted to a plastic drum shaped spindle.  The belt is a grey thick 
> rubber-band that looks like the one's you see on some home vacuum 
> cleaners.  The belt runs up a piece (30-35 cm long) of 2 inch PVC pipe.  
> The upper end of the pipe has a metal pulley holding the other end of the 
> belt.  The top ball is a one piece sphere that has a hole for the PVC 
> pipe to go into.  The ball is supported by a metal inverted V that is 
> attached to the PVC pipe.

Hi Pete!

A "comb" which contacts the belt surface is probably functioning in the
same way as one which sprays charge from a distance.  Both are creating an
ion wind with a corona discharge from the tip.  But having the wire
actually touch the surface doesn't work as well, since it paints a very
narrow stripe of charge on the belt.  With 5mm to 10mm spacing, each sharp
point paints a broad swath of charge on the belt surface.

For debugging a VDG, it helps to know that a VDG is a constant current
source.  When shorted, the current flows through the short.  When
operating normally, the potential difference rises until the current flows
through the air.  But it is the same level of current. 

So, to debug a VDG, remove the upper sphere and connect a microamp meter
between the upper comb and the lower comb.  When operating, the VDG should
put out a few microamperes.  If it doesn't, you can try various things
while watching the meter, and when you discover the problem, you'll know
immediately.

On the museum's VDG, is one roller made of plastic and one of metal?  This
is the most common configuration.  It could be that dirt or humidity is
preventing the plastic roller's surface from getting a contact charge from
the inner surface of the rubber belt.  The belt charges the roller, then
the adjacent comb spits ions of opposite polarity to the roller charge. 
In other words, the comb tries to discharge the roller, but the jumping
charge is intercepted by the belt and carried away.  If the belt's contact
with the roller isn't slightly adhesive, the "scotchtape peeling effect"
won't charge the roller.

The plastic roller and the inner surface of the rubber belt should be
cleaned with alcohol and dried by using a blow-dryer on the belt and
roller while the VDG is operating.  If the belt's inner surface has become
oxidized and polished or flakey, it might help to remove the garbage with
fine sandpaper.  Or replace the belt. 


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William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
EE/Programmer/Science exhibit designer        http://www.eskimo.com/~billb/
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