Re: van de graaff tube (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 30 Nov 2004 21:50:11 -0200
From: Antonio Carlos M. de Queiroz <acmdq@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Cc: hvlist <hvlist@xxxxxxxxxxxxxxxxx>
Subject: Re: van de graaff tube (fwd)

High Voltage list wrote:

> From: Ed Phillips <evp@xxxxxxxxxxx>

>         I realize you're "in the business" and should know better than I, but
> the current you quote for a self-excited machine sounds low to me.  I'll
> have to look up my old notes but I have a self-excited machine using a
> belt just under 2" diameter which gave something like 15 microamps.
> Far, far from an optimum design.  The column is grey PVC, 4" OD and cost
> a small fortune when I built the thing back around 1963 or a year or so
> later.  The pulleys are about 7/8" OD and turned at a pretty high speed
> through a leather belt/pulley system running off a 3450 RPM motor.
> Original upper terminal was a "more or less" 15" sphere made from
> aluminum mixing bowls, later replaced with an oblate I got from Frank
> Lee.  I asked him about his experience with separately-excited machines
> and he told me the current wasn't necessarily greater than self-excited
> but that things were much more controllable.
> 
>         I have a smaller machine I bought from Science First which uses a belt
> about 1-1/4" wide which puts out about 6 microamps.
> 
>         Antonio has published some notes on VDG output current limits but I
> haven't bothered to compare notes with my old measurements.

When the excitation roller is dry and clean, I don't see any difference
between a sprayed machine and a self-excited machine. The current can be
quite precisely calculated as 26 uA x belt width (m) x belt speed (m/s).
Or, easier to measure: 
26 uA x belt width (m) x Belt length (m) x belt turns per second.
Double this if you manage to make a current doubler at the other end
that
actually works.

My VDG notes are in:
http://www.coe.ufrj.br/~acmq/myvdg.html

Antonio Carlos M. de Queiroz