Re: Van de Graaff generator safety question (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 26 Dec 2003 08:50:45 -0800
From: Ed Phillips <evp@xxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Van de Graaff generator safety question (fwd)

High Voltage list wrote:
>
> Original poster: Steven Roys <sroys@xxxxxxxx>
>
> ---------- Forwarded message ----------
> Date: Thu, 25 Dec 2003 10:56:09 -0700
> From: Terrell Fritz <terrellfone@xxxxxxxxxxxxxxxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Van de Graaff generator safety question
>
> Hi All,
>
> I was thinking of making a Van de Graaff generator out of a 50cm diameter
> (20 inch) stainless steel sphere I have laying around.  I got it from
> Waterscapes:
>
> http://hot-streamer.com/temp/Old2001Files/2001-04/Orb.jpg
>
> http://www.waterscapesweb.com/orbs.html
>
> If I calculate the top voltage assuming 3MV/m radius:
>
> 3MV / m x 25cm = 750VDC
>
> If I calculate the top capacitance I get:
>
> C = 4 x pi x Eo x R  =  4 x 3.1415... x 8.85E-12 x 0.25  ==  27.8031pF
>
> Where I got worried was the stored energy:
>
> E = 1/2 x C x V^2  =  1/2 x 27.8pF x 750000^2  ==  7.81 Joules!!
>
> Assuming one is grounded and the thing arcs to one's forehead, assuming the
> human is the biggest resistance in the path, then all the 7.8 joules is
> dissipated in the human....
>
> My question is how dangerous is it??  Making this thing would not be fun if
> it was capable of killing anybody (well...  "easily").  If it would be
> really dangerous, I would not want to build it since it would not be fun to
> demonstrate if I had to really worry about it.
>
> It is pretty much like a 30pF 750kV cap I would think.  But I am not a
> great expert on this stuff so I thought I would ask.
>
> Cheers,
>
>         Terry

Terry:

	I've looked at the other replies to this and decided I'd add my own.
Your stored energy calculation is of course correct.  You probably won't
get that much voltage but will surely get at least half of it, and the
stored energy will be enough to give you a nasty jolt but the thing is
certainly far safer than a TC if you treat it with the same respect!  As
a reference, I have a "mini VDG" here which uses a couple of 8" mixing
bowls for the top terminal.  It will produce 6" sparks to a 6" diameter
bowl under the best conditions, 4" under most others.  When I bring my
hand or wrist up to it I get a distinct but not unbearable jolt.  As an
experiment I made a small "Leyden jar" type capacitor out of a
polyethylene pill bottle with aluminum foil inside and out.  Measured
capacitance was about 22 uufd.  When I charge it to the maximum with
this  machine (only about 80 kV based on measuring the voltage on a
large capacitor after I discharge the thing into it) it produces quite a
painful jolt, but obviously not fatal since I'm still here.  (Can't say
what the effect on the brain is because I still like to play with HV!)

	I have two other larger machines, one the Science First with 12" upper
terminal and the other a larger one I made back in the 60's with a
couple of "oblate spheroids" I got from Frank Lee.  About 15" diameter
with a 10" long cylinder in the middle to increase the capacitance.  It
will produce 18" sparks and give a nasty jolt but again, not fatal.  The
Science First machine is in between; it has a much lower current output
- ~6 uA vs 25 uA - but still gives a good jolt.

	Is all this a reason to put off building one?  Of course not, just be
careful.  You should get a delightful bang when the thing discharges.
Sometime in the future "when I have time" I intend to build one similar
to that you propose.  Simple to do and the results should be
spectacular, at least for a home-grown gadget.  Plan to use Therabond
for the belt.

Ed

P.S. The first time I got a decent streamer out of a TC was when I was
tuning a little 3" coil with small toroid.  Took a 24" streamer to the
top of my head when I leaned over to turn on the power.  Much more
painful than the VDG but then the duration was longer.