Re: Electrometer design? (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 9 Jul 2004 09:29:27 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Electrometer design? (fwd)

On 8 Jul 2004, at 10:52, High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Wed, 7 Jul 2004 11:04:17 -0600 (MDT)
> From: Steven Roys <sroys@xxxxxxxx>
> To: hvlist@xxxxxxxxxx
> Subject: Re: Electrometer design? (fwd)
>
>
>
> ---------- Forwarded message ----------
> Date: Wed, 07 Jul 2004 08:33:55 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: Electrometer design? (fwd)
>
> Malcolm Watts wrote:
>
> > I thought about removing wedges of copper from ordinary
> > Cu-fibreglass laminate to make the disks (which leaves the disks in
> > one piece). Anyone see anything wrong with that plan?
>
> As in etching off the copper? I'd thought of that.  Producing the
> discs photographically would simplify the construction, just requiring
> some accuracy with a hole cutter.  Mechanically, the discs would be a
> lot stronger and easier to balance.  I'd have thought that this should
> certainly be good for the stationary element.
>
> I do wonder, however, whether making the rotary element like this
> would block the charge travelling towards (or away from?) the
> stationary.

But it's measuring an electric field, not moving charge isn't it? The
charge movement is along a connecting wire from some ground point to
one of the plates and comes from induced charge caused by the
periodic exposure and hiding of that plate to the electric field (Y-
N)?  The reason I asked was because I was uncertain about the
influence the intervening dielectric might have. I reasoned that
relative to the permittivity of air it was so high that the air gap
would be the final arbiter (roughly analogous to an airgap in a
magnetic circuit). Also, I asked because I was seeking a definitive
answer one way or the other, preferably based on sound theory or
experience so I wouldn't waste my time constructing segmented plates
of the form I proposed that turned out to be useless for one reason
or another. Perhaps the best thing might be to do as Antonio
suggests, although phsyically cutting the PC board into a segemented
disk rather than etching as opposed to cutting a metal plate (less
appealing) might still be on?

Thanks for any input,
Malcolm
 BTW - have you got yur flyback circuit running correctly yet? It is
always useful to get feedback so we know which suggestions if any
proved to be correct or wrong. I too noted the parasitic oscillations
when the MOSFETs were supposed to have been off and I note that one
contributor suggested a series gate resistor but I seem to remember
you saying that you already had one. I think that excessive impedance
in the source lead is the main cause of those oscillations myself.
Having said that, current-mode SMPS designs *do* require primary
current monitoring and do use a source/emitter sense resistor to do
the job. Perhaps there is too much impedance in the gate lead (as
opposed to too little). What is the value of your gate resistor?