Re: [volt-nuts] Bohnenberger electrometer DANGER

[ed breya]

Here's a simplistic view that may be sufficient. Some energy (in the 
form of charge redistribution, which includes current flow) has to come 
from the capacitor, and some from the input signal, to do the work 
needed to push the leaf against gravity. When the input signal is 
removed, some of the energy (charge) stored on the leaf is returned to 
the cap as gravity restores the initial position - roughly the same 
amount of work, depending on leakage and mechanical loss, and heating of 
the protective series resistor.


With a quick review of electrostatics, you could estimate up a simple 
model and the field equations to get a more satisfying, detailed answer. 
It may be more straightforward to look at it from a circuit perspective. 
Picture it as as a very small, non-linear capacitor (the leaf structure) 
in series with a much much larger regular capacitor charged up to a 
constant DC voltage. Any actual series resistance is just resistance, 
and the mechanical loss can be represented as more resistance added in 
series. Presuming the leaf never actually touches or emits particles* or 
arcs to the reference capacitor node, it's basically a capacitive 
voltage divider, and the applied signal may be considered to be 
transient, or even AC - it steps to the applied voltage, then returns to 
zero (or open), then the cycle may be repeated. Each experiment is 
adding or subtracting charge, then reversing the process. Ideally, the 
cap would never lose its DC bias if there were no losses.


The problem is that figuring out all these details may not be trivial. 
It may be more fun to just try some experiments and see how it goes - 
you'll get some idea of how the real thing holds up, and figure what 
amount of C is OK.


*You shouldn't have to worry too much about corona discharge if the 
maximum voltage on anything is below 3 kV or so. Beyond that, it could 
cause problems.


Ed
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Re: [volt-nuts] Bohnenberger electrometer DANGER

[Dr. David Kirkby]

On 8 March 2018 at 07:19, Andre  wrote:

> Hi, re. capacitors it might be worth mentioning that the normal equation
> assumes charge and discharge through a constant current.
>

What 'normal equation' do you mean?


> Don't forget that the equation includes a non linear term so you'll need
> to take that into account (Q=CV2 iirc) where Q is Coulombs, C is
> capacitance.
>

I am puzzled by CV2.  The energy (joules) stored in a capacitor is 1/2 C
V^2, where C is the capacitance and V the voltage. I don't know if that's
what you mean.


> If this is done using something like an LM317T in CC mode or even a string
> of them (my idea) with anti-overload circuitry added externally then this
> may well work.
> Any series resistance will cause problems so you'd need quite a lot of
> regulators but there are ways to use JFETs selected by hand if you really
> wanted to
> make a test setup.
>

I am totally lost here!

If anyone has an explanation of whether any of the energy to move the leaf
comes from the battery/capacitor, or does it all come from the charge
applied to the unit, I would like to know.  If no energy (apart from
leakage) comes from the device applying the electric field, a small
capacitor is suitable, and very safe. If at least some of the energy
required to move the leaf comes from the voltage supplying the electric
field, then a small value capacitor will be no use.

This is a fairly low priority task for me at the minute, as I need to do
some real work until Friday evening. But over the weekend I will play with
this.

Dave
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Re: [volt-nuts] Bohnenberger electrometer DANGER

[Dr. David Kirkby]

On 7 March 2018 at 06:29, Andre  wrote:

> Please be VERY VERY careful. To be honest its far safer to use CCFL
> drivers and rectify them with camera diodes in series and the absolute
> minimum capacitance for the job, shunted with a high value resistor.
>

The problem with 2.2 nF is it is difficult to know what voltage is on the
capacitor at any time, since with a 10 M ohm multimeter, the time constant
is 22 ms. I have a high voltage probe around somewhere, which probably has
a 100 M ohm input impedance, but that would still only give a time constant
of 220 ms, which is too short to measure easily.

I have 47 uF @ 650 V (or it might have been 550 V) capacitor on order, but
I might go to something a bit lower capacitance if the charge storage is
not required.

I would like to know where the energy comes from to move the leaf. I wonder
if any is taken from the capacitor. In the gold leaf electrometer, with no
internal supply, it is clear the energy much come from the charge on the
plates. But when there's an electric field, that might not be the case. I
was thinking of sticking a 50 uA FSD meter inside, to see if any current is
take from the capacitor, but I don't know if 50 uA would be sufficiently
sensitive to deflect.

Clearly having an electrometer here would be useful for these sorts of
experiments, but I don't have one. I see a reference on here recently to
the Keithley 642 being one of the best, but whilst the basic meters are not
that expensive, the test head and cable are much rarer, so attract a much
higher price.

In any case, there's not a single half-decent electrometer on eBay in the
UK at the minute, and I need it before Monday.


> I have a few inverters , 10M resistor packs and diode strips here if
> anyone has a use on the understanding they are only to be used at your own
> risk, and for the intended purpose.
>
> Microwave capacitors can be deadly (you could DIE!) under the wrong
> circumstances, fibrillation can occur even with quite small shocks down to
> <8J if you get hit badly or have an undetected problem. I don't want to
> scare people but it is a serious risk.
>

It's fairly obvious to me, based on a few quick experiements last night,
that kV is not needed for this. Whilst I'm not doubting one could make an
electrometer (electroscope???) of greater sensitivity using a higher
electric field, this is good enough for a demonstration, and to learn a
bit. For quantitative measurements, I will look for a Keithley
electrometer, at a later date.


> -A
>


Dave
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Re: [volt-nuts] Bohnenberger electrometer DANGER

[Andre]

Please be VERY VERY careful. To be honest its far safer to use CCFL drivers and 
rectify them with camera diodes in series and the absolute minimum capacitance 
for the job, shunted with a high value resistor.
I have a few inverters , 10M resistor packs and diode strips here if anyone has 
a use on the understanding they are only to be used at your own risk, and for 
the intended purpose.

Microwave capacitors can be deadly (you could DIE!) under the wrong 
circumstances, fibrillation can occur even with quite small shocks down to <8J 
if you get hit badly or have an undetected problem. I don't want to scare 
people but it is a serious risk.
Had to scale back one of my projects because I had a near miss with a setup 
very much like the one described  in earlier posts and despite dual failsafes 
still got a belt large enough to require medical treatment.
(hint: the experiment is on Youtube, saying no more)
Very fortunately my systems weren't seriously affected but I probably did some 
damage.

Still have a 380J 215uF/2.5KV capacitor here and that one is staying shunted 
and under lock and key until I have the appropriate safety knowledge and 
experience. Dielectric memory is a b*t*h!

-A


From: volt-nuts  on behalf of ed breya 

Sent: 07 March 2018 00:11
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Bohnenberger electrometer

I looked at that link that Brooke put up about Bohnenberger's
Electroscope. I don't know what your specific arrangement needs to be,
but it appears you need a plus and a minus HV wrt ground in the most
general form. If so, then this would mean having to split the voltage of
a single cap, or have two caps, one for each polarity.  Then I'd
recommend using good old microwave oven caps. You could charge them both
to say 2 kV from one HV source, then switch them around so they're
stacked and grounded at the midpoint.

Ed

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