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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Re: [volt-nuts] Bohnenberger electrometer

[ed breya]

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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Re: [volt-nuts] Vibrating reed electrometers

[ed breya]

Another thing I noticed in these instruments - the highest R value used 
is E12, even though decades higher would have been appropriate in 
certain ranges. It shows that was about the practical limit for somewhat 
decent precision and cost. Filling in the desired higher ranges had to 
be done by adding complexity like more gain elsewhere, and writing the 
specs accordingly.


Ed

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Re: [volt-nuts] Bohnenberger electrometer

[ed breya]

Oops - forgot to mention a detail about microwave oven caps. Sometimes 
they have built-in bleeder resistors, which would of course spoil this 
kind of application.  Ed


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Re: [volt-nuts] Vibrating reed electrometers

[ed breya]

Yup - Keithley 640 - that must be the one. This stirred my memory 
somewhat, and I just located info on the model 642 also, which was 
apparently newer. The 642 went to (or back to) ultra-low bias MOSFETs, 
while keeping sapphire insulation and a separate input head. The MOSFETs 
need all kinds of offset, temperature, and bias current compensation. 
The 642 also uses a few digits of DVM that obscure the real 
capabilities, as I mentioned previously. The specs apparently show the 
most sensitive range as 1 pA FS, so all stuff below E-12 A depends on 
those digits to resolve. The manual recommends that extremely small 
currents below 1 fA (the third digit down) be measured in charge mode. I 
think this is to compensate for bias current and to average out some of 
the 1/f noise.


The 640 on the other hand, can apparently reach 1 fA FS (1000x lower) 
with 100 aA p-p (+/- 5%) noise on analog readout. Given a choice between 
the two, I think I'd pick the 640, and hook a DVM to the output, and 
average a whole bunch if necessary. I think the 640 uses a superior 
front-end technology that maybe could be even further improved in the 
middle and back end, while the 642 probably is as good as it can ever be 
already.


Ed

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Re: [volt-nuts] Bohnenberger electrometer

[ed breya]

For static bias, look up "electret" for ideas on some other possible 
options.


I would recommend against your option 2 capacitor - that's a dangerous 
amount of energy to store in something that may be fooled around with 
experimentally. Also, even though it's a lot of C, being electrolytic, 
the charge will eventually leak off anyway - probably faster than any 
charge loss from using the machine.


The option 2 (2 nF at 4.2 kV) seems more appropriate for this use, 
because of the much higher sensitivity attainable. It's charge will leak 
off too, but since it's likely a plastic or oil capacitor, the retention 
time will hopefully be OK overall.


I wouldn't want to take a jolt from either one. In the ultimate design, 
be sure to use some sort of series current limiting resistance to 
isolate the capacitor from the outside world. The R can be quite high 
(megohms, and of course suitable for the maximum voltage) since not much 
current is needed for operation, so the contact/fault hazard would be 
reduced from dangerous to a tingle. It would be good to also have a safe 
discharging method - another R - that can be switched or jammed in, to 
quickly clear the charge for safe keeping when not in use, or during 
design.


In the old days, optical methods were used for "gain," as in a mirror 
galvanometer, for instance. Putting some simple magnification and 
illumination (sun light if electricity is a no-no) in the system can 
increase the visibility of any deflection.


Lastly, regarding capacitors, a good option if available, is to use the 
nice HV oil caps that can be salvaged from older-era (before they went 
to switching supplies) microwave ovens. These are typically rated around 
1 uF, 2 kV AC. Two in series would do for up to 4-5 kV service. Since 
you don't want bleeder/balancing Rs in this application, it would be 
best to use identical caps, or slightly more complicated charging 
circuitry. They can bought new, but may be pretty spendy, depending on 
the project budget. I have dozens of them - saved from every microwave 
oven I've junked out over the years.


At 1 uF, these would have much better retention time, with hazard energy 
between the original options.


Ed

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Re: [volt-nuts] Bohnenberger electrometer

[Brooke Clarke]

Hi Dave:

Here's a free on line book "Magnetism and Electricity", 1877
https://books.google.com/books?id=y45PYAAJ=PA169#v=onepage=false
Chapter 6 Electroscopes and Electrometers starts on book page 74 (pdf pg 81)
but . .
Chapter 11 Voltaic, Dynamical or Current Electricity is where paragraph 214 Bohnenberger's Electroscope appears on book 
pg 169 (pdf 176).

This is the chapter for the Voltaic Pile so that's probably why since it's a 
way to testing polarity.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html

 Original Message 

Bohnenberger electrometer


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[volt-nuts] Bohnenberger electrometer

[Dr. David Kirkby]

Sorry this is not precision voltage measurement, but it is not unrelated.

As a radio club project, we are building a simple electroscope, with no
active components. The gold leave variety would work, but two bits of
alluminum foil do too.

My plan was to go one better, and build a Bohnenberger electrometer. Does
anyone here have experience of building a Bohnenberger electrometer, or
know much about them? They are similar to the gold-leaf electroscope, but
have one leaf sitting in an electric field. So the leaf moves one way or
the other, depending on the polarity of the charge. Or some other
electrometer, that does not use any active components - no ICs or
transistors.

I believe the original design was built with some sort of battery, but my
intention was to use a capacitor and charge it up. I'm not sure of what
sort of electric field / energy / stored charge is required though. I have
two obvious options for a voltage source based on what I can find at home.

1) 2 nF 15 kV capacitor which I can charge to about 4.2 kV easily (Q = 5.4
uC)
2) 2200 uF 400 V electrolytic capacitor which I can charge to 400 V (Q=0.8
C)

Clearly capacitor 2 stores a lot more charge, but for any given spacing of
plates, capacitor 1 creates am electric field 10 times higher than
capacitor 2.

Obviously if gold leaf or aluminum foil moves, that takes energy. Will that
come from the charge one puts on the top of the electroscope, or will some
come from the capacitor, so discharging the capacitor? If the latter,
capacitor 1 might discharge quite quickly due to its small capacitance,
whereas 2200 uF of capacitor 2 would not.

I do not wish the unit to be mains operated. I don't mind charging the
capacitor from the mains, but I want it to be standalone, independent of
any supply voltage.

Dave
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