A 'sticky' problem...

Discussion in 'General Electronics Chat' started by Hypatia's Protege, Jun 8, 2015.

  1. Hypatia's Protege

    Thread Starter Distinguished Member

    Mar 1, 2015
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    Kind Friends...

    In obviation of liability to failures corollary to unnecessary ‘admixture’ of EHT and semiconductors, I’ve constructed an ultra-simple, ‘bullet proof’, passive EHT indicator of a D'Arsonval movement (100uA FS), a dropping resistor and a variable shunt/calibration resistor.

    Please see file attached (immediately) below for schematic and typical application:
    [​IMG]

    Inasmuch as it must measure a (pure DC) potential difference between two terminals having no ‘ground reference’ whatever, the unit is assembled in a case/chassis which being insulated to at least 200kV from its environs (via CPVC ‘stilts’) – such that the entire assembly is ‘floating’ --- Note: the return lead is ‘ohmically bonded' to the chassis…

    The difficulty presents as ‘sticky’ and otherwise erratic and inaccurate indicator performance at EMFs much above 30kV --- Please note that, under similar conditions, said difficulty is likewise manifest in other movements and, indeed, active digital instruments (albeit, in the latter case, with different ‘symptoms’ such as ‘beeping’ ‘glitching’ erratic display, etc...)

    While it is clear that said undesirable operation owes to corona effects, the best solution would appear elusive?:confused:

    Although, for instance, ‘flooding’ the instrument with ionized air or intense ionizing radiation during operation improves performance (via, I can only suppose, neutralization/prevention of charge formation upon the movement’s dielectric surfaces) – such is both less than satisfactory and cumbersome. Similarly (although untried at this time) Faraday shielding of the unit via wire mesh, metalized/conductive glass, etc. might be expected to eliminate the difficulty altogether (via preclusion of coronal discharge from movement surfaces) -- but at the cost of impaired scale visibility (a significant 'handicap' considering good radiological/electrical safety practice often requires maintenance of several meters viewing distance).

    For clarification here’s an image of the prototype (Yes! I know! ‘Tis a bit ‘Science Fair’:eek::oops:– but'cha know, it’s not exactly a precision instrument and there really are just so many hours on the clock!:rolleyes:
    [​IMG]

    Seriously! -- I can’t ‘shake’ the feeling that I’m missing an obvious solution?
    In any event, all insight, feedback and/or suggestions will be greatly appreciated!:):D

    Best regards
    HP

    PS – Issues I’ve already considered:
    1) The return lead’s connection to the much greater surface area of the chassis sets up imbalanced loading of the PSU…
    >>Conclusion: – this in not important inasmuch as all currents involved are pure DC –AND– The PSU supplies sufficient current (60Ma @100kV) to render ‘corona loading’ insignificant…

    2) The return lead is insulated to only 40kV
    >>Conclusion: I don’t ‘count on’ the return lead’s insulation -- during operation said lead is appropriately ‘suspended’ by PTFE supports/standoffs.

    3) Perhaps the movement is defective…
    >>Conclusion: – The movement operates smoothly through its entire range when ‘floating’ < 30kV above Earth ground… --- Also, as stated above, the difficulty is present with other movements and technologies…

    Please feel free to challenge any of these 'conclusions' should they appear 'thin':D

    Again – Many advance thanks!:):):)
     
    Last edited: Jun 8, 2015
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    I encountered a similar difficulty a few years back, not with a meter movement but with a milligram-level weigh scale that was delivering erratic readings and drifting seemingly at random. After much hair-pulling, gnashing of teeth and fiddling, the cause was found: the sample pan platen of the scale (made of plastic) was being sometimes attracted, sometimes repelled, by electrostatic charges on the surface of the instrument's dust cover, which was made of polystyrene.

    The problem was solved by a generous dose of anti-static spray (I used Pledge Electronics Cleaning and Dusting Spray) to the outside surface of the instrument, the weigh pan platen, and both the inside and outside surface of the dust cover. After that, I had no more problems with erratic weight readings.

    It's possible you might get some benefit from similarly treating the chassis of your meter and the outside surface of the meter movement's window. In extremis, you could also disassemble the meter movement, treat the inside surface of the window, and re-assemble it.
     
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  3. Hypatia's Protege

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    Thanks for the reply!:)
    The chassis/case is fashioned of metal so that base is covered:D

    Good idea! That may indeed solve the charge/electret-formation difficulty!:) --- howbeit I suspect that the heavy corona discharge is likely problematic as well --- to date I have been unable to find reasonably transparent, thermally stable, conductive material with which to shield the movement...:(

    Best regards
    HP
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    OK, I'll ask - why do you need to measure 100 kV in your basement? I have a lot of weird stuff, but...

    ak
     
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  5. Gdrumm

    Distinguished Member

    Aug 29, 2008
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    I haven't a clue, but are CPVC stilts able to conduct any type of electrostatic discharge / pickup?
     
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  6. Hypatia's Protege

    Thread Starter Distinguished Member

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    Thanks for your interest!:)

    My work with radiography equipment requires design/construction of EHT anode supplies, and, hence, 'metrics'...
    Moreover such a device is useful for periodic calibration checks of my Keithley/Fluke 35080B (spectrally calculated) 'KVP divider'

    Don't know that I'd want to try it in my cellar 'tho!? Blub, blub, blub:eek::eek::eek:

    Best regards
    HP
     
    Last edited: Jun 8, 2015
  7. Hypatia's Protege

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    Thanks for your reply!:)

    Inasmuch as PVC and CPVC tubing is commonly employed as winding support/dress insulators in Tesla coil assemblies I expect it is possessed of highly favorable dielectric properties ...

    Best regards
    HP
     
  8. Hypatia's Protege

    Thread Starter Distinguished Member

    Mar 1, 2015
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    First off, thanks for your replies!:)

    Upon consideration of this matter I see that it may be adviseable to 'put a finer point' upon my inquiry...

    So... Let's make it this:

    Does anyone know of a source for, or fabrication technique of, transparent, electrically conductive glass or fused silica? --- Note: polymers are inapplicable owing to their low melting/pyrolysis temperatures and poor thermal conductivity (the material must withstand corona discharge sans formation of refractive anomalies, opacities or pitting)

    Many thanks!:)
    HP
     
  9. AnalogKid

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  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    But for many years Tesla coil builders also used varnished or waxed cardboard tubes for that. I know that Van de Graaff generator builder don't recommend using PVC or CPVC for the towers.

    Have you looked over at McMaster-Carr?
    http://www.mcmaster.com/#glass-stock/=xjs1bu
    http://www.mcmaster.com/#standard-borosilicate-glass/=xjs2om
    http://www.mcmaster.com/#silica-glass/=xjs364
     
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  11. Hypatia's Protege

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    Thank you! I'll check that out!:)

    Avoidance of excessive EMF gradients requires EHT divider (dropping) resistors to be 'lengthy' -- for instance, in the instrument in question, I use two series connected 400M resistors (measuring 20cm in length each) under dielectric oil (Spec. Shell Diala AX) -- said assembly contained in the PVC tube atop the pictured instrument...

    Thanks for your response!!!:)

    Best Regards
    HP
     
    Last edited: Jul 8, 2015
  12. Hypatia's Protege

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    Although I had no problems with it during my 'coiling days' (daze?;)) It seems some had difficulty with metal filings in certain 'thick walled' PVC extrusions --- 'twas the 'Imp of The Off Label App' come a callin'!:rolleyes:;):)

    Thanks for the links! Will check them out at earliest convenience:):):)
    Best regards
    HP
     
  13. shortbus

    AAC Fanatic!

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    There are other shapes and type of glass on the site, just picked a few that seemed like they would work.
     
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  14. Aleph(0)

    Member

    Mar 14, 2015
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    Why not tell them the truth that you are the radiation bug:D Funny how it is that a retired surgeon is a frustrated radiologist:D obtw a sump pump cheaper than the linear amp tube:p
     
  15. Hypatia's Protege

    Thread Starter Distinguished Member

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    Well... I'm bound to say: "Ionizing radiation enthusiast" fits me like the proverbial glove!:D:D:D

    Indeed, I have several (pumps that is) but a scant 10,000 square feet of cellars is no match for 32,000 square miles of water -- under any conditions!:eek::eek::eek:

    Best regards
    HP
     
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