No that is exactly correct. Another user mentioned that I should be careful
using NST for VDG seemingly because there could be short down in the inside
of the VDG to the HV brush, allowing potentially lethal currents up to the
collector. I've not been able to confirm this exact reason - maybe someone
else can chime in on this.
I was mostly only concerned about the maximum wattage I'd need for
resistors, which I realized I can actually easily calculate by assuming a
short to ground. This gives me the maximum possible current flow through
the resistor network and thus I can calculate maximum wattage.
With this in mind I've discovered my 500mw resistors are not quite beefy
enough, and I'll need to setup some parallel/series chains to get higher
wattage. The NST puts out 30ma and I want to get that down to 10s or
hundreds of microamps for safety.
I did consider using the DMM in the ground path and that seems the safest
option to me. Of course I'll need to float the case away from ground during
this process but it should only be temporary.
Thanks for the hints I'm working on potting my Diodes and resistors now,
and am using generic silicone - I'm wondering if that's going to be
workable inside 1inch electrical PVC conduit or should I switch to HV epoxy
(can't find anywhere).
On Sat, Sep 26, 2015 at 8:06 PM, David Speck <Dave@xxxxxxxxxxxxxxxx> wrote:
> Matt,
>
> OK, I see that with a NST, the center tap of the HV winding is grounded
> internally to the case.
>
> You could monitor your corona current by putting the neon transformer case
> on insulated supports and inserting your DMM in current mode between the
> case and earth ground. I would make sure beforehand that there is high
> isolation (very high resistance) between the NST ground terminal and the
> 120 VAC line input terminals.
>
> What is the purpose of the resistors that you mention? If there is any
> appreciable corona current, then there will be a significant voltage drop
> across the resistors, wasting energy as heat, and reducing the voltage
> available at the corona electrode.
>
> As I see it, the object of the corona is to spray as many electrons as
> possible (or remove as many as possible, depending on the polarity) on to
> the VDG belt. The energy with which you blast them onto the belt should
> have no impact on the ultimate potential of the VDG output. The main thing
> is to have as high a corona current as possible without getting arcing.
>
> It would be simpler to use a Variac in the NST primary circuit to set the
> HV output to the optimum level that doesn't produce frank arcing. Then,
> you wouldn't need any resistors at all in the HV output circuit. Adjusting
> the spacing of the corona electrode relative to the belt may help get
> maximum current without arcing.
>
> What sort of corona electrode are you using?
>
> (Or am I missing something?)
>
> Dave
>
> On 9/25/2015 11:30 PM, msweeney23@xxxxxxxxx wrote:
>
>> Im using a 15KV AC 30ma NST mid-tap grounded NST rectified via two potted
>> 12KV 350ma dioides on either side (4 total to be safe). I have 100 10Mohm
>> 3.7KV 500mw resistors i intend to setup in chains of 5 potted for 50Mohm at
>> approx 17KV each.
>>
>> Im using both sides of the NST to drive two pumped VDG one positive and
>> one negative (5ft tall). Im expecting approx 350KV minumum each and want to
>> study the interaction of the two polarities.
>>
>> Thus far ive only used triboelectric driven VDG so very low amperage in
>> the single microamp range. Im concerned about static behaviour allowing the
>> collector to ground to the lower brush down the inside of the VDG and thus
>> need to ensure current flow is highly restricted.
>>
>> My current 5ft VDG (14 inch diameter collector) has thrown repeated
>> 30inch perpendicular discharges to ground which is totally counter to my
>> expectations. These HV static machines are not at all behaving as expected
>> and others on this list have mentioned strange behaviour to be expected
>> along insulators. However these 30 inch discharges are perpendicular to the
>> VDG through air.. i dont understand this it doesnt match expected
>> potentials from the collector size. Its all a mystery to me and id like to
>> study it while remaining safe (i dont intend to touch these pumped VDGs).
>>
>>
>> My very first VDG doubled the expected discharge length and ive actually
>> witnessed over 40 inch repeated discharges on the 5 foot during very dry
>> conditions (minnesota winter). In some instances ive recieved non visible
>> shocks from 6 feet away. During ideal conditions the VDG constantly popped
>> 12 inch discharges out into thin air exactly 90 degrees apart but ive yet
>> to reproduce this (maybe too humid now).
>>
>>
>> Currently i dropped to a 3.5ft VDG but get lots of leakage from the
>> underside and in the dark i see streamers up from the drive motor.
>>
>>
>> Its all new to me and very mysterious :)
>>
>> ----- Reply message -----
>> From: "David Speck" <Dave@xxxxxxxxxxxxxxxx>
>> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
>> Subject: [TCML] Voltage/power dissapation drop across HV DC current
>> limiting resistors.
>> Date: Fri, Sep 25, 2015 8:02 PM
>>
>> Matt,
>>
>> what kind of HV source are you using to power the corona generator?
>>
>> There must be some place where the low potential end of the HV circuit
>> connects to ground. You could safely insert your DMM running in current
>> mode between the low end of the HV supply and ground to monitor the
>> corona current.
>>
>> More risky, but technically possible for a one time measurement, would
>> be to insert the DMM in series with the high potential end of the
>> circuit -- between the supply and the corona electrode. You would just
>> have to float the DMM and its leads away from any grounds, and be
>> certain not to touch it during operation. A quick and dirty platform
>> made of 4 clean Styrofoam hot cups with a top surface of corrugated
>> cardboard could support the meter and leads on a temporary basis.
>>
>> Dave
>>
>> On 9/25/2015 12:19 PM, msweeney23@xxxxxxxxx wrote:
>>
>>> Im facing this problem and i dont know enough to figure it out.
>>>
>>> If i have a HV source used to produce only corona (bottom brushes on a
>>> VDG) what voltage drop would occur across a resistor in series? Without
>>> this i cant determine expected power dissapation.
>>>
>>> I cant assume the corona source shorts directly to ground (the roller is
>>> grounded) there must be some specific resistance (impedence?) Of the plasma
>>> between the brushes and grounded roller but i really dont know what that
>>> would be without directly measuring it, and i cant do that with my digital
>>> Meter. I could use an analog meter to measure total current flow but at
>>> these voltages im concerned even that could pose problems (approx +/-10KV
>>> DC).
>>>
>>> Any help would be greatly appreciated!
>>>
>>> Thanks,
>>>
>>> Matt Sweeney
>>>
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