[Prev][Next][Index][Thread]
Re: Series vs parallel R's with chokes
From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: bert.hickman-at-aquila-dot-com
Sent: Thursday, September 04, 1997 9:08 AM
To: Tesla List
Subject: Re: Series vs parallel R's with chokes
Tesla List wrote:
>
> From: Gary Lau 03-Sep-1997 1118[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent: Wednesday, September 03, 1997 10:26 AM
> To: tesla-at-pupman-dot-com
> Cc: lau-at-hdecad.ENET.dec-dot-com
> Subject: Re: Series vs parallel R's with chokes
>
> >> From: Gary Lau [SMTP:lau-at-hdecad.ENET.dec-dot-com]
> >>... <describing a problem where tank circuit arcs excessively to RF ground
> >>when series resistors are used with chokes, but not when resistors are in
> >>parallel with chokes>...
>
> >From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
> >Subject: Re: Series vs parallel R's with chokes
> >Gary,
> >
> >This definately doesn't sound right, especially with only a 15 KV 30 MA
> >source - something else is probably going on here. How many total turns
> >on your primary, and what turn are you tapped at? Is the "fixed" end of
> >your primary at the innermost or outermost turn? Autotransformer action
> >can elevate the "free" end of the primary (after the tap point) to quite
> >high RF voltages, and is one reason to make the "fixed" end the
> >innermost turn. Are your safety gaps firing at all? Finally, are your
> >neon center-tap (case), safety gap ground, resonator base, and
> >strike-rail all firmly tied to a good RF ground?
> >
> >-- Bert --
>
> My primary has 14.5 turns total, tapped (capacitor connection) at turn 12.
> 10 x .030" RQ static gap between innermost turn and cap. I have ~.25"
> safety gaps on transformer and ~1" safety gap on cap. Have settled upon
> 10 gap segments based upon seeing cap and xfmr gaps occasionally firing
> at this setting. Xfmr case, fan housing, bypass caps, safety gaps,
> secondary base all tied together, tied to 8' ground rod via 15' x 2"
> copper flashing.
>
> With R's in parallel, things run pretty nicely, getting 24" blue/violet
> sparks, so I'm sure I'm in tune.
>
> My two chokes are 2.38" x 6.75" of 30AWG, side-by-side, seperated by
> about 4" surface-surface. Don't know if cross coupling matters. I'll
> try and make some measurements of self-resonant frequencies.
>
> Gary Lau
> Waltham, MA
>
> FWIW, here's my coil summary:
>
> 15 Amp Variac, 30 Amp RFI filter, 48 uF 400V PFC caps, MOV transient suppressor
>
> HV Xfmr:
> 15KV 30 MA neon sign transformer
>
> HV Xfmr protection network:
> Per HV Xfmr lead to RF Gnd: - ~600 pF capacitor constructed of .09" copper-clad
> G10 circuit board material. Vaseline for corona
> surpression (Yuk!)
> - Safety gap using 3/8" brass balls (lamp parts!)
> Per HV Xfmr lead in series: - Air-wound solenoid RFC, 30AWG x 2.38"D x 6.75"L
> 9 mH inductance in parallel w/ 3K/15W resistor
>
> Spark gap:
> 10 gaps .030" each between 14 1.25"D x 1.90"L copper pipes, cooled & quenched by
> airflow directed at gaps (2 x 1/4" air holes in PVC housing per gap) provided by
> a 4.5" fan.
>
> Capacitor:
> .0066 uF rolled poly/Al/oil, 2 layers .040" (.080" total) LDPE dielectric.
>
> Primary coil:
> 14 turn flat spiral 1/4" copper tubing, 1/4" gap between turns,
> 6.25" ID, 20" OD. Best operation at turn #12 with toroid, 300 KHz.
>
> Tank circuit wiring:
> .040" thk x 1/2"W copper strap
>
> Secondary coil:
> 4.25"D x 23.25"L 22 AWG wound on 1/16" wall dried/sealed PVC drain pipe. Many
> layers polyurethane. Low voltage self-resonant with no topload at 489 KHz.
>
> Discharge terminal:
> 4.5" x 15.5" toroid made from aluminum flex duct, stiffened and smoothed with
> 2-part wood filler and spakel, covered with aluminum tape.
>
> Ground system:
> One 8' ground rod in earth soaked with water, connected with 15'L x 2"W copper
> flashing ground strap.
>
> Performance to date:
> Max sparks to grounded target: 24"
Gary,
Thanks for the details - sounds like a very nice system! Everything
above seems to check out OK, except that the clearance between your
secondary and the innermost turn of your primary appears to be only
about 1 inch. This is a little bit tight - you may want to add some
sheets of LDPE to reduce the probability of getting flashovers. Also, I
get about 6.5 mH when I calculate the HV choke inductance instead of
9mH. A pair of these chokes in series with the pair of 600 pF bypass
caps results in an effective resonant frequency of about 81 kHz with no
damping resistors, which is comfortably lower than your operating
frequency. Adding damping resistance of two 3k/50W resistors should
knock down the Q of the filter circuit down to about 2 - which is fine!
BTW, I'm still not sure exactly what you meant in your earlier post when
you said you were seeing "terrible (2") arcing between my tank circuit
and ground (like the bottom of my secondary and the gap cooling fan)".
If you're seeing arcing to the fan in your RQ gap, you may also need to
beef up the insulation between your muffin fan and the gap electrodes.
HV RF will "track" for a much longer distance along the air-dielectric
interface, and this effect becomes worse with increasing dielectric
constant or with humidity in the case of PVC (k = 4.5 - 6).
-- Bert --