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Re: Series vs parallel R's with chokes


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"