Re: Gap Configurations and DC Excitation Questions...

Tesla List wrote:
> >From bert.hickman-at-aquila-dot-comSun Jul 21 09:46:24 1996
> Date: Sat, 20 Jul 1996 08:58:49 -0700
> From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Gap Configurations and DC Excitation Questions...
> Fellow Coilers,
> While doing some thinking about how a rotary works in conjunction with
> the tank cap, pig, and ballast, some thoughts/questions came to mind:
> 1. It would seem that if one had large enough (>100 mH) secondary RF
> chokes, capable of withstanding the large RF voltage seen on the primary
> cap, it should be possible to run with EITHER the gap or the capacitor
> across the input power source, particularly with the BIL of a pig. Safety
> gaps could still be used for added protection. Has anyone tried this
> configuration recently, and is there ANY advantage over the preferred
> configuration (i.e., gap shunting the supply side) that would motivate
> its usage? In particular, would the gap be any easier to extinguish? Is
> this a blind alley?
> 2. A while back, there was a description of a large coil which used a
> large DC source coupled with a comparatively large energy storage
> capacitor. A variable "charging" inductor was connected from this source
> to the classical spark-gap/tank combination. When the gap fired, the
> inductance of the charging inductor limited the rate of rise of current
> from the large supply capacitor into the gap, allowing the gap to be
> extinguished. The DC source and charging inductor rapidly recharged the
> tank cap (at a rate that could be significantly greater than that
> available at 50/60 Hz). This approach looks like it had a number of
> significant advantages:
>   - Maximum energy could be controlled via adjustment of the maximum DC
>     voltage and the amount of "charging" inductance
>  ** Independence from 60/50 Hz supply and rotary speed interactions
>   - ability to "tune" the system via rotary speed and charging
>     inductance settings.
>   - elimination of the "dead-time" around the 50/60 Hz zero crossings in
>     AC excited systems.
>   - improved power factor on the input power side to the pig
>   - Elimination of the primary ballast, since the secondary charging
>     choke and rotary should limit "short circuit" current
>   - Relatively inexpensive banked PFC, Filter, oil or other caps could
>     be used for the energy storage cap since it would not be seeing high
>     RF currents
> The obvious downside is added complexity and the need to be CNSIDERABLY
> more careful in discharging the caps!.
> However, the ability to set break-rates independent of phasing to the
> 50/60 Hz supply, at to operate with consistent/controlled pulsing at
> rates significantly higher than with AC excitation, may make it
> worthwhile.
> Any thoughts from those with more coiling experience?? Thanks in advance,
> and safe coilin' to ya!
> -- Bert --


Given a solid beefy, non-destructable DC supply source.  There are zero 
(0) disadvantage to using DC power in the Tesla system.  It is the number 
one and best power scource for any Tesla coil-- PERIOD!!!  All of the 
advantages you stated and a number of others are to be had from DC 
operation of a coil.

The rub is that first silly sentence!  I wish like hell someone would 
give me a non-destructable DC power source!  The best silcon rectifiers 
made woun't really standup to making the supply mentioned without many 
expensive heroics in the design of a DC power supply.

If I were to go all out,  I would use a number of 866A mercury vapor 
rectifier tubes in a voltage doubler circuit fed by a pole pig.  I would 
variac the pig for DC voltage control.  You would need a very large pair 
of filter caps at high voltage. Also needed, would be two Aircraft grade 
avionics heavy chokes with 20KV insulation.  The filament transformers 
for the tubes would have to have a minimum of 10kv insulation primary to 
secondary.  All these items are to be had at a hamfest only!!  Otherwise, 
they would be well out of the range of most peoples budgets.  This 
finished supply could probably supply just under 10KW DC for coil work.  
I still hold out hopes of building just such a system.

Richard Hull, TCBOR