Re: Triggered spark gaps for coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Gary and all,

Hmmm... I was wondering why you just outbid me on that pulse transformer...
:^)

I think it makes great sense! There should be no reason that you couldn't
use a three terminal gap (such as a home built or commercial field
distortion gap) to perform this task as long as you provided a flow of gas
through the gap to help cool it and remove ions. In fact, field distortion
gaps were often used to provide rapid pulsed switching for flashlamps to
drive older pulsed excimer lasers prior to their replacement by hollow
anode thyratrons (which will safely handle a degree of oscillatory
bi-directional current flow). For example, older Lumonics pulsed lasers
used field distortion gaps that looked a lot like little Claymore mines
(about 5.5" in diameter, and 3" thick) - every now and then these come up
for sale on Ebay. 

You need to make sure that the middle electrode is centered between the two
main electrodes and that is biased so that its potential is halfway between
the voltage that's applied between the main electrodes (except for when you
want to "fire" it). One way to do this is to use a pair of HV resistors to
create a voltage divider. Triggering is done by coupling a HV pulse (from
an ignition coil or a HV pulse transformer) through a blocking capacitor.
The main gap spacing is adjusted such that the gap will self trigger at a
voltage that's at least 20% higher than the maximum expected voltage from
the HV supply. The triggering voltage needs to be at least half of the
maximum applied gap voltage in order to reliably force breakdown. A LV
pulse capacitor, SCR, and phase shifting circuit operating off fullwave
rectified line voltage can then be used on the LV side of the trigger
transformer to control the trigger point in a manner similar to adjusting a
SRSG via a variac/capacitor phase shifter. Use a fixed font in order to see
the diagram below: 
  
                                        To HV1  
                                          |    
                                          |                || 
                   |----------------------o----------------||-------------
                   |                      |                ||  Ctank      |
                   |                      |                               |
                   |                      |                               |
       _           |                      |                               |
      | |          |              ------------------                      |
    __| |_     R   --/\/\/\/---   |    Main 1      |                      O
       HV                     |    ----------------                       O 
     Trigger        C1 ||     |                                           O
Ltank
         --------------||-----o--------------------- Trigger Electrode    O
                       ||     |                                           O
                              |    ----------------                       O
               R   --/\/\/\/---   |    Main 2      |                      O
                   |              ------------------                      |
                   |                      |                               |
               C1  |                      |                               | 
                   |                      |                               |
                   |                      |                               |
                   |----------------------o--------------------------------
                                          |
                                          | 
                                      To HV2 
 
-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com


Tesla list wrote:
> 
> Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<Gary.Lau-at-compaq-dot-com>
> 
> >From my recent web searches and technical paper readings, there appears to
> be a large variety of commercially available 3-terminal triggered spark
> gaps.  And it seems that these can easily be home-made as well.  These would
> seem to be ideally suited for use as replacements for rotary spark gaps, but
> I've not been able to find any reference to this obvious application.  The
> only reference I've seen towards "amateur" applications is in can and
> quarter shrinkers.  While RSG's do have relative simplicity on their side,
> it seems as though there ought to be some worthwhile benefits to using a
> triggered static gap.  Has anyone attempted this?  Or am I attempting to
> invent a square wheel?
> 
> Gary Lau
> Waltham, MA USA