RE: Awsome first light !! but...

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Ed:

To your question about current limiting my primary - The primary coil IS the
current limiting component.  Whenever the main gap fires, the tank current
is limited by the primary inductance.  Discharging a cap through a dead
short and through a primary coil are two very different experiences for a
cap.  In one case it is inductively limited, in the other it's limited only
by the arc resistance and ESR & ESL of the cap.

I have no direct experience with capacitor death due to over-currents but
it's not hard to see that the current pulse will be tremendously greater for
the case without current limiting.  Since over-current is a failure mode for
pulse caps, it seems prudent to soften the blow.

As an addendum to my original post, I probably should have pointed out that
I was speaking mostly for the case of using static gaps.  When running an
RSG as is the case with you, the RSG alone will NOT limit the maximum cap
voltage.  All RSG's should have a static safety gap in parallel with them,
and this will then limit the maximum cap voltage, AND it will be inductively
current-limited.

Regards, Gary Lau
MA, USA


Original poster: "Edward Wingate by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<ewing7-at-rochester.rr-dot-com>

Tesla list wrote:
> 
> Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Gary.Lau-at-compaq-dot-com>
> 
> While your safety gap may have been set for 12kV, what was your main gap
set
> to?  It sounds like your main gap was set to something higher and allowed
> the safety to fire.
> 
> As a side issue, it's generally not recommended to use a safety gap alone
> across your cap.  With no safety gap current-limiting, the discharge
current
> could potentially be bad for your cap.  Also, it's impossible for the cap
> voltage to ever be any higher than the voltage across your main gap.
> Whatever your main gap is set to will also limit the cap voltage, and the
> cap-gap is redundant.
> 
> Gary Lau
> MA, USA

Gary,

Do you current limit your primary to keep the discharge current from
damaging your cap?

I have NEVER seen a cap destroyed by a safety gap yet. I HAVE however
seen many caps go to the great beyond for the lack of a safety gap!

The first question I ask someone who has just blown a capacitor is "were
you running a safety gap on the cap?", and almost invariably the answer
is no. I personally will use a safety gap every time!

 

Ed Wingate RATCB