Re: Spark Gap Sustaining Current (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 27 Sep 2007 23:16:14 -0500
From: Crispy <crispy@xxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Spark Gap Sustaining Current (fwd)

The specific situation that I'm looking at is having a large capacitor
that is being charged and that has a spark gap across it (along with
something to limit current).  As soon as the voltage across the
capacitor reaches peak, the gap will fire, but the voltage won't drop to
0 all at once, because there is something to limit current in series
with the spark gap.  This "something" is similar to an ARSG, where
current flow will not be continuous.  However, it is different in that
it will never short out the supply.  As this ARSG-type device turns, the
large capacitor discussed above will gradually discharge and lose
voltage.  However, once the arc in the first spark gap ignites at the
gap's breakdown voltage, I want it to stay ignited until the voltage
across the large capacitor drops beneath some value (which doesn't have
to be exact).  My concern is that, as soon as the arc forms for the
first firing, I think that the arc would extinguish as soon as the
current crossed zero (at the first rotation of the "ARSG").  I don't
want this to happen - I want the arc to remain conductive for a number
of "ARSG" cycles.  I thought that I could do this by keeping a constant
minimum amount of current flowing through the gap independently of the
"ARSG" by connecting a large-valued resistor across one terminal of the
spark gap and the other terminal of the large capacitor, such that
current would continue flowing (but decreased as capacitor voltage
decreased) through the gap after it initially fired.  My question
essentially related to the practicality of this.  It should work, but
I'm wondering how large said resistor would have to be, and therefore
how much current is required to be flowing through the gap to maintain
the arc, and therefore how much static power is lost in doing so.

Thanks,
Chris


On Thu, 2007-09-27 at 20:41 -0600, Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Fri, 28 Sep 2007 08:31:52 +0800
> From: Peter Terren <pterren@xxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Spark Gap Sustaining Current (fwd)
> 
> If you supply current continuously then the spark will be continuous. 
> However, if it has a significant capacitance with an actual capacitor or 
> simply large enough electrodes (scch as a TC toroid) then then the spark 
> will discharge the capacitor and the voltage will drop. The spark will 
> extinguish in sub-microsecond times and will only spark again when the 
> capacitance recharges. This type of spark is bright white, noisy and single.
> http://tesladownunder.com/HVBrokenSparksRightPos1.jpg
> Consider a supply terminated by a resistor or inductor. Then when a spark 
> forms, the current will not be drained to near zero as the current is 
> limited and voltage remains present. The spark can be continuous if the 
> voltage supply is able to supply this current. This type of spark is a soft 
> orange glow, quiet and continuous.
> http://tesladownunder.com/Flybackbentarc.jpg
> The parameters differentiating the two include the current available 
> continuously, voltage, capacitance, resistance and inductance.
> If you try to have a continuous spark with very little current then even the 
> small capacitance of a length of wire may become significant and make the 
> sparks intermittent rather than continuous.  Alternatively, corona will draw 
> enough current to drain the voltage so no spark forms.
> Peter
> 
> 
> > ---------- Forwarded message ----------
> > Date: Thu, 27 Sep 2007 12:55:46 -0500
> > From: Crispy <crispy@xxxxxxxxxxx>
> > To: tesla@xxxxxxxxxx
> > Subject: Spark Gap Sustaining Current
> >
> > Hello,
> >
> > I have a quick question about spark gaps.  How much current is required
> > to sustain an established arc in a spark gap in "dead air"?  Let's say
> > that there are two tungsten contacts about an inch apart, and an arc is
> > ignited by a voltage of a little over 20kV.  Say that the ambient
> > temperature is room temperature and that there is not significant
> > airflow through the gap other than that which is generate by the gap's
> > heat itself.  Is the sustainability of the arc purely a function of
> > current through it?  If so, in such a theoretical gap, how much
> > sustaining current would normally be required?
> >
> > Thanks,
> > Chris
> 
> 
>