[Prev][Next][Index][Thread]

Re: Primary Qs





On Wed, 2 Oct 1996, Tesla List <tesla-at-poodle.pupman-dot-com> wrote:
>> Subject: Re: Primary Qs
>
>>From hullr-at-whitlock-dot-comWed Oct  2 22:43:19 1996
>Date: Wed, 02 Oct 1996 12:10:31 -0700
>From: Richard Hull <hullr-at-whitlock-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: Primary Qs
>
>Tesla List wrote:
>> 
>> >From MALCOLM-at-directorate.wnp.ac.nzTue Oct  1 21:55:47 1996
>> Date: Wed, 2 Oct 1996 09:03:02 +1200
>> From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>> To: tesla-at-pupman-dot-com
>> Subject: Re: Primary Qs
>> 
>> Greg,
>>       Thankyou for this info....
>> 
>> > > Q does trend up with increased gap current showing a drop in gap
>> > > resistance. It would be useful to get readings for much higher
>> > > voltages but the supply and caps on hand wouldn't allow it.
>> >
>> > This observation would agree with the data from 'Gaseous Conductors',
>> > where the author noted that the current *density* in an arc remains
>> > relatively constant as the gap current is varied.  This would mean
>> > that if the gap current were doubled, then the arc cross-section
>> > would roughly double, cutting the resistance by half.
>> 
>>      I wonder if the current density then relates to the number of
>> potential current carriers (npi) present (e.g. gas molecules) in a
>> given cross-sectional area? I wonder if your pressurized gap at 3 atm
>> (?) would allow a much higher current density than we get at normal
>> air pressure? Also, would blowing compressed air through the gap have
>> a similar effect? If this were true, it would seem that pressurized
>> gaps are more efficient than normal air pressure. What do you think?
>> Anybody measured this?
>> 
>> Malcolm
>> <snip> point noted thanks. For the normal configuration of spark gap
>> in my new machine I'm looking at using about 200uH primary with 50nF
>> at 20kV peak. Those CP caps are looking wonderful right now. Thanks
>> to Scott for organizing the purchase of them.
>
>Malcolm,
>
>All your logic seems correct about doubling the current carriers in the 
>arc at higher pressures.  I do know that at higher pressures the gap 
>width must decrease per unit voltage over atmosphere.  These generalities 
>are pretty good, but not precise.  Equations are around that supposedly 
>allow this breakdown to be computed. (in short conrolled gaps at fixed 
>pressure.)  I have been reading a number of core level treatises on 
>gaseous conduction. (and a few texts).  It seems everyone has the answer 
>and they are all somewhat different.  I have become more and more leary 
>of "the final word on the matter" theories about just what happens in a 
>gap at break down.  The gross picture is relatively well understood.  It 
>is the actual minutia that remains a mystery.
>
>Richard Hull, TCBOR
>
>
>We manufacture a line of gas filled spark gaps, and it has been my job for the 
past few years to characterize them.  Static Breakdown voltages are easily 
predicted by use of Paschen's equation which relates the breakdown voltage to 
pressure and gap spacing.  

Vb = A*P*d / ( B*Ln(P*d))

Where P = Pressure, d = Gap Spacing

A & B are constants that depend on the gas used and to a lesser degree on the 
electrode material.  I have computed the coefficients A & B from data with all 
of the common gases and many mixtures.  

Since Paschen's law is basically a statement of continuity ( The number of 
electrons leaving the anode must equal the number injected into the gap at the 
cathode.) there is a time delay between the application of voltage and gap 
breakdown.   The breakdown voltage increases with the rate of rise of the 
applied voltage.  The ratio of the static to the dynamic breakdown voltage is 
called the Surge Ratio.  

The Townsend breakdown theory from which Paschen's equation derives, assumes 
that the breakdown process starts with a single electron-ion pair.  When a high 
frequency voltage is applied to a gap as with Tesla coil operation, there is 
insufficient time for all of the charges in the gap to dissipate before the 
voltage is reapplied. The charge that lingers longest is on the insulating 
walls.  If the voltage across a gap is monitored, it is seen to decrease from 
the initial dynamic value to a lower steady state value, and as the frequency is 
increased a value will be reached where the gap conducts continuously.  
Fortunately this frequency is many megahertz for most gap designs.  

I would like to see some data showing breakdown voltage at 100 to 300 kHz as a 
function of pressure for some spark gaps.