RE: DC power again

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>


Norman:

Some of the other responses were based on AC systems.  My comments are
only directed toward a DC powered unit which I believe is what your
question is about.  You indicated your power transformer is 13 kVdc @
300 uA thus only 3.9 watts.  You are correct that you will be restricted
to a small break rate.  In DC powered systems the break rate is limited
by the power supply/charging rate.  Max BPS = Power/(2*C*V^2) where C =
Tank Cap and V = DC supply voltage(Be careful with this equation - it
assumes you have a properly sized charging reactor in the circuit).
This is obviously how you came up with 5 BPS by plugging in 10 nF and
6kvdc.

The problem is that the coil that matches your 10 nF tank capacitor will
need a much larger power supply than what you have.  At 10 nF I'll
venture to guess that your secondary is about 1-5/8" dia and your
looking for a resonant frequency of around 500Khz.  Such a coil will
require hundreds of watts thus getting your break rate well over 100
BPS.  I have a 2" and 4.5" coil which operate with break rates over 400
BPS.  My power supply is 9kVdc and 250 mA thus 2 KW.

I also built a miniature coil(5/8" dia) that draws about 18 watts.  For
a small coil like this I would need a charging reactor in excess of
30H!!  I run without the charging reactor because I haven't figured out
how to build one without spending an inordinate amount of time.  The
tank cap for this small coil is 1 nF.  Without the charging reactor
optimum break rate is around 22 BPM!!(ie 0.4 BPS).  This small coil
yields only 2-3" streamers(note inches! Not feet).  It is battery
powered and was built only so that we could take it to school and demo
for my kids classroom.  They wanted me to bring in the big coil, but I
was concerned about the safety issues - duh!!

Richie Burnett's site covers the formulas/theory for DC Resonant
Charging in depth.

Mark


-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Friday, January 06, 2006 9:17 PM
To: tesla@xxxxxxxxxx
Subject: DC power again


Original poster: norman@xxxxxxxxxxxxxxxxxxxxxxxxx

If a DC power source is used to power a Tesla coil the firing rate of
the spark gap will depend on the charging time of the cap, rather than
be nested around the line frequency.  Thus, when a DC source is used,
the size of the cap can be traded off against the number of sparks per
second . (I am assuming that the firing rate of the spark gap is set
much lower than the line frequency so that the effect of the ripple in
the DC current can be ignored.

 >From what I have read a larger cap in a properly designed Tesla
coil will yield
a higher voltage and thus possibly a longer spark.  If I increase the
cap while also suffering a decline in repetition rate will the sparks
actually
get longer?
  Is a high rep rate necessary for long streamers?

I have an old Xerox machine transformer, 13kVDC @ 300uA.  With a 0.01uF
cap charged to 6kV, the rep rate will be only about 5 sparks per second.
Will this work?

-