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Re: DC TC

In a message dated 97-02-17 03:38:19 EST, you write:
<<snip 
> You can use a DC supply to power a Tesla coil, but you 
> have to be very careful and make sure you use a diode or diode array that
> is capable of handling the full, worst-case reverse voltage. And 
> believe me, it is higher than most people think! A 12KV transformer
> will charge the capacitor to about 16.8KV Peak. When the AC reverses
> polarity the voltage across the diode goes to 33.6KV. Stress factors
> like this represent the *minimum* that you should expect. These peak
> values will occur up to once each cycle of the main AC frequency (60Hz).
> Always multiply such values by at least 1.5 (and preferably by 2) to
>arrive at a decent safety margin. I would use a 50KV diode as a minimum
> rating for use with a 12KV transformer. In my saner moments I would
> want a rating of at least 66KV for the diode. 
 
> The current rating should be at least 1 amp if you are using a constant 
> current neon transformer rated from 30 to 120 ma. Again the motto is
> "better Too Big than Too Bad!"


Tom, All,

I built a filtered DC powered disruptive Tesla coil which gave 62" sparks.  I
used 1N4007 diode series strings to form a full wave bridge, these diodes are
rated at 1kV, 1amp.   Fourteen diodes were used in each "leg" of the bridge,
for 56 diodes in total.  Of course, since two of the legs in a bridge
rectifier are always in series across the transformer, this "helps" the
voltage rating.  I used a 7.2kV, 1.5kVA potential transformer to power the
system.   A current limiting inductor was used in the primary circuit to
limit the input current, and the input voltage was controlled by a 17A, 140V
variac.  

The rotary was a 120 BPS synchronous gap.  Large chokes were used between the
filter capacitor outputs and the spark gap input to limit the destructive
currents that would have otherwise flowed.  It was important to have the
correct amount of inductance here, for best resonant charging results.  I
used the secondaries of microwave oven transformers to form the chokes--4 of
them.

A protective, RF bypass capacitor  (.022uF, 60kVDC) was installed across the
output of the power supply to protect it from kickback.  Two RF chokes were
also used for protective purposes.  No failures of any kind occured in the
system.

The system worked well, but was much more complicated to constuct and adjust
than a normal AC powered disruptive TC, and it was also less efficient.
 Surely, losses in the filter caps, diodes, and chokes, helped to trash the
efficiency.  If I remember correctly, the system used about 6kVA to produce
the 62" sparks.  These results were pretty much what I expected.  My purpose
in building this coil was only as a prelude to the construction of a pulsed,
or "staccato" disruptive system.

I would not recommend to anyone that they build a steady running DC powered
disruptive TC, unless they just want to build it for the novelty or for
research purposes.  (And there's plenty of danger from those filter caps
too!)

The DC pulsed, or staccato disruptive system is another story.  By pulsing
the DC coil, "on" a few times each second, much better "efficiency" can be
obtained.  I wrote efficiency in quotes because it is not true efficiency in
the true engineering sense that is being gained here, but rather the
practical amateur's coiling definition:  "Input power versus spark length",
which is a useful way to consider or compare coils.  By pulsing the coil, the
filter caps provide an energy storage function, and allow for the use of a
smaller input power transformer than would ordinarily be required for a given
spark length.  Using this system, I was able to obtain 54" sparks using only
about 450 watts. The sparks are not continuous in this system, a visible
pulsing is seen by eye.  Most likely, the efficiency could have been even
greater since I really didn't spend a lot of time optimizing the system.  I
wrote an article about this technique in the Vol. 15, #4 issue of TCBA News.
   

John Freau



> You should use Chokes to protect the diode/transformer.
 
> The "adjustable charge rate" is not necessary. The Spark gap takes care of 
> delivering the current at the proper moment, which is whenever the actual
> voltage reaches the Spark gap breakdown voltage.
 
> The frequency of oscillation is determined by LC, not by any other
> factor.
 
> Unless you use a full-wave bridge diode arrangement, you really
> don't gain much operationally. With a full-wave bridge you at 
> least get to have 120 Peaks per second instead of just the usual 60.
 
> What we really need here is to hear from those who have actually built such
> beasts.
> OK, guys, you who have actually *done* all this DC stuff, is it 
> worth it at all? Or is it a big bust???
 
 
 
 >> With a Tesla Coils operating on a DC power supply it seem like the
 >> toroid or sphere on top of the secondary would charge up in one polarity
 >> like a Van De Graff generator.  Would the Tesla Coil produce longer and
 >> more powerful output sparks at resonate frequency from DC?
 > 
 
> No, The Tesla coil will oscillate due to the tank circuit. It will be AC
> out, *not* DC.
 
> > Gary Weaver
 
> Fr. Tom McGahee
  >>