DC Coil breakthrough!

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

Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>" <youngs-at-konnections-dot-net>

All DC TC affectionados & interested bystanders,
 
I am the guy who reported great results with a DC SPDT RSG setup.  I am also
the guy who tried to duplicate the SPDT RSG with dual triggered spark gaps with
horrible results.  Many offered ideas to prevent both gaps from firing at once
(with resulting tremendous BANG!), but none seemed very promising.  But I
appreciated all the ideas.
 
So, the idea of copying Greg Leyh's setup seemed the next thing to try.  That
is, couple my DC supply to a normal TC hookup via a charging reactor.  Only a
single RSG would thus be needed, which could eventually be replaced with a
triggered gap.  A diode would be inserted in series with the reactor to permit
variable break rates.
 
Here is the circuit - courtesy of Bert from his recent posting (used fixed font
size):
                Lc     |\ |            || Cp
   ------o---0000000---| :|-----o------||-------- 
 +Vin    |             |/ |     |      ||       | 
         |              D1      |               0
         |                      o  Rotary       0
   Cs  -----                    X  Spark        0 Lp
       -----                    o   Gap         0
         |                      |               0
 -Vin    |                      |               |
   ------o----------------------o----------------         

So, what to use for a charging reactor (Lc) that was cheap?  I thought, why not
use a MOT secondary?  To cut to the bottom line, IT WORKS GREAT!!!  More
efficient than the SPDT setup!!  This is wonderful news!!  For all those who
have a hard time finding NSTs and don't want to mess with a SPDT RSG, consider
going this route.  Here are the details:
 
I measured 9 different MOTs of varying size and output voltage:
volts out    resistance    inductance
1600            110            10.7
1850             54            19.1
1950             90            17.4
1860             73           >20
1700             68            11.6
2100             80           >20
1820             98            10.9
1900             65            9.8
 
As you can see, MOTs are highly variable beasts.  (My Wavetec 27XT won't read
above 20 Henries. )
 
Max BPS = 1 / (Pi * sqrt(L * C), so L = 1 / (C * Pi * Pi * BPS * BPS).
 
My tank cap is 19 nF.
 
For my tank cap, L ranges from 534 Henry at max BPS of 100 down to 5.3 Henry at
max BPS of 1000.  Using my lowest L MOT of 9.8 Henry, my theoretical max BPS is
about 738.  My series diode is two microwave oven diodes in series.  Power
supply is two MOTS, each driving triplers with the final filter cap being 6 mFd
16 KV.  More on this later.  My variable speed RSG is a variac driven angle
grinder with 16 rotating brass 10-32 electrodes, so it can easily do a BPS of
1000.  
 
My first trial was with a BPS of about 100.  Disaster!  The gap would power
arc, discharging the filter cap through the charging reactor with a thick 1-2
inch long trailing arcs.  
 
So I decided to try a BPS closer to the theoretical max of 738.  Success!! 
1000 BPS was even better.  I think it is a matter of reducing the gap dwell
time so it breaks before the reactor saturates.  But even at 800-1000 BPS, if
the DC input is cranked up much past 8.5 KV, it will power arc.  Perhaps a
triggered gap can be built with wider spacing and less tendency to power arc -
we shall see.
 
Here are some measurements, all made with a 48 inch arc between twin 4 x 23
inch secondaries.
 
1) Charging reactor = 9.8 Henry
 BPS    DCkV    DCmA    DC watts    AC VA
 800    8.5    115        978        1419
1000    7.5    130        975        1375
 
One obvious thing is that my MOT DC supply is not very efficient.  Assuming
unity power factor, the efficiency is only around 70%.  I have no idea what the
real PF is.  Has anyone tried PFCs with a MOT DC supply to see if the VAs can
be reduced some?
 
2) I tried adding another charging reactor MOT in series for a total of 22
Henry:
 
BPS    DCkV    DCmA    DC watts    AC VA
 800    8.5    112        952        1370
1000    7.2    130        936        1309
 
Roughly the same performance - my measurements are not very precise.
 
3) Last case - two MOTs in parallel for a charging reactance of about 5.4
Henry:
 
BPS    DCkV    DCmA    DC watts    AC VA
 800    8.5    105        893        1295
1000    8.0    130        1040       1451
 
As a comparison, my SPDT setup took about 1,300 DC watts to do a 48 inch spark
length.  Two gaps are obviously less efficient than one.
 
So it appears the charging reactance is not very critical, and probably most
any MOT will do.  After running the above experiments over a period of an hour
or so, the charging reactor was only slightly warm.
 
As I said above, my current power supply uses dual MOTS driving dual triplers. 
Keeping in mind that the tank cap charges to about twice the DC supply voltage,
triplers are overkill unless one has a tank cap rated for 30 KV or more.  A
more efficient and simpler power supply design would be two MOTs in series
driving a voltage doubler circuit, which would put out about 11 KV unloaded. 
That was my original setup - guess I will go back to it.  With a chargng
reactor setup, there is no need for my big (lethal!) filter cap--4 microwave
oven caps in series should do the job.
 
So the good news is that one can make a well performing DC powered TC using
nothing more than a bunch of salvaged microwave ovens.  The only tricky thing
is to have a RSG with a BPS of over 600.  Or, hopefully much better, use a
triggered gap driven by a variable pulse rate drive circuit.  Comments
welcomed, especially from others who try this. 
 
--Steve