DC Coil breakthrough! (CORRECTED)

["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>

This is a REPOST - please delete the previous one which has incorrect break
rates (breathed too much ozone?)
 
Subject: DC Coil breakthrough!

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 8 rotating brass 10-32
electrodes, so it can easily do a BPS of 600.  

My first trial was with a BPS of about 50.  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 of about 400 BPS.  Success!!  500 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 400-500 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.  Helpful ideas will be appreciated.

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
400    8.5    115        978        1419
500    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
amps 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
400    8.5    112        952        1370
500    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
400    8.5    105        893        1295
500    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 400.  Or, hopefully much better,
use a triggered gap driven by a variable pulse rate drive circuit.
Comments welcomed, especially from others who try this. 
 
Addendum from previous post:  I stretched my twins as far apart as the
wires would let me - 53 Inches.  At 550 BPS and 1200 DC watts, I could
easily get 53 inch sparks.  Experiments ended when wife said I was wrecking
phone communications!  

--Steve