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Re: status report



Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Steve, Justin, all,

First of all, let me say that I have NOT ever built a DC
powered coil although I have considered it. So that would
disqualify me from making any assertive statements about
DC powered coils, at least from an emperical standpoint.
I had considered going DC when trying to think of a way
to circumnavigate the synchronization issue of a SRSG with
AC power. However after researching the issue, it seemed
to me that it was simpler and cheaper to just accept the AC
synchronization issue than to have to employ the extra com-
ponents that are necessary for DC operation (HV diodes,
inductive reactor, and large filter caps). It seems to me that
10% ripple factor is a pretty loose standard for filtered DC
and if you are going to have latitude of variation of the
break rate, you would need to have a large filter cap if you
plan on being able to run high break rates. A relatively small
filter cap would completely discharge before the next half cycle could
recharge it if running a high break rate. .67 uFd @ 18 kV = 108 J and
if you're firing 5 J/bang at even a modest 100 bps, that would mean
that you would need a minimum of 500J/sec of energy discharge in the
filter cap to keep up a reasonably steady potential "in between" the
DC "ripples", wouldn't it? It seems to me that your .67 uFd cap would
be inadequately small unless you are running a very small coil with
only 1 J or so per bang or are running a very low break rate. ?

I suppose the big issue to me with a DC system is safety, though. I
feel that Tesla coil operation presents enough hazards while energized
without the additional hazard of relatively massive filter caps that
can easily retain a lethal charge after the coil is de-energized. Bleeder
caps can be added to bleed off the filter cap's charge but it would
take a little time before the charge could be bled down to a safe
level with multi-MOhms of resistance. I know that I personally can
tend to be forgetful and forgetting about that filter cap still having a big
charge on it after the coil is powered down could have fatal results!

I know some have successfully built nice DC resonant Tesla
systems and my hat is off to those individuals. John Freau has
done some excellent work with his "Stacatto" DC coil system, too.
I think everyone would agree that it is a bit more of a challenge to build
a DC system than an AC one, though, and for a beginner, it's definitely
better that he/she start off with an AC powered coil. As far as
the AC synch issue, I found that with a pole pig for a power
source, even synchronization wasn't necessary ;^)

David Rieben


----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, December 28, 2005 5:29 PM
Subject: Re: status report


Original poster: "S&JY" <youngsters@xxxxxxxxxxxxxxx>

Justin,

I disagree with David's claim for the need of big (dangerous) filter caps.
You can choose the amount of AC ripple you are willing to tolerate on your
DC output (e.g. 10%), then calculate the filter cap based on the DC load
your setup will draw (e.g. 0.25 amp).  The equations and "how to" may be
found in the Power
Supply section of Radio Amateur's Handbooks.  You can get used ones from
Ebay and other used book dealers--lots of good info for coilers.

I run a DC coil that performs quite well,
and the filter cap is only 0.67 mF at 18 KV (which is still dangerous) made
up of 24 microwave oven caps in series-parallel.

That aside, DC resonant charging gives you a lot more "fun factor" because
you can control
break rate and power fairly independently.  But to get the most fun out of
DC, you need a rotary or triggered spark gap.  And if you
design your system right, you don't need to ballast your HV transformer.

I would not recommend using a NST to run a DC system.  NSTs are designed to
run mostly at lower voltages, once the gas filled (neon) lamp is conducting.
With a DC system, you are likely to be running your NST without a proper
load at times which will lead to insulation breakdown inside the NST. Best
to go with MOTs or a Potential Transformer.  I use an oil immersed MOT
6-pack which works just
fine.
--Steve Y.