Original poster: "S&JY" <youngsters@xxxxxxxxxxxxxxx>
David,
Here are some measured values for my DC coil. MMC is .041 mF @ 36 KV. At a
break rate of 200 BPS, my DC supply was producing 7.4 KV with an average
current to the resonant charged TC of 0.1 amp. I use full wave
rectification, so the ripple frequency is 120 Hz.
10% RMS ripple at 7.4 KV is 740 volts. The Peak to Peak ripple is 740 * 2 *
sqrt 2 = 2093 volts.
Filter cap needed in microfarads = 0.1 amp * 10^6 / (2093 volts * 120 Hz) =
0.4 mF. So my 0.67 mF cap is is more than adequate.
At 400 BPS, the power supply is outputting 6.4 KV at .142 amp. Filter cap
needed for 10% ripple calculates to be 0.65 mF so my 0.67 mF filter is still
OK. Usually I don't exceed 300 BPS when running my coil.
I think in your discussion below you have not considered that the filter cap
is being recharged 120 times a second, with 0.1 amp * 7.4 KV = about 740 DC
watts average being transferred into the filter cap.
Ideally, because of resonant charging, the MMC will charge to twice the 7.4
KV, which is 14.8 KV. However, due to less than ideal conditions, I believe
it only charges to about 13.4 KV. (I use a RSG to first charge the MMC from
the power supply, then discharge the MMC into the TC primary. The MMC
doesn't get fully charged before the gap stops conducting.) So Joules per
bang are = 0.5 * C * V * V = 3.68. At 200 BPS the average power into (and
out of) the MMC is 3.68 * 200 BPS = about 740 watts. For these
measurements, I was producing 54 inch leaders between twin coils.
I hope someday you actually build a DC coil - they are much more satisfying
then plain old boring AC coils!
I hope this helps,
--Steve
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, December 29, 2005 10:24 AM
Subject: 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. ?
> <SNIP>
>> 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.
>
>
>
>
>
>
>