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RE: Parallel resonant DRSSTC (long) Longer :o))

Original poster: Terry Fritz <laptop@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve and Dave,

At 06:24 PM 1/8/2005, Steve Conner wrote: 
Just my $.20 worth on this-

............
3) Dave Sharpe said
>Having <100A versus 1kA + going
>through your power devices might improve reliability just a slight
>bit...

This is true. But remember that in the DRSSTC we need a very fast and
violent delivery of power to a resonator with a Q of about 10. Typically the
"peak" power is from ~25kW for a tabletop coil to ~500kW for a Steve Ward
sized monster. So high currents in the power devices are unavoidable if you
want good performance from a pulsed SSTC. In fact they are the cause of the
good performance!

I have noted that DRSSTC builders try to use very high coupling and fast ring up times to smash high primary currents into the system with a minimum pulse widths... Basically, hit it as hard as you can and stop before the IGBTs blow up from RMS heating or current ring up... Conventional coilers often do this too to get that low "surge" impedance...

Consider a simplified coil whose primary is simplified to one of two modes of operation:

A: 10 cycles of 1000 peak amps over 100uS. We can assume square waves, a 1 ohm load and 1BPS operation here. So the RMS current is 1000 amps and the power delivered is 100 watts (1000^2 x 1 x 100uS). But lets suppose we also have a coil...

B: 40 cycles of 500 peak amps over 400uS. The power is now (500^2 x 1 x 400uS) 100 Watts...

Same power to the load. One is fast and high current. The other is slow and 1/2 the current. Which one will give better sparks? IMHO - either one could, or could not...

Why do I say this? Because it is not the primary current, but the "power" that makes sparks. John's formula of:

Spark length = 1.7 x SQRT(input power)

works on both high and low primary current coils... High coupling does help on "lossy" coils for sure, but it does not matter much on well constructed low loss coils. I certainly noted when Dan reduced his primary current with thinner wire. His IGBTs did not blowup due to the lower current, but the "output was not affected much"!! The primary current is all he changed (but I would guess the pulse width increased a little)..

In the case of the OLTC, we needed super high current due to the low primary voltages. But in the DRSSTC, we can ring up as long as we want!! 100uS, 1000uS, 10S ;-)) We can ring until something blows or the I^2R system losses equal the input power... Just because there "are" 10,000 amps IGBTs, does not mean we "need" them... OK, maybe Greg does ;o))

The fact that the DRSSTC is not a "power limited" device, like a capacitor discharge powered coil, invites all kinds of new possibilities!!!! The first one "I" see is that coupling and primary current can go way down!! I think I mentioned that the models like coupling even down into the 0.03 territory in that case!! Lower coupling also reduced stress in IGBTs due to streamer ground strikes and such.

So I am going a bit against the norm in my case. Since peak IGBT current is "expensive", I am trying to get the same performance but keeping the peak currents as low as possible. If we can get 10 feet from 200 amps peak or 10 feet from 1000 amps peak, the 200 amp case is vastly easier, cheaper, and more reliable... Of course, I can modify "mine too" to kick unimaginable currents if I am wrong ;-)))))

I should also point out something interesting. My "pure" CW coil uses a -50dB noise free RF signals to make about a 6 inch brush at 900 watts input.

http://www.hot-streamer.com/TeslaCoils/MyCoils/CWCoil/400watts.jpg

I note tube coils do MUCH better because they ARE disrupted, at the 120Hz line frequency:

http://hot-streamer.com/temp/PB210006.JPG

But my "pure" CW coil does make 15 inch long arcs under a certain condition! If I mess with it just right I can hit the "fast" current limit circuit that instantly cuts power to the FETs. As it switches off, there is a long "POP" with a single big arc!! So It seems that the "disruptive" part is the key to many things...

I would guess that disrupting a happy CW or gently rung up coil somehow "rips" an arc air into the air. The space charge around the terminal may have a "dV/dT" limit between "displacement" current and "real" current ;-)) This sudden disruption of the happy CW like system probably accounts for a coil's ability to "reach out" with longer darty streamers. But the coil needs "power" to support an ionized region to build onto and grow long streamers from too.

It is all really pretty unknown. We could never really control a coil well enough to know. MMCs gave us a very stable cap. Rotary gaps are pretty "stable" but they burn power like crazy and vary day to day... We know the theory, but we could just never "control" things well enough. The OLTC helped a lot, but it was just "too different" from the typical coil. Even though we did not learn much from it, I am happy that the OLTC "technology" seems to have helped out ;-))) I don't think E-bay has had a good price on big IGBTs since!!!

However. Now with the DRSSTC we can control ring up, current, voltage... We can control everything to a vastly higher and "repeatable" degree never imagined!! The most exciting thing about the DRSSTC (at least for "me") is that we can now "screw with" Tesla coils at a very basic level!! If the thing wants one cycle, then three, six, two, seven... we CAN do that!! Unlike the old days of rolls of magnet wire, oily caps, carriage bolt gaps... One can almost sit at a Godlike computer control panel and fiddle with parameters at a whim. Imagine an experiment like Paul's big computer studies of coils. Run over the Internet. But now an actual coil is fully computer controlled so one can test vast numbers of parameters until the best is found!!! It would be still be a stretch to let folks run their coil parameters in JAVATC on a "real" coil. But maybe... Since Dan's big DRSSTC run. Maybe those without coils could enjoy running a could over the Internet!! If this guy can do it with a chicken...

http://www.subservientchicken.com/

"Network Solutions - The domain name "subservientteslacoil.com" is available." :o)))))))

I am not sure what is right or what is wrong in all this. But now we can find out!!! The DRSSTC is VERY BIG!!! It may not be as "easy" as throwing all your caps in the trash and getting Geek Group caps*, but the whole Tesla coil world has changed!!!

I think once we get a lot of "boards" around, "everyone" will be able to play with them. Dan has boards. I will have many little boards for my own screwy version, and soon there will be other folks... Eventually, we will find all the "best methods"... Of course, "low cost" is super important!! It is all still super young, so stay tuned ;-))

*Hey!!! Has anyone tried running a nice DRSSTC coil with a "party cup" cap yet :o)))

PCC-DRSSTC (Party Cup Cap - Double Resonant Solid State Tesla Coil). Probably just have to try it to be sure the thing survives as the cap incinerates itself... Or, if the currents are low, Party cup caps my be perfect!!! :O))

Cheers,

        Terry