Off-Line Tesla coils (OLTC)

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Today I substantially rethought the front end of my off-line Tesla coil.
The schematic is at:

http://hot-streamer-dot-com/temp/Image1-1.gif

Although a bit "rough" at the moment, the basic system seems sound.  The
cap values in the old design were simply "poor" choices considering
commonly available caps.  The advantages of this change are as follows:

1.	The AC line is now very highly filtered from the coil's Fo frequency.
No line filtering is really needed.

2.	The primary cap cost is substantially reduced.  Ten CD-940C6W4P7K series
caps would work.  Such a system would probably be tuned by adding or
removing primary caps since the super heavy duty single turn primary "coil"
can't be tapped for tuning.

3.	The BPS rate can arbitrarily be anything the rest of the circuit could
support.  Since it is now a true DC charger, there is no need to worry with
60 or 120Hz BPS rates.  It is far simpler theoretically now that the old
way which was tuned to the line frequency.

4.	There is now no real power limit.  :-))  If you beef up the components,
10 to 20+ kW operation is possible.  True pole pig power levels are
relatively easy to obtain.  You basically just get larger storage caps and
increase the BPS rate (turn the knob up on the 555 timer).  You could
simply add components to an existing system to increase the power.  Perhaps
just bigger fuses and more electrolytic caps since the other parts could
already be made high power with little added trouble.

5.	The caps are now relatively easy to get new or surplus.  The primary
caps need only be cheap metalized ones since so many are in parallel.
However, high BPS rates may need metal foil types.  Not a big deal either
way.  Large electrolytics are common surplus items, but DigiKey sells these
too ;-))

6.	The IGBT really does not care if you increase the BPS rate, so nothing
else really changes.  You may be able to go to a smaller primary cap to
increase the Fo frequency.  Its value no longer depends on the input circuit.

7.	The primary tank circuit is no longer loaded by the charging circuit.
That allows more and cleaner energy and more power throughput.

The only real disadvantage is that the two large 5600uF caps (250 joules
each) need a lot of care.  Fusing, soft start, safety discharging, etc. is
needed (don't wire them backwards ;-)).  The large caps values are really
not necessary and are arbitrary, but we need HIGH ripple currents.  That
naturally calls for thousands of uF of capacitance in common big caps.
Ripple current is their key specification here.

Right now, the system as shown should do 6+ foot streamers (very
theoretical).  The ESR in the caps and primary coil are very low.  With the
IGBT operating very efficiently, only the streamers should be burning off
much power.  I do have a bunch of power resistors in there too, but they
are mostly for protection in case something in "Rev A" should go bad...
Eventually, they should be removed.

Much of the control circuits would be based on my previous SSgap work with
the electronics hand held and connect via fail safe fiber-optics.  Probably
IR glass fiber since the added cost would probably we worth it.

Thanks to Jim Lux for some great ideas!  BTW - I just could not get
photoflash caps to work.  The specs just did not "fit" and they are
polarized which was mess to deal with.  

Cheers,

	Terry



>Hi All,
>
>I have been doing some studying of off-line Tesla coils.  These are (would 
>be) Tesla coils that have "no" HV transformers but run directly off line 
>voltages.  What I found so far (purely by computer modeling) is somewhat 
>encouraging.  While a far cry from a 38 foot streamer unit, it may easily 
>approach an NST powered coil in power.  The only real new components would 
>be an IGBT solid state spark gap and rather odd MMC caps.  Everything else 
>is quite conventional.  So far, the design would run off 240VAC at around 
>700 watts.  This is the schematic as it now stands:
>
>http://hot-streamer-dot-com/temp/Image7.gif
>
>The the top is 120-0-120 VAC (single phase split 240 as is common in US 
>homes).  That voltage is rectified by two diodes and charges two 100uF caps 
>(50uF primary capacitance) through some ballast resistors.  This gives a 
>firing voltage at 60 BPS SYNC of:
>
>240 x SQRT(2) x 2 = 679 VDC
>
>The capacitor waveform looks like this:
>
>http://hot-streamer-dot-com/temp/Image3.gif
>
>Since the caps need a full AC cycle to charge, the natural firing rate is 
>limited to 60 BPS.  The current through the charging diodes looks like this:
>
>http://hot-streamer-dot-com/temp/Image4.gif
>
>Only about 4 amps RMS and very easily in the range of any 240VAC circuit.  
>One concern is the RF spikes at about 20 amps which would have to be 
>filtered.  The resonant frequency is 16 kHz.  A sweep of the circuit revels 
snip....