OLTC

AD-OLTC (Alternate Drive - Off line Tesla Coil) 

Alternate Drive - Off line Tesla coil. An upgrade to MY OLTC too double the power.

The Primary coil 2 coils of 2 turns of 5mm copper tubeing .

Primary cap 2.45 uF

The Secondary 2100 turns 0.125mm enameled copper wire wound on a glass storagejar 4" in diameter, 11" tall.. 

The Topload 10" Stainless steel sphere. (Two salad bowls from Ikea, but don't let on)

Drive 4 x IGBT's, fired Alternately in pairs. 

Output 21" so far

BPS 10hz - 10Khz

Circuit Diagram   (not complete and subject to change)

The New controller , To allow for driving the IGBT's alternately, I had to construct a new driver. As I was making a new driver I thought I may as well include all the features I may need in the future.
E.g 0-10Khz BPS settings, Interruption with variable mark space and TT mode (more later).
Quenching is now set with a 5 turn pot on the front panel.

Traces from inital testing at low voltage

In each picture the top trace is Primary Capacitor Charge and the
bottom Trace is the secondary voltage, pickup from an unearthed scope probe.

The Top Picture is the coil running at low frequency around 200Hz.

The bottom picture is the coil running at 4Khz showing some strange charging problems, and the bangs happening before the coil has stopped resonating. These two observations are probably connected. As the effect changes at different high BPS as if the oscillation on the secondary is becoming out of phase with the next bang on the primary. 

The bottom trace also shows that the charging inductor needs to be of a smaller inductance, to charge the capacitor up faster. This may also increase the charging voltage by flyback effect. 

A new topload from Ikea, now 10" in diameter (was 8") means that I can now use a 2.45uF primary cap.

The primary is only roughly tuned as yet and as I haven't changed the charging choke (yet), I can't run at full power, but with an interrupted (10hz 1:10 mark/space) drive of around 5Khz (had to change charging fuse to 5A one) , I'm now getting 17"+ strikes to earth easily and still some power / frequency to go yet. (pretty good with an 11" secondary :-) ) The IGBT's were cold after about a 30 Sec run and the sparks were still growing at the 5Khz mark.

The new top load is scary, the field it creates is much larger than the 8" one, standing 1M+ away from the coil I get 1/4 sparks from me to the earthed case of the controller..

I've added a new Charging choke, to give me a quicker charge, and a little more voltage on the primary caps. It also is giving me much longer sparks at lower BPS's. The new choke is 2.5mH, made from a whole real of 1.25mm enammeled wire. I had to wind it to fit the space available 1" x 3".
I think the older choke (RHS of picture) was saturating and net being as high impedance to hf as it sould have been.

After the choke change I found that the coils performance had increased. I had set up and earthed rod with 1" of wire pointing towards my coil at the top the gap was set at 20". The strike directly to the rod at the bottom of the picture must be 21" at least. (I put this down to gravity :-) ) 

This photo was taken at around 5Khz with the interrupter set at around 50%, after a 20 sec run the IGBT's were warm to the touch.

I have also added a strike rail over the top of the IGBT's heatsinks to protect the IGBT's. It's only a matter of time till I hit them...

Next a test of TT mode.....

Click for a Video of the coil running.

TT mode Running, At the Derby Teslathon.

TT mode allows the OLTC to Play Twinkle Twinkle little star whilst running.
(Silly I know, but I was told it was not possible.)

Running at the Derby Teslathon I had loads of problems with strikes to the mosfets and strike rail resulting in the damage to the secondary.

This is a rather terminal strike to the secondary. No matter how I repair it, it will leak sparks. :-)

This is only of the problems in putting more and more power through a coil, eventually it self destructs.

So a major rebuild is in order...