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Sam's HIGH POWER Ignition Coil Driver. Record pulses, driver dead:-(

  Hello everybody!


 It’s true what they say… You are not having fun until things start to
break!
 I had my High Power Double Car Ignition Coil Driver all tuned up and
nicely built inside a neat little plastic box. I had all the
electronics on a good quality epoxy board and the SCR on a proper
heatsink.
 On the metal top I had the 1100Watts oven heater element, properly
mounted on large heatsinks. The driver ran off 220V, some 2000Watts
and outputted nearly 1000Watts. The coils buzzed with the sound of
power and you could actually feel the box vibrate with the pulses. The
frequency of them was controlled off a potentiometer. On the lowest
rating I would get some 5-6cm long high frequency arcs. VERY hot and
well capable of burning insulators and driving nice Jacob’s Ladders.
On the Highest setting the frequency was somewhat lower and I would
get unstable arcs (more like fast sequences of sparks) at up to 10cm
in length.
 Life was good… It ran nice and I could leave it on for hours without
having to worry about anything overheating (the resistance got VERY
hot, the coils got warm, and the SCR got mildly hot).
 I finally made a high voltage rectifier of a stack of two HV
Television 24kV 3mA rectifiers on each output. I found out that
although the output from the coils was NOT DC (they ring with each
discharge, much like our beloved Tesla Coil. They build up the HV by
induction, as the turns ratio is only 1:100), putting the rectifiers
the wrong way around would cause almost no voltage to get trough and
they would overheat. Putting them the right way around had no decrease
in the output and allowed the driver to charge BIG caps… I was
charging plastic bottle caps and getting 5cm long discharges off them.
It was deafening (I never dared trying to charge them to the full
output because at only half the maximum voltage they made so much
corona that the sizzling sound of it could be hear far away. Ozone
filled the room. I think they’ll punch trough at the maximum output).
 I could now run the plasma globe off the DC output of one of the
coils and safely touch it (darn it hurt before). The bulb kept on
spraying after it was disconnected from the power supply for a few
secs, because of DC charge. I even stood on an insulating platform and
used a large metal screwdriver (getting hit in the skin burns, getting
hit when you are grounded will send you flying away. I talk from
experience) and charged myself to the DC output… All the hair in my
body rose and my shirt stuck to my chest… It felt funny:-)… I’m not
saying it didn’t hurt, but it was nevertheless cool.
 So, With such a great driver, you might be wondering why the heck I
did the following:
 Two things always bugged me. One was that the stupid resistance would
get hot enough to boil coffee in a few minutes. The other is that the
coils, being High-Power oil cooled ones, didn’t overheat. I always
felt there was room for improvement. I decided to do a major leap and
limit the driver with a MOT. My MOT came from a 1000Watts Samsung
Microwave oven. Considering the fact that the actual magnetron pulses
10 – 20% more than the effective heat it produces on the food, and it
is only about 80% efficient (together with the voltage Doubler), I
assumed that the MOT was rated for >1800Watts. And, seeing as it could
run for hours at that power, It was obvious that I was getting a lot
more than 1800W when I shorted the output and the whole thing smoked
in less than 30secs.
 Before using the MOT I measured 2500V at 1.98A on the secondary
output. As it seemed way too high, I measured the input: 220V, OFF
SCALE on my 20A meter.
 Well, all seems to indicate that the MOT was running at 4500W (no
exaggeration here! I really did measure these outputs. Those MOTs
should not be considered shunted transformers).
 Seeing as my driver contains parts rated for much less than 20A, I
was already considering the breakdown of something. I expected the SCR
to go because it was the only thing that got really hot before, and I
had replacements.
 I set the output power on the driver on the lowest setting (normally
Higher Frequency sparks, 5-6cm long), shorted the MOT, and used it to
limit the input.
 When I flipped the switch it made a loud hollow noise (the switch
welding because it was only meant for 5A) and the driver hummed VERY
loudly, much louder than the shorted MOT could possibly hum.
 The coils mechanically shook the box. The power was awesome!
  Before that test run I specifically put the wires over 12CM apart
because I was SURE that it couldn’t possibly arc that far on the
lowest setting.
 Well, it ran for less than 5 seconds. A bright, loud orange arc flied
across the gap and than, a small “pop” and it was silent again.
 The driver was destroyed. I am not sure about what is broken but it
obviously is more than just an SCR.
 Now I come back to my original statement: You are not having fun
until things start to break.
 Did I have fun?
 HECK YEAH! I killed my driver but it was truly inspiring to see it
run at that power level. I am now redesigning everything from the
ground up with all-new components, a larger SCR, a better trigger, etc…
 I know if it had survived it might have sparked 15, maybe even 20cm
(8inches) far at the highest settings. I have decided to limit all
runs to 3000Watts and below, and concentrate on a better driver. I
want more reliability and a better frequency range. Than I will go
Terry’s way and use a step-up transformer to pulse more voltage on the
coils. I will make my advanced prototype with X-2 Polyethylene
capacitors, I will make it in a 5uF 600V bank.
 I can see 50Cm sparks just around the corner… Perhaps with a better
coil and such I can go further, but for now I will concentrate on the
current design. Than, finally I will parallel those coils for less
voltage and more amperage output, and maybe even try running a TC off
it.
 This is evolving into something much greater than I ever expected.
The power of the driver is really awesome now, and it will go further.
But I need *YOUR* help to make it better.
 The current problems I would like help with are:

 Why does changing the size of either caps affects the output
negatively? Could the coils be resonating and any change in cap size
causes it to disrupt the resonance pattern?

 Does anyone have a good diagram for a SCR trigger circuit that will
trigger larger SCRs (I have a 20mA gate current one now, moving onto
100mA gate current) at a wide, adjustable frequency range?

 How can I get SCRs to work at higher voltages? I can’t find anything
larger than 600V 50A here. The 50A are more than enough, but I would
like at least 1000V for my next design.

 If I put a 50A SCR and a 50A diode on the driver, will the higher
resistance of them cause loss of input? Should I try to keep their
values as low as possible (within working limits) to maximise
efficiency or can I make them bigger?

 How about the Diode snubber network? I heard someone used it to
protect the SCR but I don’t quite see how it would work and be used.

 What voltage are they outputting if they make 15cm long DC sparks?

 Thanks for anyone who helps me with this… Soon I will have the full
construction manual for my final prototype (the one I just blew). I
will distribute it for free so that anyone who wants to can have his
own and try to improve it. But I will not make the schematics
available until I am done with it, and without the expertise of you
people who have worked with this before, it becomes very hard for me
to progress. So far I had to design and build this entire driver off
My knowledge and experience. Many parts have being lost to find the
exact values. Having some advice would make a big difference!
 Well, the good news is that Reinier still has a working driver so we
can have some fun while the Advanced prototype (an expensive
joint-effort between us) doesn’t come.
 All parts will be sacrificed with honours by High-Voltage capacitive
discharge :-) the diodes, SCR and resistance blow apart in a very
interesting way!


 Sam Barros.
==
 Sam Barros,
sambarros-at-yahoo-dot-com
ICQ#:15156975

 "The Less You Know, The Better You Sleep"...
 "Evolution Stops When Stupidity Is No Longer Fatal"
 "If At First You Don't Suceed, Increase The Amperage"
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