blah blah blah blah. and rhubarb too.
you guys need to play with some high voltage supplies.
memories of fooling around with tesla coils and 15kv transformers.
strike an arc from the trasnformer directly. it starts at about an inch away, then you can draw the ionised arc out to several inches, depending on air currents. its how a jacob ladder works (hot air rises, the arc heats the air, the arc remains established). it doesnt make much noise, just a dull sizzling sound. whole 15mA of current...
then add a capacitor.
crack crack crack...
whats happening?
the capacitor stores energy.
its "soaking up" the current, storing charge as potential on two plates.
bigger the capacitance, the more current it takes to charge. or really, energy, as it can charge slowly with a low current, or quickly, with a high current.
and capacitors have this strange behaviour. given a short circuit, they can discharge virtually instantly, releasing that charge in nanoseconds or less. because of the amount of energy released in such a short time, the currents are correspondingly large. we are talking a few hundred amps. not tens, not milliamperes...HUNDREDS of amps.
once it charges to the potential required to arc across the gap, BANG. the arc is acting as a short circuit, and the capacitor discharges instantly. crack.
thats all this "plasma ignition" is.
it doesnt work with a resistive plug because the resistor reduces the current flow.
the cap cant charge fast enough if a resistive lead or boot is used.
the cap cant discharge fast enough with a resisitive plug.
the resistor serves two purposes. reduce or suppress EMI, and to stretch out the duration of the short spark that a CDI produces otherwise...because it is just a capacitor discharging into the small resistance of a ignition coil. the smaller the winding resistance, the more current it can deliver, but the shorter the duration of spark.
refer to jennings on discussions about spark duration...it IS an issue with CDI.
back to our plasma ignition, or jamming a capacitor in parallel with the spark gap.
bloody tesla, doing all this 150 odd years ago...
a typical ignition coil, be it an induction type as used in cars, or simple step up types as used in CDI, is a COIL. an inductor.
and what do we get when we have an inductor and a capcitance in parallel?
a tank circuit, a tuned circuit. it oscillates.
in a typical ignition system, the arc only travels in one direction. its a DC pulse. some systems also make use of the resultant back EMF from the coil and use it to charge the CDI capacitor a bit, assisting the power supply to charge it sufficiently.
with my neon sign transformer, its a sine wave of 50hz, and the ionised air lingers around long enough at every zero crossing to keep the spark lit. it blows out quite easily once the arc is stretched out further than it can initially jump across. a DC arc, its almost impossible to blow out. sadly, in a CDI system, its such a short pulse that we actually have to slow it down with some resistors.
but this capacitor changes things.
it can charge instantly, discharge instantly.
every time the potential at the gap gets high enough to arc, yes, the capacitor discharges. the coil builds a huge magnetic field...
that then collapses, and dumps a huge current back into the capcitor, that soaks it up instantly... and then discharges again...
you dont get a single spark. you get a ringing, an oscillation of a few Mhz or so. it only lasts briefly as the resistances in the circuit quickly dissipate the power, quenching the arc. more resistance, the shorter the duration.
at 50 hz, you get 100 discharges per second. you hear them as a cracking sound, like a gunshot when the capacitances and hence currents start getting large. try discharging a 200,000uf cap bank charged at just 400V (through your thumb...owwwwww. i didnt stop shaking for five hours...). its pretty violent.
i dont have access to a cap bank that large that can take Kv... but give me enough wine bottles and i shall try!
each "crack" is actually an oscillation of a seriously high frequency, that frequency being determined by capacitance and inductance. its very high with the small cores used in CDI ignition coils. low inductance. its a very low frequency in auto ignition coils that use induction, therefore have high inductance.
the addition of any resistance to the circuit quenches the oscillation too fast. thats why plasma ignition doesnt work if you use resistive components.
and, that being said, back to the neon sign transformer. without the capacitor, the arc can be stretched right out before extuinguishing because its coming from a sinewave, and the actual voltage across the arc is very low, its acting like a short circuit. as the sinewave crosses the zero point, it only does so very briefly compared to the complete cycle. ionised air hangs around long enough to remain ionised.
but with the capacitor, it cannot be stretched out any further than the maximum striking distance, (one inch at 15Kv) as the capacitor is causing the arc to have a definite potential to reach before ionisation can occur, so it has a definite on and off period. the ionised air dissipates before the next arc can start. open the gap any larger, and the arc goes out. (and blows up the test tubes you used to make the capacitor with as all that energy attempts to go somewhere.)
pretty simple really. nothing magical.
wears out your spark gap pretty quickly. you definitely want iridium/platinum plugs for that... the hard to find type, with both electrode and ground strap of the precious metal, as the arc is travelling both ways when a ring starts up.
neither the capacitor or resistor change the total energy in the system. that is delivered and limited by the power supply. all they do is alter how that energy is administered at the spark gap.
the voltage doesnt increase, its already at a fixed maximum potential based on the charge in the CDI capacitor, and the turns ratio of the coil (or the inductance and rate that the magnetic field collapses +turns ratio when using an induction/auto ignition coil...which is why we also put a capacitor across the points in old ignition systems. it quenches the applied voltage in the primary fatser, causing faster magnetic collapse, causing faster moving field lines, that cut the conductors of the secondary faster and produce more EMF... is that maxwells or faradays thing? the EMF in a conductor is proportional to the speed and the number of magnetic lines cutting that conductor)
for a given potential through a given resistance, only so much current can flow. ohms law.
one day, im going to rebuild that tesla coil...
