Ignition coil arc initiation with protected low voltage continuation?

Question

I'd like to initiate a spark plug arc with an ignition coil and then power the resulting (highly conductive) plasma with an ultracapacitor (low voltage, say 12V to 24V, and high current) without damaging the ultracapacitor during the initiation.

It would do something similar to this circuit:

But instead of relying on a 20KV power supply, it would be preferable to use a low voltage supply

I'm trying to avoid any arc-circuit resistance beyond the arc itself and the negligible ESR of the ultracapacitor.

I find the aforelinked PDF description of the above pictured circuit somewhat confusing. It says the 20KV capacitor discharges through the inductor upon the Arc Strike Signal closing the switch -- but doesn't say anything about the timing of circuit in relation to the switch re-opening the circuit (which is presumably what causes the ignition spark). Moreover, why would it not be sufficient to dispense with the capacitor in this role and dispense with the timing problem by increasing the induction of the coil, lowering the Igniting Power Supply voltage and then closing and opening the switch at "leisure" to produce the magnetic field and then the spark as with an ordinary ignition coil? It's almost as if the description is off -- and that the capacitor is there only as a "snubber capacitor", to absorb the back EMF away from the Arc Power Supply.

Thinking along these lines:

Might the ultracapacitor serve as its own snubber capacitor if the polarity of the back EMF discharges the inductor-side plate? Also, if the inductor has reasonably high inductance, it seems the Arc Strike Pulse Unit's power supply can be the same (kind of) supply (say lead acid battery) that charges the ultracapacitor. Pehaps, something like this (I was unable to find a spark gap in the circuit lab parts so I used a voltage controlled switch for the arc):

^{simulate this circuit – Schematic created using CircuitLab}

You close SW1 to charge the capacitor. Open it. Then close SW2 to establish the inductor field. Open it and the arc is initiated while the inductor's back EMF draws a small amount of charge from the inductor-side plate. The sacrificial fuse then breaks the arc plasma's circuit after it is delivered a short, high current, low voltage pulse.

You can play with the switches and simulate the behavior at this falstad URL.

Is this conjectured circuit reasonable?

Answer 1

Here's my suggestion for some experimental work.

Since you are looking for a DC discharge maybe all you need is a single high voltage pulse of quite short duration to trigger the arc. So, maybe try a camera flash circuit put through a small step up transformer to initiate it. To protect the supercap a couple of high value inductors and behind them some zener diodes to limit any voltage higher than the capacitor charging voltage.

You will likely need a scope that can handle high voltages to see exactly what is happening - there could be some weird resonances involved. Also, initially, leave out the supercap and just see if you can trigger a one-shot arc successfully. If you want to go beyond that, google "stun gun circuit" for your high voltage generator of more simply try something like this from eBay Ignore the hype - you will get more like 15-50kV from it - the wider the discharge electrodes the higher the voltage and the slower the repetition rate

Answer 2

I've done some work on a project with similar requirements, a DIY TIG welder. The crux of the issue are the contradictory requirements to deliver both high current at low voltage and low current at high voltage to the same electrodes. This is difficult, because you need switching devices which can block several kilovolts yet pass tens or hundreds of amps to do this in the naive way.

The typical solution appears to be discharging a high voltage capacitor to an inductor in series with the high current low voltage supply, as in OPs schematic. Usually this is done with a spark gap. As the energy rings down in the parallel LCR circuit thus formed, a high enough voltage to strike an arc is briefly present at the output. While conceptually simple, this has a few drawbacks:

• It requires a high voltage power supply to charge the capacitor to several kV.
• Arc initiation cannot be triggered exactly when required without substantial extra circuitry.
• It's not solid state.

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My solution is to use a custom step up transformer in series with the low voltage high current supply:

^{simulate this circuit – Schematic created using CircuitLab}

The transformer is optimized first and foremost for a very low resistance secondary winding. Low resistance is critical for passing large currents trough the secondary winding once the arc is struck. Thus the turns ratio of the transformer is fairly small and the primary only has a single turn, allowing the high voltage secondary to be constructed of just twenty turns of thick copper tape. In order to still get the required voltage gain despite the 1:20 turns ratio, the primary forms a series resonant LC circuit with a tank capacitor. When driven at its resonance frequency, the current trough this circuit rises with every passing cycle, until the primary voltage peaks at several hundreds of volts, the output electrode at several thousands, and an arc forms.

One neat but still untested feature of this topology is the ability to use the same H bridge for both arc initiation and controlling the main welding current, just by changing the switching frequency:

^{simulate this circuit}

I'll still have to investigate if it's feasible to use the high voltage transformer as the choke for PWM (as shown above), or if using a separate inductor for current regulation makes more sense (just letting the HV transformer saturate when the arc is present).