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This is a follow-up to my previous question. I am trying to increase the spark gap distance of an arc lighter without increasing the supply voltage - I'm using a 3.7V li-ion battery to power it. I'm getting a 3.5mm gap at the moment.

Schematic

I'm using two MOSFETs in parallel to decrease Rds(on). The collector current is about 1A when the secondary of the transformer is sparking. I tried decreasing R1 to increase the collector current. This worked, but didn't make a significant difference in spark gap distance.

I noticed that there's a lot of noise at both sides of the primary. I filtered out the noise at the supply using a 47uF decoupling capacitor. The signal at the collector does not appear to be a square wave - it is very choppy and includes 125V spikes. I tried to filter this out using a revere-biased 3.6V Zener diode with a current limiting resistor (limiting the current to around 20mA). This did not fix the voltage spikes and the choppiness of the primary input. The below picture shows the collector voltage before and after sparking/conduction.

enter image description here

I also tried different frequencies and duty cycles, and this one seemed to be the best. I have considered a DC-DC converter to step up the voltage into the primary, but that would be too bulky based on the currents I'm dealing with.

Any suggestions? I'm open to redesigning the circuit.

w00t
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  • Do you have any additional i/o lines? If so, maybe implement a voltage doubler. – Samee87 Feb 23 '17 at 21:02
  • Could a DC voltage doubler connected to the battery work at 18kHz? I can't see how an AC voltage doubler would fit into the primary side of the circuit. An AC voltage doubler on the secondary wouldn't work because it's in the kV range. – w00t Feb 23 '17 at 22:05
  • Just add a voltage tripler to it... This is getting 4-5 inch sparks... https://hackaday.com/2018/06/05/lightning-generator-from-electric-lighter/ – Craig N Apr 04 '20 at 08:41

2 Answers2

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The maximum length of a spark is limited by the breakdown voltage of air -- about 3 kV/mm. If you want a longer spark, you'll need to increase the voltage.

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The size of the spark is a function of the turns-ratio of the transformer and the rate of change of current in the primary.

What did your collector current rise to when you reduced the base resistor?

So while you may have increased the max current, you might not be turning it off as fast as you think. BJTs turn off slowly when base current is removed, though with the MOSFETs at the emitter I suspect the current will cut off at the rate the MOSFET turns off.

What are you driving the PWM with?

I suggest you drive the PWM signal with something that can turn OFF (pull down) the MOSFET gate better than your current solution. A simple solution is a buffer such as an AND gate [one with a stronger output than your current solution], or some smaller MOSFETs to drive these bigger ones. The smaller MOSFETs will present less input capacitance to your driver and will turn on faster. The best solution will be a dedicated driver design.

Another simple check is to remove one of the MOSFETs. This will half the input capacitance and should result in faster switching. Be sure the reduction in conductance and the increased power dissipation in the remaining MOSFET doesn't cause any issues.

Good luck!

rohmeooo
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  • When I reduced the base resistor to 5 ohms, the collector current rose to 1.3A. I'm driving the PWM with a regular signal generator + a pulldown resistor. Removing one of the MOSFETs results in less current and a shorter spark gap. When I remove the transformer from the circuit, I can see a perfect 18kHz square wave at the emitter/drain connection. This leads me to believe that the switching speed of the MOSFET is fine. Putting the transformer back in re-introduces the noise. Are you attributing the noise to a slower switching speed? – w00t Feb 23 '17 at 21:59
  • What is driving the PWM?

    When you remove the transformer, what do you put in it's place? If it's an open circuit it makes sense that it switches quickly. This is called "clamped inductive switching." Switching on an inductor is way harder on the switch than switching on nothing (floating) or a resistance.

    Also, what is the inductance and turns ratio of the transformer? Perhaps 18kHz is simply too fast (too much inductance). A part number or datasheet would help.

     – rohmeooo Feb 24 '17 at 21:08
  • The "noise" is the transformer behaving exactly as it should. That huge spike gets transformed to the secondary side and becomes your spark. – rohmeooo Feb 24 '17 at 21:27