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Why do capacitors increase arc lengths? Answered

I have seen a lot of videos with people increasing the arc length of a nst or mot with capacitors and i was wondering why this works. Also, is the capacitor increasing the voltage (higher potential difference, so longer arcs) or the amps (more ionisation so you can draw the arc longe)r?


It is increasing both. Most of those videos are using 50 or 60Hz resonance. Current limited transformers (to a small degree all transformers since you cannot get 100% coupling) like NST's and mots have built in current limiting for keep them from overheating and also frying the neon sign or magnetron from too much current. It does this by a shunt that bypasses some of the primary flux as more current is drawn, which has the effect of dropping the output voltage until it balances out (a 15kV/60mA NST will only be about 7500v when 60mA is drawn). Electrically this is equivalent to an inductor in series with the transformer primary. When you put the right size capacitor in series with the primary it will cancel the effects of the shunt (the capacitive reactance is equal to the inductive reactance at either 50 or 60 hz). Usually the cap is sized to where it either doesn't completely cancel it out or a resistor is also in series to limit resonant rise. Otherwise the rise continues until windings are fried from either too much current or too much voltage. That is, unless it's a pole transformer and a couple of arc welders for inductors with a pulse capacitor on a 100A circuit https://www.youtube.com/watch?v=TIF_NmUsCzI


6 years ago

AC arcs reverse every 8.33ms in the US and cause current to go through
zero for a power line and more often for higher and high frequency.

AC means Alternating Current. 
DC means Direct Current  which almost always has an output capacitor.
When an arc is established the ionized air gets hot and forms a plasma making it easier to to pass current  between metal terminals.
( no more energy is needed to ionize cold air ).

DC arcs will maintain current flow as far as a hot plasma can extend.
Do you know why arcs are for the most part curved upwards ?

Yes.. Rising hot air causes a convection air current  But the arc electric
flow generates a magnetic field which tends to expand and if not
limited will cause the plasma to expand thousands of times faster
then convection.   Two examples ;
  • Power line emergency switches use the magnetic arc principle of high current to blow the arc out  when they open by the expansion beyond an arc's ability to maintain the plasma.
  • Working over the batteries in my electric Volkswagen a metal wrench touched 48 Volt pole terminals before I could move the wrench was flashed to an expanding plasma arc ( I did close my eyes ) when I opened them the arc had extinguished and the vehicles ceiling was on fire !
Now... AC arcs cause the the plasma currents to reverse
ie go through an instant of zero current  making it harder to maintain
a nice long plasma Arc like that of a DC current arc.


so you mean when the wave goes through the 0v part, the capacitor substitutes it?

How about this, with a capacitor acting as an additional source of current
the plasma has more ionic flow to keep the plasma hot.  That means
the arc can be a bit longer.

Depends on how you wire it -- certain ways of hooking up a cap will increase voltage (like a voltage doubler) while others will increase current (store up voltage for longer before dumping it all when the air ionizes)