transformer question

[benet]

I've heard around a lot that spark length he's based off of wattage. But if that is true Why does it matter what transformer is used? Because a transformer just changes the voltage to amperage ratio it never actually changes the wattage. I feel like I'm missing something pretty major here.

[Mads Barnkob]

the old spark length estimation formula, only uses power as an input.



http://deepfriedneon.com/tesla_frame6.html

Efficiency of a spark gap is however lower at lower voltages, so there is a consensus to use 5-15kV, above that it gets problematic to make a good enough resonant capacitor.

[MRMILSTAR]

I assume that your question has to do with transformer output voltage. For transformers supplying equal power the reason that voltage matters, at least for a SGTC, is that it becomes quite difficult to make a spark gap fire reliably at low voltage. The "best" voltage level is debatable but I would want to use something in the range of 10 KV to 15 KV RMS. Its fairly straightforward to make a reliable spark gap in that range. Its also in the range where the required capacitance with the proper voltage rating can be achieved without tremendous expense. Lower voltage can be used but the spark gap design becomes more difficult.

[benet]

Sorry I should have been more specific I'm talking about the spark length of a Tesla coil. Does the same hold true in that case?

[Twospoons]

Because a transformer just changes the voltage to amperage ratio it never actually changes the wattage. I feel like I'm missing something pretty major here.

What you're missing is that a transformer can also be thought of in terms of impedance change.  Impedance matching is an important part of maximising power transfer - i.e. watts.

[benet]

I'm still a bit confused why use the transformer to change the impedance instead of just changing the size of the secondary coil So that it has the same impedance size the power main?