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Re: how much does current matter?



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Ben,

Original poster: ben eells <squeels2171@xxxxxxxxx>

In my attempt to build a more powerful tesla coil I have come across this question. How much will the output current of your NST affect the length of your streamers. I'm using the program Tesla map to help design this coil and it calculates the max streamer length by the output wattage of the NST. Unless I'm mistaken output wattage can never be higher than input wattage.

This is true, however, input wattage can be higher than input specs. The current rating for your NST is for a dead short. When driving a capacitive load, it is possible to get more than the current rating from the NST especially with a SRSG.

The current output from the NST does matter and higher current rating (everything else the same) means higher VA rating and more power available to charge a larger Cp and keep the BPS around 120 (higher bang energy means higher power processed and potentially longer sparks)

An empirical equation that John Freau developed is

spark_length (inches) = 1.7 * sqrt(real_power_at_the_wallplug)

The 1.7 factor is sometimes refered to as the Freau factor (FF) and can be thought of as achievable. Some don't do as well and others exceed this factor. Small power levels have a harder time achieving the FF.

For static gaps, simulations shows a power factor (PF) of approximately 0.5 (with the gap set so the gap breakdown voltage is same as the actual Vs peak). So for a NST with a VA rating of 900, the VA can be adjusted for the actual Vinput applied by multiplying it by the square of (Vinput_actual / Vinput_rating). Multiply this adjusted VA by the PF of 0.5 to get an estimate of wallplug power.

For SRSG, it is possible to get more than the VA rating in real watts. I draw 2000 watts from a 15/120 NST by running the input at 140Vac with a SRSG running at 120 BPS. The VA rating is 1800. With 140Vac input, the VA goes to 2450 so with 2000 watts of real power draw, the PF is 0.82

Gerry R