# Re: Spark length formula and NST's, was ^^(my) First Light ^

```Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 5/26/01 4:18:49 PM Eastern Daylight Time, tesla-at-pupman-dot-com
writes:

> > Hello, everyone
>  >
>  > 45"?? didn't he say he had a 12/60? I thought the max u could get with a
>  > 12/60 was 36"?! Maybe I'm wrong, what is the formula for that again? I
>  > think
>  > it was John Freau's Formula...
>  >
>  > ---------------------------------------
>  > Jonathon Reinhart

Jonathon,

One must consider that an NST can draw a lot more than its rated
power.

Much more than 36" can be obtained from a 12/60.  I get 42" from
a 12/30 NST.  Gary Lau gets 61" from a 15/60.  The key is that
an NST can draw a lot more than its rated power if resonant
charging is used, or if a 140volt step up type variac is used to
power the coil with an LTR cap.  My formula is meant to be
used with the *actual* input power, not the NST rated power
(not with the VA either, since non-pfc'd NST's may draw a lot
of current, much of which does not translate into true power).

The formula is:

spark length inches = 1.7*sqrt input power (wallplug watts).

My 12/30 NST is rated at 360 watts.  But with a 0.015uF cap
for LTR, and using a 140V step up variac to power the coil, it
draws 620 watts, (VA is a little higher using PFC).  It gives
42" sparks.

1.7*sqrt 620watts = 42.3 inches

John Freau

```