RE: Another Terry's filter question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

The power dissipated in the resistor is actually 
the (current)squared times R - even less than you 
thought.  However, a couple things tend to make 
the actual current higher than what you may expect.

The RMS current from your NST is rated at 30mA 
maximum to a short circuit or resistive 
load.  But when the NST is charging a capacitive 
load, the actual current can be far higher. 
Boosting the mains voltage above nominal with a 
Variac further contributes to this.  We don't 
presently know how to estimate this in advance, 
but it may be several TIMES the rated current.

The bypass caps in the R-C filter also contribute 
to the power dissipated in the resistors.  Each 
time the main gap fires, the main cap discharges 
through the primary coil, but additionally, the 
bypass caps discharge through the resistors.  The 
full magnitude of Vbang is applied to the two 
resistors, and is not limited to 30mA since it's 
supplied by the bypass caps.  The power 
dissipated in each resistor from the bypass cap 
is (BPS * .5 * C * Vbang/2 * Vbang/2) 
Watts.  Assuming a static gap @240BPS, two 500pF 
bypass caps, 21kV Vbang, each resistor will see 
6.6 Watts ABOVE what the charging current contributes.

The placement of the safety gap has changed in 
recent years.  Before Terry popularized his 
implementation with the MOV's, safety gaps were 
generally placed at the NST side of the low pass 
filter.  But he also wanted to use MOV's as a 
last-ditch clamp in addition to the safety gap, 
and MOV's can't take as much abuse as a safety 
gap.  I'm speculating that the safety gap is at 
the output side to take the brunt of a primary 
streamer-strike, leaving the more fragile MOV's to guard against lesser events.

Regards, Gary Lau
MA, USA


> Original poster: norman@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> I recently posted a question to the list about
> Terry's filter.  I wanted to know
> if my string of twenty, 56 ohm, ½ Watt 
resistors (in place of the 100 Watt 1k
> resistor) would fry when the safety spark gap 
fired.  I mistakenly assumed that
> the safety gap was across the NST rather than 
the primary gap.  If the safety
> gap were across the NST, and fired, the primary cap would discharge back
> through each of the 1k resistors, resulting in a large momentary current
> through the resistors.  It was this large 
momentary current that concerned me.
> But, since the safety gap is not across the NST, the current that will flow
> through the 1k resistor is only the 30 mA of 
the NST.   30 mA times 56 ohms is
> only 0.168 Watts per each ½ Watt 
resistor.  The resistors are at 1/3 of their
> rated power and will survive.   Several people 
replied that the resistors would
> run hot.  Am I missing something?  Also, I do not think that the 1k resistor
> will ever see the full 7500 volts of the 
NST.  7500 volts across a 1k resistor
> calculates to 7 amps - from a 30 mA NST??  But again I may be missing
> something.  Perhaps the small filter capacitor 
(charged to 7500 volts) on one
> end of the 1k resistor, and the firing 
(shorted) primary gap on the other side
> of the resistor, might momentarily place 7500 volts across the resistor?
>
> So, now that I realize that the safety gap is not
> across the NST, my question is
> why was it designed that way?  Since I want to protect the NST and not my
> resistors, why not put the safety gap across the NST?
>
> Norman Scheinberg
>