NST Shunt Removal (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 10 Jun 2007 16:05:07 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: NST Shunt Removal

Hi All,

My 4.5" diameter coil spark lengths are far beyond a 12/60 NST capability.
http://www.classictesla.com/photos/ba45/ba45.html
And these pics don't really show the lengths I've seen with this coil.

The NST I use was modified by fixing some hv windings and removing 1/2 
of the shunts (back when I documented the NST depotting procedure. So, 
the 12/60 Franceformer is no longer a 12/60. At the time, I never 
finished the measurements on the NST. Well, I rebuilt my hv divider 
today using 6 100Meg HV resistors (mfg p/n EX2G1006, $1.66ea). These are 
made by Ohmite and good for 7.5KV each (I have 3 in series on each leg 
for 22.5KV at 300Meg). Anyway, following calibration, I tested an 
unmodified 15/30 Franceformer. All came out as it should. Here's the 
data for the unmodified 15/30 NST.
Rp=0.8 ohms, Rs=16230 ohms.

OPEN CIRCUIT MEASUREMENTS:
V_pri = 124.1 V
I_pri = 0.74 A
P_pri = 24 W
VA_pri = 91 VA
PF = 0.27
V_sec = 15340 V

SHORT CIRCUIT MEASUREMENTS:
V_pri = 124.0 V
I_pri = 3.83 A
P_pri = 39 W
VA_pri = 474 VA
PF = 0.08
I_sec = 25.74 mA


Knowing that the hv divider is working well, I decided to unbolt the 
modified 12/60 and run it through both open and short circuit 
measurements. Here is what I found: btw, Rp=0.45 ohms, Rs=4660 ohms.

OPEN CIRCUIT MEASUREMENTS:
V_pri = 124.2 V
I_pri = 1.53 A
P_pri = 50 W
VA_pri = 190 VA
PF = 0.26
V_sec = 13380 V

SHORT CIRCUIT MEASUREMENTS:
V_pri = 121.3 V
I_pri = 20.3 A
P_pri = 428 W
VA_pri = 2457 VA
PF = 0.18
I_sec = 206.7 mA

So, as you can see, this NST is no longer a 12/60, closer to a 13/200 
and that explains the arc lengths being so good, or really, "not that 
good" considering I am about 20" shorter than I should be for this power.

Dr. Resonance was wondering why the little coil was doing so well and 
had asked me offline. I let him know the NST was modified, but I didn't 
have the data to give him at the time. I've probably pushed that NST to 
the brink of saturation, but, it has held up well, it just keeps on 
running. It's reliability is a pleasant surprise considering what I know 
"now" about the modifications.

I took out exactly 1/2 the shunts. I_sec jumped from 60mA to 200mA (more 
current than I intended). I was hoping removing 1/2 the shunts would be 
a linear jump to 120mA, but that was just a shot in the dark. If I were 
to modify an NST again by removing shunts, I would only remove 1/4 of 
the shunts. 1/2 seems a bit much. This NST is more like a PT or 
bombarder now. I really don't know how long it will last. Last week, I 
lost an XBOX 360 and my main computer (video section on motherboard 
committed suicide). This was due to a small 2" coil that I ran without 
an RF ground. Big mistake. However, my NST Terry Filter smoked with this 
4.5" coil Thursday. The coil and NST still run great, but the Terry 
Filter has some burnt resistors. Looks like I'll have to check all those 
MOV's!

Just thought this data might be of interest to anyone considering shunt 
modifications. I know those that happen to manage NST shunt removal like 
the fact they have (more power, baby!), but not everyone makes the 
measurements following the modification. And considering what I found, 
1/2 the shunts removed might be stretching (or not depending on your 
view of things).

Take care,
Bart