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Re: [TCML] Spark gaps, Solid state switches and diodes

Hi Stephen,
Stephen Hiscock wrote:
Bart and fin wrote:
"I get around 600 something turns....... "
"Yes, 648 turns at 32 tpi. I would expect Ipeak to range 185A to 1130A (15/30 up to 15/180). The primary details are important as geometry greatly influences Lpri and thus Ipeak. Top loads play a role also since primary resonance is based on an accurate f2 with all components in place. So if the system is listed in some detail, then Ipeak is better guesstimated."

The figures I gave you are wrong, but How were the figures above calculated?
I assume your asking about the currents given.
Ip = Vp x sqrt(Cp/Lp), where Vp = 15kV x sqrt(2) = 21.2kV. C and L are from the cap and primary figures.

Say we look at five 15/30's with 32nF. The primary should tune in at about 5.9 turns with an L of 22.3uH.
Ip = Vp x sqrt(Cp/Lp) = 21200 x sqrt(32nF/22.3uH) = 803A.

I meant to say that the Primary is 30 to 100 micro Henries (not microFarads) sorry silly mistake. and the Secondary is 30.25 inches winding length at 32 turns/inch ( again sorry) so 968 turns total
the former is 6.5inches diamater
The top toroid is 6 inch diameter ring and 6 inch diameter inner ring and 18 inch total outside diameter
eg: (circles represent the toroid) = in courier font it should look ok
 OOO           OOO
 OOO           OOO             |<-6->|<--6-->|<-6->|
|<----18 inches---->|

The secondary should be roughly 185kHz assuming 22 awg at 968 turns with the following toroid.

yeah the Primary circuit would have two of my transformers as a minimum (and 6 at a maximum) I fired it up the other night I used 2 transformers rated at 15kV and 30mA each
so 15,000 volts at 60mA.
12nF is my primary capacitance (but as a maximum I could have 36nF)
The primary was tapped at the 10th turn out from the middle. (and as a maximum could be the 4th turn - but its not working properly)
The primary is 12.5 turns total Turns
The dimensions are 13.5 inches inside diameter and 37 inches outside diameter.
with 1 inch between turns.
as it is at the moment I'm only getting 6 inch sparks at the best tuning!
Yipes! That's a problem to deal with, but not related to the question (think "gap" for that problem).

I'm seeing a tune point of 9.6 turns with the 12nF, so your 10 turn tune point as listed above is close enough that I know I'm near.
So in this situation:
Ip = Vp x sqrt(Cp/Lp) = 21200 x sqrt(12nF/60.85uH) = 298A.

And with this you can see how L and C dictate Ipeak as compared to the 5 NST case earlier. If a six NST case, then near 1000A.

(but sometimes I would hook up extra capacitance and transformers)
each transformer I have is 15,000 @ 30mA so my capacitance for each transformer should be 6nF right?
Not necessarily. 6nF is almost resonant with a 15/30 NST. We try to stay away from transformer resonance as the voltage is allowed to rise above and beyond what the NST secondary winding insulation can handle (NST's can die if the static gap is opened up wide enough). So we try to go a little larger than resonance if possible. Roughly, 1.5 x Cres value (but it does vary from coiler to coiler at about 1.3 to 1.5 x Cres on average).

You should adjust the gap without the cap across the NST first (just the gap and NST on a workbench). Open the gap until the NST cannot fire. Then close up the gap until the gap just fires consistently. Leave it there from then on. This simple procedure ensures your gap voltage is clamped at it's maximum cold value (and saves NST's from a needless death).

The tuning point for the biggest capacitance I can't verify because I think my capacitor died.
But it should scale up right when I repair the capacitor bank?
just like the primary transformers? eg if I use 2 x the capacitance then the peak current should be twice as large?
Yes, if you go from 12nF to 24nF "and also" adjust Lpri accordingly for resonance, then yes, the current will be 2X. But if you simply add the C without tuning Lpri, then it will be less.

Is that enough info for a ball park Ipeak figure so I can work what IGBT's I need for a SISG?
It's fine for now. However when listing the primary in the future, give tube size and if the spacing is edge to edge or center of wire to center of wire if you happen to list that spec (actually with ID and OD, tube size is really all that's needed). The ID's and OD's were the most important and you gave those. The info we had was enough to guess on the tube size and spacing reference.


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