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Re: Does it Matter?

Original poster: "Barton B. Anderson" <tesla111@xxxxxxxxxxxxx>

Hi Tyler,

I'll take a stab:
(btw, used a 4x16 toroid to simulate some reality),

Case-1 is the 4x18 w/#26 at 1400 turns.
fo = 175 kHz
hd = 4.5:1
turns = 13.5 (you need more turns on your primary).

Case-2 is a 3x20 w/#28 at 1100 turns.
fo = 306 kHz
hd = 6.67:1
turns = 8.15
dcr = 56 ohms
zo = 24.6 Kohms
les = 12.5 mH
ces = 21 pF

Case-3 is similar to case-1 with larger wire = 4x18 w/#24 at 900 turns.
fo = 306 kHz
hd = 4.5:1
turns = 8.23
dcr = 21.5 ohms
zo = 24.4 Kohms
les = 12.6 mH
ces = 21.2 pF

It's all personal preference, but I would note that in case 2 & 3 above, zo, les, and ces are all similar. Dcr is however more than double on case 2. Given those specs above, I would pick case 3 with a wider secondary, larger wire size, less dcr, and since there is little change in the effective inductance. But then, if I had #26 wire on hand, I would just build case 1 and build a primary to suit. Doesn't really matter too much. The performance between these coils is hard to compare.

Regarding your question with the sec to pri height relationship. The two coils are mutually inductively coupled. It is their proximity to one another which sets up the transfer rate of energy between the two coils which is partly a function of the mutual inductance between the two coils. The mutual inductance can be looked at mathematically as a ratio of electromagnetic coupling between the two coils. The ratio is known as the "coupling coefficient" or "k". Typical values of k are 0.13 to 0.22, with the average nearer to 0.15 (on the "classic" 2 coil systems). The closer the two coils are, the faster the transfer rate (number of cycles required to transfer all the energy from point A to point B) because the mutual inductance is greater (k is higher). It typically takes 2 to 4 full cycles for full energy transfer from one coil to another. The faster the transfer rate (fewer cycles), the higher the stresses on the secondary. If too high, secondary breakout occurs. This is typically seen as racing sparks down the secondary. Note, if we decrease the proximity between the two coils (add more space) the transfer rate decreases as does the sec stress. We could also add so much space that "no sparks" achieve breakout. So, there is an adjustment. We want to transfer energy as fast as possible without over-stressing the secondary. This adjustment is known as "adjusting the coupling". It is usually performed by raising/lowering the secondary in relation to the primary (but move whichever coil is easiest to move). Hope some of that makes sense.

Take care,
Bart B.

Tesla list wrote:

Original poster: Tyler Pauly <rpggod714@xxxxxxxxx>

Hello,
 I have a coil in construction. Powered by 9kv,30ma
NST, copper pipe spark gap, 13.5 nF MMC (11) caps, and
primary of 12 turns of .25 OD copper tubing in flat
spiral. Two questions:

1. What would be best for my sec.: short fat(18x4 PVC)
coil with 28 AWG-it would be about 1400 turns- or
slightly taller and skinnier with 26 AWG about 1100
turns? I know that there are no set rules for best
results, but I'd just like your thoughts.

2. I read a post about arcing between primary and
secondary coils. This reminded me about a question:
Where do you put the secondary in relation to height
to the primary i.e. the secondary starts 2" below the
primary, it starts 1.75" above, etc. what does this
change? I'll only need to ask these questions once so
bear with me.

            Thanks for your patience,

                      Tyler