Re: [TCML] poor coil performance again- help!

[Mddeming@xxxxxxx]

Hi Thomas,
 
I would like to add a couple of points to what Bart has pointed out:
 
1. For most TC purposes, the operating frequency is a consequence of  design, 
not a target. Frequency varies inversely as the SQRT(L x  C). The larger the 
secondary, the larger the L and the larger the internal  C, therefore, the 
lower the frequency. Likewise, tiny secondaries can't provide  much L or C and 
therefore have very high operating frequencies.
 
2. The "rules" for L and C are:
 L(pri) x C(pri) = L(sec) x C(sec) and
 Vout(max) = Vin x SQRT(Lsec/Lpri) 
 
3. I have observed that the best spark formation occurs when the  topload 
capacitance equals or exceeds the internal capacitance of the secondary  coil. It 
seems that having more of the energy "out there where it's needed" aids  in 
leader formation. This condition is almost always met by the toroid  sizing 
rule-of-thumb, " minor diameter = sec diameter, etc.". Of course, if the  toroid 
is too big, then most of the energy just gets stored there, and no  breakout 
occurs. 
 
Hope this helps.
 
Matt D
 
In a message dated 4/24/08 10:08:31 P.M. Eastern Daylight Time,  
bartb@xxxxxxxxxxxxxxxx writes:

Hi  Thomas,

Regarding LC values, there are maybe preferences by some, but  ratios, 
not so much in the secondary. It seems larger L in the primary  helps to 
keep gap losses lower than they would be for a small L primary  due to a 
higher impedance (affects current peaks). But this is still  speculation 
and it may simply be the gap itself for currents  involved.

In the secondary, energy is stored in the inductance and  capacitance of 
the coil. Obviously, more L or C, the more energy that can  be stored, 
but "how" you increase those values makes a difference and in  my little 
head, geometry (shape and size) plays a "very" significant role.  For 
example, say I have a 5:1 h/d coil using 24 awg at 1111 turns (5" x  
25"), and thus L, C, dcR, acR, Q, etc.. all have some value.

Case 1  = I double the "length" of the coil (5" x 50", 2222 turns):
>From the  original coil, L increases 200%, C increases 150%, dcR 
increases 200%, acR  increases 135%, Q decreases 110%.
Even though I doubled L, I introduced  large losses both at DC and at RF. 
Although L and C increased, the coil is  lossy.

Case 2 = I double the "wire length" to match Case 1 but return  h/d to 
5:1 (10" x 50", 2222 turns).
>From the original coil, L increases  200%, C increases 200%, dcR 
increases 200%, acR decreases 115%, Q  increases 115%.
Better than Case 1 and the reason is the geometric shape as  far as h/d 
is concerned. The coil will work, but still lossy.

Case  3 = I double the coil size as in Case 2, but now I increase the 
wire size  to have the same turns as the original (10"x50", 1111 turns).
>From the  original coil, L increases 150%, C increases 150%, dcR 
decreases 150%, acR  decreases 130%, Q increases 130%.
Here is better coil than Case 1, Case 2,  or the original. It's h/d is 
identical to the original, but it's geometric  size has changed. We get a 
good LC increase, some major reduction in DC  and RF impedances, and a 
nice jump in Q.

The LC ratio is not that  important to me as is the geometry. Both L and 
C store and release energy.  The larger the coil is, the more energy that 
can be stored and released,  but note that it's "shape" or h/d is also 
important for maximizing not  only LC but also minimizing losses. The 
basic guides like the range of  turns, h/d, etc. are mostly empirical in 
that a range of h/d has been  found to work well via experience, but it 
also can be supported with  numbers. Someone only looking at say the 
above data might think that  larger wire is always better, but that is 
"not" the case. Turns, wire  size, and coil geometry are important 
aspects for an efficient coil. These  aspects will dictate LC ratios.

Your 22 awg is perfect for a nice range  of coils. Suppose you select a 
1000 turn coil and you want to use 22 awg.  22 awg is .0279" including 
insulation. So, .0279 x 1000 = 27.9" length  coil. Say you want a 4.5 h/d 
ratio. 27.9" / 4.5 = 6.2" diameter. This is a  decent medium sized coil. 
So look for a form similar in diameter. It might  end up right at 6". 
That's ok. It only means the h/d ratio will slightly  increase to 4.65 
which is fine. You end up with 6" x 27.9" winding length  at 1000 turns 
using 22 awg. Run some numbers and you'll find that a 12/60  NST running 
a static gap will be LTR at .02uF. This coil would need about  a 10 turn 
primary. Throw a decent airflow gap in and you'll have a nice  coil. This 
coil would have a Q a little above 300 which is pretty  good.

If you go with higher power on "this" coil (which it could handle  200mA 
nicely) and if you try to stay LTR with the transformer, then the  
primary turns decrease, but that's the trade-off and it's not  
necessarily a bad one if kept within reason.

BTW, above when I say  "energy is stored", don't misunderstand. It's not 
stored over several  energy transfers or cycles, it's stored during a 
single energy transfer  event. The remaining stored energy simply 
transfers back to the primary  tank circuit minus losses encountered then 
back to the secondary until  there's no more energy to keep the gap 
conducting. For C, energy is stored  in the turn to turn capacitance, 
turn capacitance to ground, top load,  external objects to turns, etc.. 
For L, energy is stored in the magnetic  field.

For frequency, the lower the better as RF losses are reduced.  But this 
isn't usually a big worry unless the wire size is small and coil  is 
small. Frequency affects AC resistance of the coil and Q. Some will say  
Q isn't important because when the coil sparks, Q drops like a rock.  
Well, yes their right, but I don't care about Q during sparking, I care  
about Q only after the gap quenches and before a spark occurs. So for  
me, it's important to keep secondary Q as high as possible and to  
totally ignore spark time Q because it's not relevant to the coil in any  
way. The off-time is when Q has a clean slate and if the secondary has  
an "off-time" Q of 300, then that is a nice number to have. 300 Q in a  
medium sized coil is what I personally shoot for (or better).

Take  care,
Bart

Ryckmans, Thomas wrote:
> Thanks, I think I will  just design a new one. By the way, are there
> specific rules about  ratio of L and C in the primary and the secondary?
> In other words, for  both resonant cicuits, what should be the relative
> contribution of L  and C to resonance? And what resonance frequency
> should I aim for? As  low as possible?
>
> Many thanks
>
>  Thomas
>   


 



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