Re: What is the difference between LTR and reso-caps ?

["Barton B. Anderson" <tesla123-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Alan, 

Tesla list wrote: 
>
> Original poster: "Kelly & Phillipa Williams" <kellyw-at-ihug.co.nz> 
>
> Hello, 
>
> I am a little confused about the difference between "resonant capacitors" 
> and 
> "LTR capacitors." 
> What are the definitions of these two terms?

Resonant Capacitor is a capacitor which is equal in reactance to the
transformer reactance. The reactances when equal will cancel out oneanother
because they are opposite reactances (in a perfect world). "Ideally", when this
occurs, current is maximum, in phase with the transformer voltage, voltage is
max across L or C, and the impedance is minimum. Resonance can occur at any
frequency, however, the frequency which a resonant capacitor is attempting to
match is the 60 or 50 Hz line frequency. 

LTR (Larger Than Resonant capacitor). It is just that, actually larger in
capacitance than the line frequency cap size. Larger capacitance can store more
energy, however, they take longer to charge considering equal current and
voltage. LTR's have been used by several coilers with success with NST's, but
the actual advantages is best to come from the coilers who have run in this
mode (NST protection, running with a sync gap, etc..). 
>
> Is it correct that with a reso-cap, the transformer amperage is fixed, but 
> with 
> an LTR cap, you can raise the amperage? (addition of more NST's) 
> This is probably wide of the mark.

No. Both transformers will charge at a rate determined by the capacitance and
their current available. Increase current (more NST's), and you increase the
"rate" of charging the caps (any cap size). The size of the capacitance and
voltage level will determine the energy stored in the cap. The voltage at
conduction (when the gap fires) will identify the energy in joules by the
equation: 
j = .5 * C * Vp^2 = increase Vp or C and you increase cap energy. 

An LTR cap will have more energy. During charging, every capacitor will charge
as fast as it possibly can at the rate it is allowed. The larger the cap is,
the slower it charges to a breakover voltage and the slower the BPS will be.
The bang size (cap energy) will also be more than a smaller cap at the same
voltage. 
>
> With a power source of 15kV and 120mA, (NST's) 
>
> Resonant capacitor size is 25.46 nF (.02546uF) 
> and with this value capacitor I cannot lower the amperage??
> And LTR cap size is somewhere in the region of 50nF 
> And with an LTR sized cap I can raise and lower the amperage?? 
>  

These are explained above. 
>
> Which size produces the biggest spark for 120 mA? 
>  

All depends on your system, your losses, etc.. The bigger cap will have the
larger cap energy and should give the biggest bang increasing spark length, but
not always. The larger cap typically reduces the primary turns and can increase
gap losses (considering nothing changes on the secondary). But, the primary and
secondary may be designed for the bigger cap, your transformer may be adequate
to charge it, and would then do just fine. 

If the cap is too small, it can charge WAY over the voltage applied to it if
the static gap is set wide enough. For example, if the charge time is really
fast and your gap is set for 60KV, the cap will continue to charge until it
reaches the breakover voltage (obvisouly, dangerous to the cap and NST) but
will still have a relatively fast BPS rate due to it's fast charge rate. The
breakover can occur anywhere along the AC cycle (except zero crossing). Where
the break will occur along the AC cycle is determined by the charge rate and
how long it takes to reach 60KV. 

A big cap will do exactly the same except the charge rate is slower and the
bang is bigger once it reaches the gap breakover voltage. 

With an RSG, the break is determined by the gap timing. Here is a problem for
NST's. In the above paragraphs, the static gap will arc when the breakover
voltage is reached, but in an RSG, the timing is in control. In an extreme case
of a really small cap that charges very fast with a powerful transformer, the
voltage can reach extreme levels between RSG timing breaks and can kill
components (NST's, caps, etc..). 

BTW, I looked at some simulations with a parallel static gap accompanying an
RSG, and in this situation where the cap charge time is fast, the RSG timing is
slow, the static gap takes over. This makes the RSG useless (IMO). But again,
this is with extreme cases. 

A bigger cap will have a longer charge time where an RSG could be used,
however, a non-sync RSG will break at different points along the AC cycle and
perodically not conduct allowing the voltage to build, again killing
components. SRSG's can be set to fire at the same points along the AC cycle,
thus if set correctly, should break continuously (in a perfect world). 

Regardlesss 
>
>   
> Also, 
>
> I calculated that an Archimedes flat spiral primary coil using 0.1875 OD 
> tube (3/16 in) 
> 10in inside diameter,                    (8" secondary form, so 1" 
> clearance) 
> with 0.5 in spacing, and 20 turns, 
> would have an average radius of 13.25" and a width of 9.125". 
> This works out to an inductance of 152.26 uF - microHenries OR  0.15226 
> milliHenries - mH. 
> Is this far too large? 
>  

Sounds about right. Should be fine. 
>
> (high input voltage, small capacitor, large primary surge impedance for low 
> gap losses, 
> with large secondary form 8" OD 40" wound with 24AWG to approx 1600 turns.)

Yes, this coil sounds like it will do very well! 

Take care, 
Bart 
>
>   
> Thanks A LOT, 
>
> Alan Williams