Re: LTR question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

Steve,

I think what is confusing you here is the defenition of 'resonant
capacitor'. 'Resonant' does not refer to the LC primary resonator circuit -
it referrs to the transformer capacitor circuit. A resonant cap will reach
peak charge at the same point as the AC sine curve reaches peak voltage.
When the SG fires, then the resonant rise process will begin in the LC
circuit. The reasons for using an LTR cap are stated below. I agree with you
that there would be no point in using an LTR primary circuit, because then
the coil would not be resonant, just badly tuned.

> I have been reading a lot of posts lately about LTR (lower than resonant)
> capacitors.
LTR is LARGER than resonant. STR is SMALLER than resonant

> This term does not make sense to me. Any capacitor in combination
> with an inductor will form a tank circuit which will resonate at a
frequency
> determined by the values of the L and C. Is LTR refering to some sort of
> stagger tuning where the primary tank circuit is resonant at a slightly
> different frequency than the secondary tank circuit? If so, to what end?
No. The point of using an LTR capacitor is for 3 reasons - one is if you
need the primary to be smaller, and this will mean that because of a bigger
cap you can get a smaller primary. The other advantage of using an LTR cap
is to increase the power into the primary. With a faster than 100 (or indeed
120) break gap this means that you can get more energy into the primary
coil. Also, the cap sees less voltage from the transformer because of the
reasons mentioned at the bottom.

> In either case, a common suggestion that is offered to achieve this "LTR"
is
> to use a slightly smaller or larger capacitor than one would normally use.
>The same result could be achieved by merely changing the tap position on
the primary inductor
> rather than going to the trouble of changing the capacitor.
This is what is done (except with a larger not smaller capacitor). If the
cap value is increased then the tap can be moved further into the primary,
thus decreasing the inductance. this decrease in inductance and increase in
capacitance means that the primary resonates at the same frequency as
previous resonant cap.

> Another reason often cited for using this "LTR" value of capacitance is
that
> it lowers the peak voltage that the capacitor is exposed to. This does not
> make sense to me either. The resonant frequency of the tank circuit should
have
> little effect on the voltage passing through the tank circuit.
The charging curve for a capacitor is an inverse sinusodial curve. This
means that when the current is high (in the early stages of charging a cap)
the voltage across the cap is extremely low. With a resonant cap, the cap
reaches peak charge at the same point as the voltage sine curve from the
xfmr reaches 90 degrees. With an LTR cap the cap is still charging when the
sine curve reaches 90 degrees, thus the peak voltage across the an LTR cap
is lower at any given point than with a resonant cap.

>If stagger tuning is being referred to, then there may be some rationale
here. Any
> insights?
I have never heard of stagger tuning being used in TC primary circuits.

I hope this helps - any other questions just ask :)

Regards,
Jason