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Re: Series or Parallel resonant ?
Hi Dale and all,
In a typical TC schematic the base of the secondary is grounded, and
the top of the secondary has capacitance back to ground. So my initial
opinion was that this is best represented as a parallel resonant
network. That is how it appears on paper anyhow.
However, as you correctly pointed out, voltage is actually induced
into the lower few turns of the secondary due to transformer action
with the primary circuit. This changes the "equivalent circuit" such
that it appears as a series resonant circuit with a voltage source at
the base of the secondary inductor. See below:
______________
! ! <--- Top of secondary
! !
! !
) !
) Ls __!__ Cs
) _____
) !
! !
_!_ !
/ \ !
\___/ Vac !
! !
! !
!______________! <--- Ground
Vac is the voltage induced into the bottom hundred or so turns by
coupling with Lp. Ls is the secondary inductance, Cs is the total
secondary capacitance. This is obviously a SERIES RESONANT CIRCUIT.
Of course you are right, it is me that is getting confused !
The situation is even more clear in the case of the extra coil in a
magnifier. This coil is end fed from a stiff voltage source and also
behaves as a series resonant circuit. In this case Vac represents
the output of the driver.
I agree that the primary circuit is parallel resonant when the spark
gap is in conduction.
You are the first person who has spotted this mistake. Thankyou for
pointing it out, Dale. I will correct the web page as soon as I can.
You lost me with the bit about secondary discharges being represented
as an equivalent Sec series resistance. I am not too hot on the 1/4
wave theory of the secondary. Although I have heard that loading at
the top of the secondary causes a rise in base impedance.
Any more comments on this topic ?
Cheers,
-Richie,
> Original Poster: Dale Hall <Dale.Hall-at-trw-dot-com>
>
> One befuddlement:
> The Secondary is referred to as being parallel resonant.
> At parallel resonance, voltage is minimum across LC
> and minimum current is sourced from a parallel generator.
>
> In a classic TC, Primary energy and its loss resistance
> is coupled as a low impedance series generator into the Secondary,
> effecting a Series Resonant condition.
> The generator current is maximum (requiring a low Z source),
> limited by Rdc_inductor+SeriesEquivCoupledPri&ArcLoad,
> the LC junction (top of Sec) voltage is maximum at resonance.
>
> Discharges represent resistance in parallel with both Sec L & C,
> that can be represented as an equivalent Sec series resistance.
>
> Comment ?
> Am I confused ? perhaps series=parallel depending on how viewed ?
>
> The Primary LC is parallel resonance since energy is provided
> in parallel from precharged Pri Cap prior to a brief switch closure
> to transfer stored energy.
> When the switch is open the generator charges the Cap in
> a dormant, not part of the resonant period, isolating its losses (?).
> Perhaps the confusion is due to the impulse excitation of TC's Vs
> continuous
> nature of the generator (signal source) so a comparison loses meaning ?
> Help ?
>
> Regards, Dale