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RE: Series or Parallel resonant ? ( was: New pics and scope waveforms)

Hi Malcom and All listening,

Oops, I stand corrected, 
brain was thinking generator perspective, spoke LC perspective :(  

Malcom correctly states (re-phrased), 
at resonance for both Series and Parallel excited generator modes
 circulating LC current is maximum producing maximum voltage at Xc=XL.
 Line current, i.e. from generator source, is opposite.

Series resonance:
1.) defined as the Freq where XL=Xc, only 1 freq meets this.
2.) Current source generator drive is a lowZ in Series w/LC.
The Generator passes the high series circulating current, adding energy. 
XL=Xc cancel and generator internal resistance adds to
 inductors Rdc and Cap ESR.
Since the generator is in series providing new current plus passing 
circulating current,
 the voltage across LC is limited only by the ability of the
 generator to source current via low internal impedance (else lower Q) or
 less than infinite Q due to losses (load producing parallel resistance
 which can be transformed to represent equivalent series resistance).

Parallel resonance:
1.) defined as 
    a.) minimum generator current, or
    b.) maximum impedance, or
    c.) V & I in phase
    a., b., c., each represent separate distinct (but close) resonant
frequencies for Q=<10, above which the three converge to one freq.
Applic note: Q before spark load is likely >10, afterward <10.
The voltage across LC is supplied by the parallel generator.

2.) is modeled as a Voltage source in parallel i.e. across LC. 
LC Reactance is highest at resonance, diminishing before and after.
So, generator source line current is highest either above or below Fres.
Approaching Fres a parallel generators source line current dips to a minimum

due to rising voltage across LC as circulating current increases to max
(XL-Xc=0 so Igen = Vin/Rdc).

Hope I have this right now, and that it is relevant !

Regards, Dale

-----Original Message-----
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, December 19, 1999 4:23 PM
To: tesla-at-pupman-dot-com
Subject: RE: Series or Parallel resonant ? ( was: New pics and scope
waveforms)

Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz> 

Hi Dale,

> Original Poster: Dale Hall <Dale.Hall-at-trw-dot-com> 
> 
> Hi Malcom,
> Thanks for your response.
> Please see my earlier reply to Richie's feedback.
> 
> I disagree with your response:

> >The truth is that both 
> >types exhibit a current maximum and a voltage maximum across 
> >their individual components at resonance. 
> 
> I believe the essence of whether LC parallel components
>  are series or parallel resonant depends upon where 
>  the source generator is in the circuit.

How one classifies the circuit depends on how one applies energy 
to it, that I agree with. However, any tuned circuit has a circulating 
current which is a maximum at resonance and byu Ohm's law, the 
voltage across each individual component in it is a maximum also.

 Series resonance: generator is part of the series LCR.
> All Rdc's are in series.
> 
> Parallel resonance: generator is across "Both" L and C physically
parallel.
>   The generator sees L's resistance different than in the series model
>   in that C is across the generator, L+RdcL is across the generator.

The generator sees the shunt impedance of the circuit in this case 
whereas in series resonance, the generator sees the ESR. 

> In this context, I stand by my original statements.

I must differ on the contention that voltage is minimum across a 
parallel circuit at resonance.

> I agree this is all very confusing, I hoped to make it clear (?) sorry.
> 
> There appears to be easy confusion stemming from components in parallel
> contrasted to how they are driven,
>   generator in series with parallel components (Igen=max) or
>   a generator driving across parallel components (Igen=min).

Agree

> In your radio set front end example
>  assuming parallel resonance,
>  the incoming signal (a generator) appears across both L & C at same time,
>  the voltage must be minimum else the current couldn't be minimum
>  to represent minimum load to the incoming signal at Fo.

Aha!  Suppose the voltage was a minimum. The circuit would 
present a near short circuit to the aerial. Aerial current is a minimum 
as you say but the circulating current in the tuned circuit is a 
maximum.

 I agree with 
> >the secondary is more driven at Fr than by an impulse,
> sorry if I was misleading....
> 
> even the Primary simply and highly selectively at Fr extracts the
> fundamental
>  of the square impulse provided by a spark gap switch.
>  Current injected by the gap switch is accepted by the Pri LC
sinusoidally,
>  not as rectangular impulse, due to the frequency selectivity of the LC
>  and that rectangular waves are composed of sinewaves which LC is able to
> isolate.
>  The LC actually highly rejects all energy outside the fundamental
resonant
> freq.
>  High order harmonics see a very high impedance producing little current
>  due to their divergence from the highly selective resonant frequency. 
> 
> Experiment: set up a lowZ (simulating Rgap) square wave generator driving
>  a HI-Q toroid inductor and mica cap in series resonance.
>  The fundamental sinewave provides most of energy at resonance benefiting
> from 
>  Vpk_fund = 4/Pi*Vpk_squarewave peak {flat top] amplitude !
> 
>  Placing a high value load resistor across LC demonstrates
>   affects of De-Q'ing, equivalent series resistance, etc.
>   simulating Spark loading,  without sparking !

I think a resistor is a poor representation of an arc discharge as the 
characteristic is wrong.

Regards,
Malcolm
<snip>