Weird coil configuration, does it classify as parallel LC circuit?

[Dejan567]

I have two spiral-wound conical coils that are 1:1 in turns ratios and mutually coupled and shorted together at both ends. I am applying a signal generator signal at one end of the shorted coils and measuring with my oscilloscope at the other end. I am assuming this behaves as a parallel LC circuit. At resonance, do I expect to get a voltage multiplication effect?

https://www.electronics-tutorials.ws/accircuits/parallel-resonance.html

"Note that the Q-factor of a parallel resonance circuit is the inverse of the expression for the Q-factor of the series circuit. Also in series resonance circuits the Q-factor gives the voltage magnification of the circuit, whereas in a parallel circuit it gives the current magnification."

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-6/q-and-bandwidth-resonant-circuit/

"A practical application of “Q” is that voltage across L or C in a series resonant circuit is Q times total applied voltage. In a parallel resonant circuit, current through L or C is Q times the total applied current."

Chat-GPT 3.5 response is given attached.

Can you clear up my confusion? Is my circuit a parallel LC circuit? Should I expect a voltage multiplication at resonance?

[moffy]

Please provide a photo and schematic of what you are talking about, is this what you mean by conical coil?: https://www.coilcraft.com/en-us/edu/series/what-is-a-conical-inductor/

[Dejan567]

Sorry I can not provide a photo at this time as part of this project is related to work in the private sector.

Yes, that is indeed what I mean by conical coils. I have two of them shorted together at their ends and with 180 degrees difference in orientation. I supply a sine wave input in the kHz range on one end and take the output from the other end. No other circuits attached currently. They are very tightly coupled. This makes them a form of parallel LC circuit (with series LC and R of course added).  I am wondering if this circuit, based on theory alone, should produce voltage magnification at the output or not? I get what seems like conflicting answers from texts above.

[moffy]

Sorry I can not provide a photo at this time as part of this project is related to work in the private sector.

Yes, that is indeed what I mean by conical coils. I have two of them shorted together at their ends and with 180 degrees difference in orientation. I supply a sine wave input in the kHz range on one end and take the output from the other end. No other circuits attached currently. They are very tightly coupled. This makes them a form of parallel LC circuit (with series LC and R of course added).  I am wondering if this circuit, based on theory alone, should produce voltage magnification at the output or not? I get what seems like conflicting answers from texts above.

I am sorry but it is still difficult to work out the windings relative to one another and their electrical connections, a photo of a sketch showing the mechanical winding and their electrical connections would be informative.

[Dejan567]

Please find very rough sketch attached. It is basically two conical coils shorted together at both ends and driven by AC signal.

[wasedadoc]

Please find very rough sketch attached. It is basically two conical coils shorted together at both ends and driven by AC signal.

Just as two resistors or two capacitors in parallel, two inductors in parallel behave as one.

[Dejan567]

Please understand. This is not just two theoretical coils. It is two conical coils that have stray capacitance and resistance as well. I need clarification if this is a parallel LC circuit and if there would be voltage multiplication or not if I tap the same connections as the input at resonance. The resources I listed above make me confused on what to expect or what is true.

[Kim Christensen]

As per your diagram:
If the two coils are coupled and connected in phase, then they'll act like a single coil as stated by waseadoc.
If they are coupled and connected in antiphase then they'll appear as a shorted transformer.
Either way, neither are considered to be a resonant LC circuit.

Perhaps you are thinking about it's self resonant frequency? (The frequency at which an inductor resonates with its parasitic capacitance)
I guess that could be considered to be a LC in a way, but probably a low Q one.

[Sensorcat]

All real inductors show self resonance. It depends on the operating frequency whether or not this is a concern. Since you mentioned using a kHz signal from your source, is this frequency the operating frequency of your application? Can you specify this frequency? Many inductors in practical use have resonance in the MHz range, so it is possible that you don't have to worry about resonance and the coils getting capacitive, because that never happens in your application.

[Dejan567]

Not sure I agree with this assessment. If a coil has it's own self-capacitance then isn't that a type of LC circuit by definition?

Yes, I am looking and asking about the self-resonance of the two shorted coils (what other resonances are there)?

At self-resonance, do I expect a voltage multiplication? It seems confusing to me based on the resources I looked at. What would the Q-factor show as an output voltage/current in relation to the input voltage?

I am supplying a 720 kHz sine wave at the input and tapping the output at the same locations. The application calls for this circuit to be at resonance. I am trying to understand what the output voltage would be in terms of the input voltage. Would the ratio be greater than 1?

[wasedadoc]

That circuit will not give voltage multiplication even at resonance.  The voltage across the effective single inductor is always the same as the voltage from the generator.  The current taken from the generator will be a minimum at resonance.

If you want voltage multiplication you need a configuration where the inductance and capacitance are in series, not in parallel.

[Dejan567]

Well that's the weird part because now I am measuring it and getting a voltage amplification at it's resonant frequency....

How would this be possible? With two shorted coils in parallel does it matter where the tap the input and output to explain this voltage multiplication? To me, it shouldn't matter too much since they are shorted and in parallel configuration....yet, I get voltage multiplication at output....

[wasedadoc]

Well that's the weird part because now I am measuring it and getting a voltage amplification at it's resonant frequency....

How would this be possible? With two shorted coils in parallel does it matter where the tap the input and output to explain this voltage multiplication? To me, it shouldn't matter too much since they are shorted and in parallel configuration....yet, I get voltage multiplication at output....

Provide us with a diagram showing how you have this coil, generator and measuring instrument attached which you say is showing voltage multiplication.

[CatalinaWOW]

Based on your links, each one of these conical inductors is effectively a series string of parallel LC circuits.  The implication is that the unusual geometry of these coils decouples each turn from the next.  I can't offer an opinion on how good that assumption is.  To first order the LC circuits are each a single turn of the coil with its self capacitance.  Very high frequency and high Q because the only parallel resistance with that parallel LC is leakage through the wire insulation.  I would not expect voltage multiplication in a single one of these coils. 

If the big ends of the coils are connected to each other they will be in phase.  Coupling is probably not much of a factor unless you have them tucked inside of each other.  In that case the simple assumption that the coupling between non adjacent turns of the coil may be compromised, but I still wouldn't expect significant amounts of voltage multiplication.

[fourfathom]

Consider the output impedance of your signal source vs the impedance of the parallel L/C circuit over frequency. I wouldn't call this "amplification " but the measured voltage will peak at self-resonance.

[Sensorcat]

Well that's the weird part because now I am measuring it and getting a voltage amplification at it's resonant frequency....

You must take the whole circuit into account, not only what you see. Your generator has an output impedance between the acual generator circuit and the output of the instrument. You set the amplitude of the signal at the first node, measure it at the second. wasedadoc's statement about the voltages beeing the same implies that the output impedance is neglectable.

[wasedadoc]

Well that's the weird part because now I am measuring it and getting a voltage amplification at it's resonant frequency....

You must take the whole circuit into account, not only what you see. Your generator has an output impedance between the acual generator circuit and the output of the instrument. You set the amplitude of the signal at the first node, measure it at the second. wasedadoc's statement about the voltages beeing the same implies that the output impedance is neglectable.

My statement does NOT imply that.

[Dejan567]

I will try to post a diagram later if still needed. The conical coils are tucked inside each other with the big end (the side of the conical coils with a larger loop diameter) is connected to the small end of the other coil. I am inputting a signal on one side of this parallel connection and connecting the oscilloscope probe to the other end of the parallel connection. I want to see how to explain this voltage multiplication I am seeing on the output. The coil turn ratios are basically 1:1 and again both shorted together.

[wasedadoc]

I will try to post a diagram later if still needed. The conical coils are tucked inside each other with the big end (the side of the conical coils with a larger loop diameter) is connected to the small end of the other coil. I am inputting a signal on one side of this parallel connection and connecting the oscilloscope probe to the other end of the parallel connection. I want to see how to explain this voltage multiplication I am seeing on the output. The coil turn ratios are basically 1:1 and again both shorted together.

Your two coils wired together present two and only two points to the rest of the world.  Your signal generator must connect to those two points.  Your oscilloscope must connect to two points.  The only two points are those of the coils and connected signal generator.  Therefore the generator output and oscilloscope input must be on the same points.  No voltage multiplication.

[Dejan567]

Please find as much as I can share for now. I have requested to share more. It shows the input signal I am using at my signal generator, how I connect to the conical coils and the output on my oscilloscope at resonance. The red lead is attached at the top of the shorted coils and the black lead is attached at the bottom. Signal generator ground is floating but I get same results with it connected to Earth ground, just more stable waveforms. Oscilloscope probe is 10x attenuation so please take that into account.

Please ask any questions and I will clarify.

[mag_therm]

Refer to the Coilcraft data sheets for the conical inductors.
https://www.coilcraft.com/getmedia/299d39b5-dce9-4982-b5c7-799e9a3da4d8/bcl.pdf
Because the diameter reduces along the coil length, the distributed Inductance reduces, and the distributed interturn Capacitance reduces.
In a certain GHz range, the coil has approximately a uniform broadband parallel resonance.
Refer to the two graphs for S11 and S21, when a conical coil was connected as shunt element on a 50 Ohm line.
Phase is not given, so assume a resistive loss.
I used qucs to model a pi attenuator in S parameters so I could better understand the losses.  ( Note: I had trouble using two of the online Z to S converters).
 The attenuation by the coil is roughly 4% in voltage.

Thanks Dejan, Very interesting, I did not know about these inductors until today.

[Sensorcat]

Your two coils wired together present two and only two points to the rest of the world.  Your signal generator must connect to those two points.  Your oscilloscope must connect to two points.  The only two points are those of the coils and connected signal generator.  Therefore the generator output and oscilloscope input must be on the same points.  No voltage multiplication.

No, the circuit has three relevant nodes, and the source node of the generator is internal to it, so it cannot be connected by the oscilloscope in normal use. See my attachment to clarify this. Generator output and maesured signal on oscilloscope clearly different. No voltage multiplication, of course.

[wasedadoc]

From the photos I see one connection (from the centre conductor of the BNC cable on the generator output?) to the narrow end of one coil.  I do not see connections of the outer of that BNC, the scope probe, the scope probe ground, the other end of that coil and the two ends of the other coil.

[wasedadoc]

Your two coils wired together present two and only two points to the rest of the world.  Your signal generator must connect to those two points.  Your oscilloscope must connect to two points.  The only two points are those of the coils and connected signal generator.  Therefore the generator output and oscilloscope input must be on the same points.  No voltage multiplication.

No, the circuit has three relevant nodes, and the source node of the generator is internal to it, so it cannot be connected by the oscilloscope in normal use. See my attachment to clarify this. Generator output and maesured signal on oscilloscope clearly different. No voltage multiplication, of course.

1. The point internal to the generator prior to its 50 Ohm output is not accessible.  For the physical connections the composite coil has two points, the generator has two and the scope has two.

2.  The schematic cannot match what the OP claims his words and photo show.  The generator says 10 Volts peak to peak.   Even if that is referring to a 50 Ohm load it means 20 Volts peak to peak inside the generator.  Yet the scope is saying the probe is experiencing 26.4 Volts peak to peak.

[Dejan567]

The two ends of each coil is connected to the other two end of the other coil. That is, both coils are shorted together.

The signal generator input is at the top of the shorted coil configuration and the bottom of the two coils is the oscilloscope probe. Both signal generator GNDs and oscilloscope GNDs are connected together and not touching the shorted coils and then also connected to Earth GND.

Also, the oscilloscope is 10X attenuation so the output voltage is 10 x 26.4 Volts.