Weird coil configuration, does it classify as parallel LC circuit? - Page 1
Forum section: electronics / projects

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 03:53:36 am
content: 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?

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2024, 04:07:41 am
content: 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/

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 04:56:33 am
content: 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.

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2024, 06:10:56 am
content: Quote from: Dejan567 on May 12, 2024, 04:56:33 amI 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.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 02:39:40 pm
content: Please find very rough sketch attached. It is basically two conical coils shorted together at both ends and driven by AC signal.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 02:52:32 pm
content: Quote from: Dejan567 on May 12, 2024, 02:39:40 pmJust as two resistors or two capacitors in parallel, two inductors in parallel behave as one.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 03:01:26 pm
content: 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.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 12, 2024, 03:21:10 pm
content: 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.

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 12, 2024, 04:19:37 pm
content: 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.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 04:40:35 pm
content: 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?

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 04:57:54 pm
content: 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. Thecurrenttaken 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.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 05:03:57 pm
content: 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....

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 05:07:56 pm
content: Quote from: Dejan567 on May 12, 2024, 05:03:57 pmProvide us with a diagram showing how you have this coil, generator and measuring instrument attached which you say is showing voltage multiplication.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 12, 2024, 05:10:00 pm
content: 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.

author: fourfathom
author_rank: Super Contributor
timestamp: May 12, 2024, 05:12:34 pm
content: 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.

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 12, 2024, 05:17:55 pm
content: Quote from: Dejan567 on May 12, 2024, 05:03:57 pmYou 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 05:21:59 pm
content: Quote from: Sensorcat on May 12, 2024, 05:17:55 pmMy statement does NOT imply that.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 05:26:00 pm
content: 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 05:38:50 pm
content: Quote from: Dejan567 on May 12, 2024, 05:26:00 pmYour two coils wired together presenttwo and only twopoints 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.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 05:41:36 pm
content: 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.

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 12, 2024, 06:27:52 pm
content: Refer to the Coilcraft data sheets for the conical inductors.https://www.coilcraft.com/getmedia/299d39b5-dce9-4982-b5c7-799e9a3da4d8/bcl.pdfBecause 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.

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 12, 2024, 06:37:57 pm
content: Quote from: wasedadoc on May 12, 2024, 05:38:50 pmNo, 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 06:39:27 pm
content: 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 06:56:51 pm
content: Quote from: Sensorcat on May 12, 2024, 06:37:57 pm1. 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.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 07:10:20 pm
content: 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.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 07:30:32 pm
content: Quote from: Dejan567 on May 12, 2024, 07:10:20 pmLike pulling teeth!Finally you have revealed that you have a series LC circuit. The series C is the input capacitance of the scope probe. Yes, in that case you can expect the probe to experience a higher voltage than the generator is producing. But maybe not 264 Volts. Almost all scopes have a menu to select 'x1', 'x10' and maybe others. If that is set to 'x10' the scope has already done the calculation and the displayed 26.4 is correct and should not be multipled by 10 again by you.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 07:43:57 pm
content: I will have to correct the point about 10x and my oscilloscope. As a sanity check, when I connect my 10 Vpp input signal to my probe I see 10 Vpp at 1x. When I switch to 10x, the Vpp displayed drops down to ~ 1V. So I still need to account for this in my measurements manually. Perhaps there is something in my oscilloscope that can be adjusted in its settings to account for this itself, but the measurement I showed did not do that so there is a 20x voltage multiplication.Also, from Chat - GPT:"However, it's important to note that this voltage multiplication occurs across the individual components (inductor and capacitor) and not across the entire circuit. The total voltage across the series LC circuit will still be equal to the applied voltage."Can you clarify as I am measuring across the entire series LC circuit?

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 08:19:09 pm
content: Quote from: Dejan567 on May 12, 2024, 07:43:57 pm1. The x10 probe attenuates the signal. The 10V p-p at the probe tip becomes 1v p-p at the scope's BNC socket. Look in the scope's manual and I expect you will see there is some setting to inform the scope whether you have switched the probe to x1 or x10.2. You arenotmeasuring across the entire series LC circuit. The generatorisacross the entire series LC circuit. The scope isnot. Your measuring apparatus is in series with the inductance. The input capacitance of the probe and scope is the series capacitance. It is the voltage across that only capacitance, not across the inductance and capacitance that the scope is displaying.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 10:02:05 pm
content: Yes I am aware what the x10 probe is doing.Thanks for clarifying the input capacitance of the oscilloscope probe, that is helpful.What would be the formula to calculate V_out based on the series LC configuration and a given V_in? Assuming of course there is non-zero R.

author: fourfathom
author_rank: Super Contributor
timestamp: May 12, 2024, 10:26:29 pm
content: Quote from: Dejan567 on May 12, 2024, 10:02:05 pmThis is apparently not a series LC, but a series L into a shunt C, which happens to be an impedance-matching circuit. With the proper component values and frequency, the voltage step-up can be as much as SQRT(Rsource / Rload). With a 50 Ohm source, and a 10 Meg load (the oscilloscope input R) this will be 447 x the input voltage. With real-world inductor loss the step-up will be significantly less.In this case we can probably ignore the inductor self-resonant parallel capacitance. The typical 'scope input capacitance is about 15 pF when using a 10x probe. I don't know what your inductance is.I can't go beyond this without pure guesswork. What are you really trying to do, and what are you attempting to measure or accomplish? Why two parallel inductors? What frequency range?And your first diagram showed a parallel resonant circuit, not what you later described. If this stuff is new to you that's nothing to be ashamed of, but without more information it's going to be hard to give you useful advice.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 10:32:18 pm
content: Well everything you measure voltage across with an oscilloscope is considered a shunt, isn't it?This is not new to me, was just trying to understand how I was getting voltage multiplication with this setup. I don't think we can ignore the self-resonance of the coils since there is a definite resonance point. If we were to ignore that then we wouldn't expect a voltage magnification spike for a range of input frequencies.I'd rather not get into specifics of the application at the moment.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2024, 10:47:13 pm
content: With the circuit topology you actually have, the self resonance of the coil with its own parallel capacitance makes it higher impedance whichreducesthe voltage displayed on the scope.

author: Dejan567
author_rank: Contributor
timestamp: May 12, 2024, 10:52:51 pm
content: So seems like there are opposing forces? Some factors are reducing the output voltage others are multiplying it?Just trying to become less confused by estimating what the output voltage would be for a parallel LC circuit followed by a shunt C. It seems like there are alot of conflicting factors that could lead to a range of effects depending on exact values. I don't have the L or C values of the coils/scope but the resonance point is around 700 kHz.

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 12, 2024, 11:46:48 pm
content: Quote from: wasedadoc on May 12, 2024, 06:56:51 pmThat's what I explained above. However, for the behaviour of the circuit the accessibility is unimportant. There is a voltage drop across the internal resistor depending on the load, so it does not matter it you prefer to see only two nodes, it's three. And because the node at which the oscilloscope probe connects and the signal source are not the same, your claim that the voltages must be the same is wrong. It is staring to get funny, because in Reply #18 you insisted that the two voltages must be the same, and now you argue against me with the measurement of the OP that they are different.It was not my intent to show something that matches the - still obscure - configuration the OP has, it was my intent to demonstrate that your claim from Reply #18 is wrong. Do you know that most generators have a setting for 50 ohm and Hi-Z output that does not change the output circuit, but only the amplitude display?

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 13, 2024, 12:03:07 am
content: Quote from: Dejan567 on May 12, 2024, 10:52:51 pmIt is still unclear what you actually have, partly because you did not show everything and partly because you do not use standard nomenclature. For instance, the two coils are not shorted, they are in parallel. May sound like hair-splitting, but there's a potential problem when you are creative with your own words and we don't realize it. No offence!May I suggest that you disconnect the coils and measure the unloaded output of the generator with your ocilloscope in an otherwise unchanged configuration? This will make some points clear, for instance how the instrument settings affect your result. There is this output impedance setting I mentioned above, and I wonder if your scope acconts for the 10x probe or not. Do you perhaps have 10/1x switchable probes?

author: Dejan567
author_rank: Contributor
timestamp: May 13, 2024, 12:22:01 am
content: Yes probes are switchable 1x - 10x. That means with my current arrangement as I mentioned before the 10 Vpp with the probe shows ~ 1V.Two coils being shorted together at both ends means they are in parallel connection so not sure what the confusion or difference is there.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 13, 2024, 12:42:18 am
content: Quote from: Dejan567 on May 12, 2024, 10:52:51 pmOf course there are, which is why in a circuit with this many unknowns, you are best to do exactly what you have done :ie Measure it,ideally under conditions as close to your desired operating, as possible.A signal generator (+ series RS) is ok for quickly finding any peak, but you should use yourintended driving circuitwhen chasing details like operating Q.

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 13, 2024, 12:57:01 am
content: Quote from: Dejan567 on May 13, 2024, 12:22:01 amDid you actually test that? Since all this is really looking weird, it's time to question everything. Checking the measurement without the coil is one way to do this. So you have, with the generator set to 10Vpp, and probes set to 10x, the scope set at 1x probe attenuation, 1Vpp measurement with coil disconnected and 26.4Vpp with coil connected, nothing else changed?Next test would be to work without the earth GND connection, as it is not needed for this measurement, but sometimes causes problems.Also, you could measure and plot the peak-peak voltage over frequency.Quote from: Dejan567 on May 13, 2024, 12:22:01 amThe point is that usually no one in EE wouldcallthe parallel coils shorted, Knowing that you do, means that there could be misunderstanding lurking somewhere in this thread, unnoticed.

author: Dejan567
author_rank: Contributor
timestamp: May 13, 2024, 01:35:57 am
content: QuoteYes, I tested it exactly as you stated and have mentioned this at least twice.QuoteI tested with and without earth GND and it is just more stable with earth GND connection.QuoteI have a background in EE, with my usage of phrase was trying to emphasize the connections with the signal generator. Perhaps better use of words could have been given, sure. Again, no difference in what I said and the alternative.

author: fourfathom
author_rank: Super Contributor
timestamp: May 13, 2024, 04:41:53 am
content: Quote from: Dejan567 on May 12, 2024, 10:32:18 pmWith the series L and shunt C you would absolutely expect a "voltage magnification spike", that's the whole point of the LC impedance-matching network. With your test circuit,as I understand it, the self-resonance will result in a voltage dip (null) at the SRF (Self Resonant Frequency). There are ways to characterize inductor L, R, and SRF -- this has been discussed on this forum, and it's not very difficult to get fairly accurate numbers.If you can't tell us about your application, can you at least give us a proper description of your test circuit?FWIW, here's a simulation of an LC network (ideal) that matches a 50 Ohm source to a 10 Meg / 15 pF scope probe. The generator delivers 1V to a 50 Ohm load, and the matching network boosts the voltage to over 400V at the load. I assume that your circuit won't have that inductor value, but the principle should be obvious.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 13, 2024, 07:09:46 am
content: Quote from: Sensorcat on May 12, 2024, 11:46:48 pmWhat I wrote applied to the information available to me and everyone else except the OP at the time. Namely that the sig gen was across the coil. That was stated in words and shown in the diagram in Reply #4. (There is only one way to connect the scope and retain the sig gen directly across the coil.) Now that the OP has revealed that was untrue, the configuration is very different and any previous post by myself or anyone else may be inaccurate. Your own Reply #21 models the same incorrect configuration of both sig gen output and measurement being across the coil.I am well aware that many generators have a fixed output impedance (typically 50 Ohms) and the different settings really mean the external load. My own sig gen does that. However the non-zero output impedance of the generator was irrelevant at that stage of the discussion. Whether comparing to the voltage on either side of that impedance, the voltage on a scope in the configuration that the OP led everyone to believe was in use could never exhibit multiplication. (As was the intent of your Reply #21).

author: Njk
author_rank: Frequent Contributor
timestamp: May 15, 2024, 10:26:21 pm
content: There is something magic in these inductors and resonances.In 90's I was working for a small company and once upon a time a chess master came to our office. He used to travel around the country with the purpose to perform simultaneous game show. He was thinking about how to improve the show. So he asked if we could design a hi-tech chess board with a computer interface. After some thinking, I came to the conclusion that the only reliable way would be to use the RFID method. Then I did some research and found a nice RFID tag from Temic, in the form of tiny glass tube. That's easy to plant that tag in a chessman. But I was not sure about actual availability, lead time and cost for that part and finally decided to design everything from scratch as that likely will be faster. And I succeeded. A working prototype was built in two months (see the attached images).Two surprises were encountered in the process. In RFID, the energy is wirelessly transferred between two resonant tanks. That's obvious, the Q-factor in the transmitter tank depends on energy consumption by the receiver tank. The more energy is consumed, the less the Q factor is (and therefore, the less is the voltage/current amplitude across the tx resonant circuit). But it not always so simple. In reality, when the rx tank is almost short circuited (max. current consumption), it effectively becomes aperiodic and disturbs the tx tank less. That results in increased AC voltage across the capacitor of the tx tank, not decreased.Soon after I'd found a better job and left the company. Much later I learned that the guy who was assigned to that project had a problem to reproduce my prototype. I used a high frequency space-grade capacitors (from my stockpile), while he tried to use cheap random caps, arguing that the design is actually not of HF (100 kHz). But that's actually not a surprise that the components quality matters.To the OP: The person who's most capable to solve your project problems is you. Everyone else is less aware of your particularities and therefore is much less capable. If you can't sort this out then nobody can. It's as simple as that.