Question relating to parallel inductors.
Ash Small, Tue Dec 06 2016, 12:35PMI've read the laws applying to series and parallel inductors, and it makes perfect sense.
My question is do these laws still hold if the inductors are wound on a common core?
Re: Question relating to parallel inductors.
Kizmo, Tue Dec 06 2016, 12:50PM
Kizmo, Tue Dec 06 2016, 12:50PM
Ash Small wrote ...
I've read the laws applying to series and parallel inductors, and it makes perfect sense.
My question is do these laws still hold if the inductors are wound on a common core?
Short answer is no, i think only parameter that does behave the same way in both cases is DC resistance.I've read the laws applying to series and parallel inductors, and it makes perfect sense.
My question is do these laws still hold if the inductors are wound on a common core?
Re: Question relating to parallel inductors.
Ash Small, Tue Dec 06 2016, 12:57PM
Thanks Kizmo, I suspected as much.
What's the slightly longer answer?
Ash Small, Tue Dec 06 2016, 12:57PM
Thanks Kizmo, I suspected as much.
What's the slightly longer answer?
Re: Question relating to parallel inductors.
hen918, Tue Dec 06 2016, 02:39PM
The slightly longer answer is that if they are wound on the same core you have yourself a transformer. If they are wound in phase (in the same direction) and connected in parallel, you will have just doubled up the copper on what is essentially one winding (this is to be avoided though, because and discrepancy in the number of turns, or more specifically amount of flux cut through, causes inefficiencies as electrically turns are shorted due to their being a difference in voltage between them). The inductance *should* be the same.
If your (same length) windings are connected in series you will have doubled the length of the windings and the inductance will increase proportional to the square* of the turns. (voltage will increase linearly, of course).
If your windings are in anti-phase and in series the number of turns will have to be subtracted and this "n" used to find the new inductance.
In anti-phase parallel, you will have effectively shorted turns on your inductor.
Care should also be taken not to locate unshielded power inductors near each other when the windings (as they are slightly coupled by their proximity) are in anti-phase, for the same reason.
*Edited, meant square, not square root.
hen918, Tue Dec 06 2016, 02:39PM
The slightly longer answer is that if they are wound on the same core you have yourself a transformer. If they are wound in phase (in the same direction) and connected in parallel, you will have just doubled up the copper on what is essentially one winding (this is to be avoided though, because and discrepancy in the number of turns, or more specifically amount of flux cut through, causes inefficiencies as electrically turns are shorted due to their being a difference in voltage between them). The inductance *should* be the same.
If your (same length) windings are connected in series you will have doubled the length of the windings and the inductance will increase proportional to the square* of the turns. (voltage will increase linearly, of course).
If your windings are in anti-phase and in series the number of turns will have to be subtracted and this "n" used to find the new inductance.
In anti-phase parallel, you will have effectively shorted turns on your inductor.
Care should also be taken not to locate unshielded power inductors near each other when the windings (as they are slightly coupled by their proximity) are in anti-phase, for the same reason.
*Edited, meant square, not square root.
Re: Question relating to parallel inductors.
Dr. Slack, Tue Dec 06 2016, 02:41PM
The longer answer includes the mutual inductance of the inductors, the voltage induced in one because of the dI/dt in the other. In fact one way to calculate the mutual inductance is to measure the individual inductors, measure the combined inductance, and whack the measurements into a suitable formula.
For instance, consider the windings on a common mode choke. Each would measure some finite value, but because of their mutual inductance, when put in series, their inductance is 4x that of one, and when in anti-series their inductance is approximately zero.
Dr. Slack, Tue Dec 06 2016, 02:41PM
The longer answer includes the mutual inductance of the inductors, the voltage induced in one because of the dI/dt in the other. In fact one way to calculate the mutual inductance is to measure the individual inductors, measure the combined inductance, and whack the measurements into a suitable formula.
For instance, consider the windings on a common mode choke. Each would measure some finite value, but because of their mutual inductance, when put in series, their inductance is 4x that of one, and when in anti-series their inductance is approximately zero.
Re: Question relating to parallel inductors.
Ash Small, Tue Dec 06 2016, 05:13PM
Thanks for the replies. I've been doing some research elsewhere, and found something similar, and found a reference to mutually coupled inductors, and a formula relating the total inductance and the mutual inductance. I don't think it was explained exceptionally well though.
I can see how wiring, say, the primary and secondary of a step down transformer in parallel would create issues.
I also remember something about shorting out the secondary, or leaving it open circuit when measuring different parameters, so am I correct in assuming that if I short the secondary, the primary would be the only measurable inductance? (ie, would the secondary no longer have any effect on the inductance of the primary? )
Ash Small, Tue Dec 06 2016, 05:13PM
Thanks for the replies. I've been doing some research elsewhere, and found something similar, and found a reference to mutually coupled inductors, and a formula relating the total inductance and the mutual inductance. I don't think it was explained exceptionally well though.
I can see how wiring, say, the primary and secondary of a step down transformer in parallel would create issues.
I also remember something about shorting out the secondary, or leaving it open circuit when measuring different parameters, so am I correct in assuming that if I short the secondary, the primary would be the only measurable inductance? (ie, would the secondary no longer have any effect on the inductance of the primary? )
Re: Question relating to parallel inductors.
2Spoons, Tue Dec 06 2016, 09:43PM
With partial coupling of different parallel inductances you can have some very odd things happening. I found one scenario where current would flow backwards in one inductor, due to coupling from the other. This all came about from me trying to find the best way to construct high power, low loss PCB coils.
Try it in a simulator : 1:2 turns ratio, and coupling >0.5. Inductors in parallel.
2Spoons, Tue Dec 06 2016, 09:43PM
With partial coupling of different parallel inductances you can have some very odd things happening. I found one scenario where current would flow backwards in one inductor, due to coupling from the other. This all came about from me trying to find the best way to construct high power, low loss PCB coils.
Try it in a simulator : 1:2 turns ratio, and coupling >0.5. Inductors in parallel.
Re: Question relating to parallel inductors.
hen918, Tue Dec 06 2016, 10:09PM
By shorting out the secondary and measuring the primary, you are measuring the total leakage inductance. This is the inductance that is not coupled to the transformer / inductor. It is drawn in models of real transformers as a separate inductor in series with the primary, as this is effectively what it is.
By leaving the secondary open, and measuring the primary, you are measuring the magnetising inductance. This is the inductance that's effectively in parallel with the primary, and is drawn as such on the real diagram of a transformer. Whatever is driving the transformer has to cope with the current flowing through the magnetising inductance even when the transformer is unloaded. This is why large power supplies are very inefficient at low loads.
hen918, Tue Dec 06 2016, 10:09PM
Ash Small wrote ...
...
I also remember something about shorting out the secondary, or leaving it open circuit when measuring different parameters, so am I correct in assuming that if I short the secondary, the primary would be the only measurable inductance? (ie, would the secondary no longer have any effect on the inductance of the primary? )
...
I also remember something about shorting out the secondary, or leaving it open circuit when measuring different parameters, so am I correct in assuming that if I short the secondary, the primary would be the only measurable inductance? (ie, would the secondary no longer have any effect on the inductance of the primary? )
By shorting out the secondary and measuring the primary, you are measuring the total leakage inductance. This is the inductance that is not coupled to the transformer / inductor. It is drawn in models of real transformers as a separate inductor in series with the primary, as this is effectively what it is.
By leaving the secondary open, and measuring the primary, you are measuring the magnetising inductance. This is the inductance that's effectively in parallel with the primary, and is drawn as such on the real diagram of a transformer. Whatever is driving the transformer has to cope with the current flowing through the magnetising inductance even when the transformer is unloaded. This is why large power supplies are very inefficient at low loads.
Re: Question relating to parallel inductors.
Ash Small, Tue Dec 06 2016, 11:32PM
Thanks for the reply. Do I take it then, that if leave one coil unconnected, in a situation where two inductors share a common core, that it no longer interacts with the other inductor?
Ash Small, Tue Dec 06 2016, 11:32PM
Thanks for the reply. Do I take it then, that if leave one coil unconnected, in a situation where two inductors share a common core, that it no longer interacts with the other inductor?
Re: Question relating to parallel inductors.
klugesmith, Wed Dec 07 2016, 05:14AM
If you leave one coil unconnected, it can still resonate with its self-capacitance and affect the "black box" behavior of the other coil. Might be easier to measure in an air-core design.
There are inductive devices called variometers, unrelated to aviation instruments with the same name. Arrange two coils that can be rotated with respect to each other, to sweep the coupling factor k between almost 1 and almost -1.
Suppose each coil alone has inductance L, and they are connected in series. Then the combined inductance ranges from almost 4L (max positive k), through 2L (k=0, e.g. orthogonal orientation), to almost 0 (max negative k). I think a parallel connection would behave in a similar way.
Image from
klugesmith, Wed Dec 07 2016, 05:14AM
If you leave one coil unconnected, it can still resonate with its self-capacitance and affect the "black box" behavior of the other coil. Might be easier to measure in an air-core design.
There are inductive devices called variometers, unrelated to aviation instruments with the same name. Arrange two coils that can be rotated with respect to each other, to sweep the coupling factor k between almost 1 and almost -1.
Suppose each coil alone has inductance L, and they are connected in series. Then the combined inductance ranges from almost 4L (max positive k), through 2L (k=0, e.g. orthogonal orientation), to almost 0 (max negative k). I think a parallel connection would behave in a similar way.
Image from
Re: Question relating to parallel inductors.
Ash Small, Thu Dec 08 2016, 11:50PM
Thanks for the replies. The answers I have recieved vary considerably from when I asked the same question in a different way sometime ago.
I suspected the answer was fairly convoluted
Ash Small, Thu Dec 08 2016, 11:50PM
Thanks for the replies. The answers I have recieved vary considerably from when I asked the same question in a different way sometime ago.
I suspected the answer was fairly convoluted
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