Inductor Behavior : Why Orientation Does Not Matter

Passives Inductors, Coils, Chokes
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We have received a lot of inquiries asking if inductors are electrically polarized. The simple answer is “no”, however, there is a particular concept that may cause confusion to those learning about inductors. While there is no defined electric polarity for inductors, direction of current does matter because of a phenomenon called inductor kickback. Kickback occurs when a very high voltage (thousands to tens of thousands of volts) is generated after applying voltage due to the magnetic fields breaking down in the coils.

Many analysis problems when learning about inductors include polarity symbols, however, this does not mean the components are actually electrically polarized (here is a post I made about component polarity a while back for some more context: Component Polarity). This is done in analysis to make things a little easier to understand. From what I know, based on my education, electromagnetism is taught about the same time because it is important to know why there is kickback. Teachers often use the word polarity quite a bit when describing magnetic poles as well as using symbols in analysis. This might add on to the misconception if not explained well enough. This post is to provide greater context on why inductors are not electrically polarized.

Inductor Orientation

Inductors are composed of coiled wire usually wrapped around a solid material which is typically ferromagnetic or has high magnetic permeability. The fact is, an electromagnetic field is generated in a particular manner based on internal windings and the direction the current is flowing. If you want more context on how that occurs, I’d recommend reading this post: Electromagnetism Basics. The magnetic fields are a storing the energy required to produce a high voltage when the fields collapse when voltage decreases across the inductor (this is critical for many circuit designs and may damage sensitive components). The coiling inside inductors will always be the same regardless if you flip it around. A clockwise coil traveling upward will be going clockwise going upward when turned upside-down. If you follow the coil top to bottom, it goes counter-clockwise. Again, if you flip the coil upside down it will go counter-clockwise from top to bottom.

Regardless of inductor winding direction and orientation, the polarization of the magnetic fields does not matter for kickback because the current resulting from the voltage produced across the inductor on a connected circuit will always travel the opposite direction. This is why individual inductors do not have electric polarity. The real issue is applying current in the right direction based on the desired circuit behavior. Here is a quick animation showing kickback:
kickback
Here are the relevant equations used for analyzing inductors:

V_{L}=L\frac{dI}{dt}
I_{L}=\frac{1}{L}\int_{0}^{t}V_{L}d\tau+I_{0}

Assume the inductor in the animation above was rated at 10mH and the switch was held down for some time then released. Assume that current flowing through the inductor (given some resistance in series) was 5mA. Since opening the switch causes the current to go to zero quickly, the following can be calculated assuming opening the switch takes 1ns (can’t be zero):

V_{L}=10mH*\frac{-5ma}{1ns}

The change in current is -5mA, the result would be -50,000V which is certainly enough to jump an air gap 1mm wide causing a spark. This particular behavior for inductors can be both useful and very dangerous to other components. A buck/boost converter often uses inductors to increase the voltage output (along with other components that limit spikes). The negative effect is sensitive devices such as transistors and other semiconductors can be permanently damaged by such voltages, so a fly-back diode is needed.

Winding Marking Caveats

While it is true that all inductors aren’t electrically polarized, some manufacturers include markings and coil winding information in the datasheet. It would make sense that a manufacturer would wind inductors the same way when using the same form factor (axial and radial through hole, SMT footprints, and other designs), however there are no guarantees for this. Many packages will indicate where the “start of a coil” is with a dot on the surface of the part and may even indicate in the datasheet which way the coils are wound. As a quick example, here is a part that documents both: LPS3015-104MRC ; 2457-LPS3015-104MRC-ND The datasheet says on page 2 what direction the coils are wound and the dot does indicate where the coil starts. This is not suggesting “electrical polarity” but they included the information for better detail.

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Very nice article thanks, love the animation.

It’s been ages since I thought about it, but IIRC about the only time you need to pay attention to coil start/end markings is when connecting coils in series or parallel in generators, motors, electromagnets, etc.

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At a fundamental level, as presented here, the orientation does not matter to the function of the inductor. After having designed many switch mode power supplies (SMPS), I have come to appreciate the winding marking on the inductor. Here is why:

  1. In many simple topologies, one lead of the inductor is connected to filter capacitance of some sort while the other is connected to the switching node of the SMPS.
  2. There are very fast voltage transitions and transients on the switching node.
  3. If I can ensure the start of the winding (the wire leading to the inner most windings of the inductor) is connected to the switching node, the outer most windings will be connected to the filter capacitance - which provides a low impedance to the “ground” node.
  4. The outermost windings now provide additional electrostatic shielding of the high-transient voltage noise present on the switched, inner windings. Thus I get added benefit of reduced electromagnetic radiation (EMI) and improve my chances of passing FCC regulations.

While every manufacturer is different, if possible, I use inductors with winding marks and add the corresponding mark to the PCB so as to decrease EMI.

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