Beginner Concepts: A Quartet Of Videos On Inductors

Inductors can be found in many of the devices you use every day, but if you’ve been working only with DC in your projects there’s a good chance you’ve never needed to know anything about them. Now’s your chance to pick up on the basics with this video tutorial series. [Afroman] put together four short videos that we’ve embedded after the break. Set aside fifteen minutes to watch them; you’ll be glad you did.

The first in the series starts out by explaining that an inductor is a coil of wire that serves a similar function as a capacitor with one major difference. A capacitor stores voltage, while an inductor stores current. In the second video, [Afroman] hooks up some inductors to a square-wave generator, then measures the resulting current characteristics using an oscilloscope. He shows the difference between inductor core material (air core versus ferrite core) and illustrates the properties that make inductors so useful as filters. The third video covers filtering circuits, and the fourth is the best explanation of why you need a flyback diode when driving a motor (an inductive load) that we’ve seen yet.

18 thoughts on “Beginner Concepts: A Quartet Of Videos On Inductors

  1. Capacitors do no store voltage, they store
    electric charge, and inductors do not store
    current, they store energy in a magnetic field.
    I understand the desire to simplify the
    concepts involved, but these sort of errors
    will just end up confusing beginners even more.

  2. It’s a nice introduction, but I think a few points are missing from it.

    First, adding the values of inductors in series only works when where is no magnetic coupling between the inductors, so you should either used closed inductors, set them at right angles to each other, or simply set them reasonably far apart.

    Also, it makes sense that the inductance increases when you use more turns on a given core. However, that’s only half the story; the inductance increases with the square of the number of turns! This also makes sense, intuitively, because every new turn influences the existing turns, and is influenced by the existing turns. At the same time, the resistance only increases linearly with the number of turns (actually, the length of the wire). However, if you try to fit more turns on a give core that is already full, you would have to use thinner wire, so the wire is both longer and thinner, so in the case of a full core, the resistance increases with the square of the number of turns.

    Finally, a given core (other than an air-core inductor) can only sustain a given magnetic field before it saturates. When it starts to saturate, the core starts to disappear magnetically, and the inductance decreases when the current increases further, as if you connected an air cored coil in parallel to the ferrite core inductor.

    An air-gapped (meaning, a non-closed core) is much less susceptible to saturation, but closed cores generally give you the most inductance for a given number of turns.

    There is much more to know about inductors, like the frequency characteristics of the core material and the parasitic capacitance between the turns, which determine the natural resonance frequency of an inductor, but those are really advanced topics.

  3. These are interesting videos, but I find them quite simplistic, especially “this wave” + “this inductor” = “this wave”. What are the waves? The missing point is Faraday’s Law, which tells you how the rate of change of current through an inductor varies with the voltage put across it (or, what the voltage will be across an inductor as a function of the current through it). Simply saying that an inductor changes a wave’s shape misses the point.

  4. @ironring: of course, how could I have missed that: the most important part is explaining U = L * dI/dt, meaning that the voltage across an inductor equals it’s inductance times the rate of change of the current. Using this, it’s also easy to explain why you need flyback diodes when switching inductive loads (which was, apparently, explained in the missing fourth part of the series).

    When you switch off an inductive load, you quickly interrupt the current. As the factor dt (the time it takes to switch off) approaches zero, the term dI/dt, and therefor the voltage across the inductor, approaches infinity. Clearly, no electronic component, or airgap, for that matter, can handle an infinite voltage. The energy has to be removed from the coil, and it will find a way. The voltage will rise until some path is created, which usually means the semiconductor that does the switching will go in an often destructive avalanche breakdown mode.

    Truly understanding U = L * dI/dt is the most important part of understanding inductors, just like I = C * dU/dt is important for understanding capacitors.

    By the way, the energy stored in an inductor equals E = 1/2 * L * U^2, which is remarkably like kinetic energy, E = 1/2 * M * V^2. When explaining the behavior of inductors, I often refer to water analogies; there is a flow of water, and water has mass, so there is a certain amount of kinetic energy stored in the moving water. If you would try to suddenly stop the flow of the water, you will have to remove that energy.

    1. You can also measure inductance with an oscilloscope and a signal generator. A pc can be used for the generator. Set the output to sine wave and adjust so you have 1v p-p on the scope. Put the inductor in parallel with the signal input and adjust the frequency until it reads .5v p-p on the scope. Divide 4750 by the frequency in khz and you get the value of the inductor in uH.

  5. In the fourth video, the author says that with the diode anti-parallel to the load, some current returns to the power supply. This is not true, simply because there is no closed path through the power supply (note that the MOSFETs body diode is reverse-biased).

    Also, in the example, the voltage spike seems to be limited to 40V. I think there are 2 possible explanations for this. First, the MOSFET can’t turn off instantly, so dI/dt will be limited, and apparently, it is sufficiently low to limit the result, U = L * dI/dt, to only 40V. When the MOSFET would be driven harder, the dI/dt would increase further, and the spike would go up.

    The other possible explanation is that spike actually is (much) higher than 40V, but the bandwidth of the oscilloscope is insufficient to measure it properly. Note that this is not related to the actual switching frequency; a spike, by definition a short signal with very high rising and falling slopes (high dU/dt), contains very high frequency components.

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