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Homework Help: Inductors: what do they do?

  1. Nov 24, 2005 #1
    I've been studying circuits for almost 3 months now, and I can pretty much carry out analysis of circuits with inductors. I know the basic relationships for an R-L circuit etc, how to derive the equations and so on. In other words, I can solve textbook problems.

    But what does an inductor actually do? You see, for resistors, one can rather easily attain a visceral appreciation of its effects. I find it hard to 'picture' an inductor at work. The textbooks I've read are all really abstract; storing 'magnetic fields' and 'back emfs'. One I read even attempted drawing a comparison with fluid flow; how an inductor acts as a dam for energy because of the back-emf... I don't get it tho.

    Could someone please explain the function of an inductor in as lay a manner as possible? Like for what reasons would a person want to include an inductor in a circuit.
  2. jcsd
  3. Nov 24, 2005 #2


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    Well an inductor is like a capacitor. They both store electrical energy. The capacitor stores its energy in an electric field, whereas an inductor stores its energy in a magnetic filed. Like a capacitor, the inductor opposes changes in a variable in the circuit. The indictor likes to maintain a constant current. Whereas for a capacitor, it likes to maintain a constant volatge.

    Because of this feature of "constant current":
    When current through an inductor is increased or decreased, the inductor "resists" the change by producing a voltage between its leads in opposing polarity to the change.

    http://www.ibiblio.org/obp/electricCircuits/DC/00322.png [Broken]

    http://www.ibiblio.org/obp/electricCircuits/DC/00323.png [Broken]

    Notice the polarity change of the inductor with respect to increasing/decreasing current?

    Where does the magnetic field come from? In comes from current flowing in the coils (of the inductor). The more current in the coil, the stronger the magnetic field will be, and the more energy the inductor will store.

    --hope it hepls
    Last edited by a moderator: May 2, 2017
  4. Nov 24, 2005 #3


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    To the last question. That definitely cannot get a layman answer. With enough study in the future, you will appreciate that there is an opposite of the capacitor. We really can't do without them in some cases, but their use has been minimized through some smart tricks, especially in IC design where micron-sized inductor fabrication is practically impossible.

    The inductor is what the textbook says, it stores magnetic fields. It's hard to give an analogy with inductors, because the dam analogy only works well with capacitors. Inductors as well as capacitors only 'work' when there is an alternating current/voltage source. In DC, capacitors are nothing but an open circuit; an inductor is nothing but a piece of coiled copper (short circuit).

    You know from Ampere's law that charges moving in a wire will create a magnetic field. Then a constantly changing current flow AC will create a constantly changing magnetic field.

    You know from Faraday's law that a changing magnetic field will produce a voltage in looped wire. So electrons moving from the current source into the inductor will all of a sudden start seeing another 'voltage source' caused by their own movement.
  5. Nov 24, 2005 #4


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  6. Nov 24, 2005 #5
    Thanks. Ranger, those were helpful diagrams. I appreciate it. And I'll go have a look at the sites.
  7. Nov 24, 2005 #6


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    One of the characteristics that make inductors useful is that their reactance (opposition to AC current) increases with frequency. (unlike a capacitor, for which the reactance decreases with frequency).

    Thus inductors when combined with capacitors become useful when you want to make filters that let only chosen frequencies through, such as cross-overs for speakers or tuner circuits in radios.
  8. Nov 24, 2005 #7


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    I quickly scanned the thread and found no mention of a low-pass filter, so let me throw that in with what you guys have already put up. Kenny, you can Google or wiki that term and see what you come up with...I don't have the time right now to elaborate.
  9. Nov 24, 2005 #8


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  10. Dec 16, 2010 #9
    Hey guys, total noob here. I appreciate the fact that I have to RTFM, but honestly I'll never be able to get to that level of understanding. So if any of you could help me understand the purpose of an inductor in the following device, I'd appreciate it very much!

    Here is a picture of the device in question


    The device is an LED driver that has an input voltage (is that Vf?) of 0.9-1.5V. It boosts the voltage to 3.0-3.4V @ ~150mA. (is that Vo?)

    So I can see that there is a capacitor, what appears to be a rectifier, and two more components that I imagine are the booster and something else. But what function does the inductor serve? I always see an inductor on boosters.

    Thanks for your replies.
    Last edited by a moderator: Apr 25, 2017
  11. Dec 16, 2010 #10
    Many LED drivers are in effect small Switched-Mode Power Supplies (SMPS). Their inductors are used as energy-storage elements, but their exact function depends on the type of circuit used. To provide voltage increase, the "flyback" principle is often employed, but there are different methods to suit differing requirements. You should be able to find plenty of information on this by looking up SMPS design, e.g. on Google.
  12. Dec 16, 2010 #11


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    :smile:I remember taking a course in electric circuits in my early college years. I was whipping out RLC circuits and imaginary power equations like there was no tomorrow, I got an A or B in the course, and in the end, had absolutely no idea of what a capacitor , inductor, or vacuum tube was used for, or how they worked! I just solved the equations and did the math. I then went on to be a structural engineer. This site gives a brief description of an inductor and its use that even a dumb structural guy can understand, no maths involved::wink:

  13. Nov 26, 2012 #12
    I don't know much at all about it but I've learnt some stuff recently that may already know and stuff but anyways. Correct me if I'm wrong on anything please so I can learn, I'm a beginner. What they do is they resist any change in the amount of current flowing through them by slowing down the current as the magnetic field gets used to it by getting stronger. When the magnetic field gets to the strength it "wants" to go to for the amount of current the magnetic field does not interfere with the current. When the power source for example is turned off the magnetic field resists the drop in current flowing through the inductor (so the current keeps on going for a while after). I'll guess I'll post more about it when I know more.
    Last edited: Nov 26, 2012
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