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Old 02-22-2013, 05:55 AM
Mercfh Mercfh is offline
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Exclamation "Why" Does inductive Kickback Happen?

I understand that it does, and i've looked up lenz's law.....but I guess im a little confused at the physics of why this phenomenon happens? Could anyone explain in some detail why exactly this happens? As well as Back EMF voltage?

The physics are kinda confusing me...and i've searched google and can't really find a "WHY".

Thank you.
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Old 02-22-2013, 06:57 AM
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crutschow crutschow is offline
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I find a simple way of understanding this is from an energy viewpoint:

A inductance provides an impedance (proportional to L) to a changing current (as for example the current generated by an applied voltage V across the inductance L). The change in current is di = V dt / L. The energy absorbed by this impedance is stored as energy in the magnetic field with energy equal to 1/2 LI^{2}.

If the voltage source is removed, this stored energy in the field attempts to keep the current moving at the same level (rather like the inertia of a moving mass). It will generate a voltage proportional to the impedance in series with the inductance to maintain a current until all the inductive energy is absorbed by the impedance.

If you try to suddenly open circuit the inductor when it's carrying current, it will generate a very high voltage in an attempt to keep the current flowing, often sufficient to generate a spark (or zap a transistor) until the energy is dissipated.

Note that the current wants to continue flowing in the same direction, so the terminal with the current flowing out will become positive with respect to the other terminal. This, of course, is of opposite polarity to the initial generation of the inductor current, where the current direction is from the positive voltage applied to the inductor to the negative terminal. Thus the voltage polarity reverses at the inductor terminals when the source is removed, even though the current flow direction does not. In effect the inductor goes from absorbing and storing energy when a source is applied, to supplying energy when the source is removed.

Does that help?
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Last edited by crutschow; 02-22-2013 at 08:37 AM.
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Old 02-22-2013, 12:53 PM
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This may also help:
http://www.allaboutcircuits.com/vol_1/chpt_15/1.html
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Old 02-22-2013, 03:40 PM
davebee davebee is offline
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I visualize a circuit as a stream of electrons flowing along a wire.

When the circuit is interrupted, as in breaking a wire, the flowing electrons pile up at the end of the wire, and the excess charges that accumulate at the end create the voltage spike.

This visualization assumes that electrons have inertia, but the analogy really isn't that far off from the physics explanation, where the action of the collapsing magnetic field on the electrons acts just like inertia does on everyday objects.
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Old 02-22-2013, 03:48 PM
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ErnieM ErnieM is offline
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A simple version: an inductor stores energy in it's magnetic field due to the current flowing thru it, and "due to the physics" this current cannot change instantaneously but must ramp from one value to another. The voltage across an inductor is related to the change in current, and the voltage direction is such that it tends to oppose the change of current; this is because the change in current is storing energy in the magnetic field of the inductor.

So if you slam an inductor open you are attempting to instantly change the current... and an instant change of current results in an instant large change in voltage. A large change of current is a large di/dt. The energy in the inductor has to exit somewhere, and the voltage will increase to the point where it finds an exit, either in a catch diode, or by blowing up (shorting) the transistor trying to open circuit it.
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Old 02-22-2013, 04:01 PM
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I made a circuit to look at the results of 1/2LI^2 = 1/2CV^2
It demonstrates how the equations work and makes sorting your junk box inductors easier.

http://forum.allaboutcircuits.com/blog.php?b=478
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