# DC inductor

Discussion in 'Homework Help' started by amilton542, Jan 30, 2011.

1. ### amilton542 Thread Starter Active Member

Nov 13, 2010
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64
how can an inductor supplied with DC create inductance? the voltage isnt changing its constant so i cant see how its going to create an EMF or change polarity

2. ### beenthere Retired Moderator

Apr 20, 2004
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You may have in mind a case of inductor used in DC power supply lines that is called a "hash filter". Indeed, it has no effect on the DC power, but the presents an attenuating impedance to a source of high frequency noise that might otherwise affect the supply regulation, or bleed into another circuit fed from that supply.

Do you have a specific example in mind?

Dec 26, 2010
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For as long as an inductor carries a steady current I, it stores an amount of energy equal to 0.5*L*(I^2) in its magnetic field.

A practical inductor with DC resistance R will drop a DC voltage equal to I*R, and will also dissipate power equal to (I^2)*R.

In this steady state however, there will be no induced voltage in the coil, unless it is subject to some external varying field.

That is what happens when a coil carries a constant current. Not a lot, you might think, but you have to remember that an inductor is a passive device. To make it react requires a stimulus, such as an AC signal, or frequently in DC circuits a switching action produced by an active device such as a transistor.

When the current through changes, it develops an EMF in such a direction so as to oppose that change. This can produce very significant voltages, particularly in the case where the current through the coil is reduced very quickly, perhaps by a transistor turning off. This effect can be very useful, as in motor ignition circuits or switched-mode power supplies. It can also be a nuisance, as in back EMFs from relay coils.

If you want to find out more about inductors, try reading up on the subject. There is interesting material on this web site (see link), and elsewhere. Most likely, as you find out more about it you will find that these ideas make more sense. Good luck.

Last edited: Jan 30, 2011
4. ### amilton542 Thread Starter Active Member

Nov 13, 2010
496
64
i read that chapter a while ago and thats what i dont get, how is the voltage changing polarity in a DC inductor when its constant.what they've got there is pretty much the same as for AC inductors, but i thought they only oppose the supply when theres a reactance due to a frequency. DC inductors cant have reactance because the voltage isnt changing.unless its something to do with passing through a loop for a loop in the coil.
Also ive read the typical inductor, light bulb and an open switch theory and it states the with the DC inductor the light bulb will remain on for a while after the switch has been opened but with the AC inductor i swear it said it will behave in a totally different way.

Dec 26, 2010
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Nothing much happens to an inductor under truly constant conditions, so I wonder if you may have misunderstood something. Are you perhaps thinking of the back-emf which results when an inductor is disconnected from a supply? This is a transient condition, not steady-state.

It is difficult to help further without more information on the material which you find confusing. Can you please explain in more detail, or even better post a link to the article.

6. ### amilton542 Thread Starter Active Member

Nov 13, 2010
496
64
ill summarise what im trying to get at. excluding reactance what are the differences between an AC inductor and a DC inductor.

7. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
719
Nothing really. Both oppose a change in current through them, the higher the frequency/change, the more the inductor "resists" with counter-EMF. A MOSFET switching DC off is a BIG change in current, which is why switch mode power supplies work. A square wave is the sum of very high frequency sine waves.