Inductors,Chokes & Capacitors

Discussion in 'General Electronics Chat' started by Lucky13, Jan 24, 2009.

  1. Lucky13

    Thread Starter New Member

    Dec 29, 2008
    1
    0
    Hello everyone, this is my first post and I am very new to electronics, so be a little easy if my questions is way off base.


    I was reading some info that was explaining Inductors and it made this statement

    "
    Inductor (coil)

    An inductor is a coil of wire which may have a core of air, iron or ferrite (a brittle material made from iron). Its electrical property is called inductance and the unit for this is the henry, symbol H. 1H is very large so mH and µH are used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes). They pass DC easily, but block AC signals, this is the opposite of capacitors.



    For some reason I don't really understand this statement that is underlined & in bold type. Well then I started researching info about capacitors and I really got confused. Statements like there behavior in AC or DC circuit just made it even worse for me. The statement above really gets me because (remember I am new to electronics) I have built a few simple power supples for CNC machines using a transformer, rectifier and capacitor. If a capacitors blocks DC voltage how does it work in a power supply like this?


    As far as I knew up to this point a capacitor was a storage tank, the bigger it was the more current it would hold, the high the voltage rating the more voltage it could hold up to. I know there is more to them than that but it looks like I don't understand the way they really work, and how they function.


    Is there anything anyone could explain that might help me with this. Of course just pointing me to info would be great, but a little discussion about the subject would be great.


    Jess
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I'm squeezing two to four hours of lecture into a couple paragraphs here, so please bear with me...

    A capacitor will charge until the voltage across it equals (for all practical purposes) the applied voltage. At this point, no more current will flow, since there is no difference in potential. This is how it can "block" DC. Since an AC voltage changes, the potential across the capacitor changes as well as the charge changes. The opposition to AC, which we call "capacitive reactance," decreases as the AC frequency increases. As frequency increases without bound, capacitive reactance decreases toward zero. As frequency increases toward zero, capacitive reactance increases without bound.

    An inductor stores energy in a magnetic field rather than in an electrostatic field like the capacitor. (Current is not stored. Energy is stored.) As the current through the inductor changes, the magnetic field changes. This in turn induces a counter-EMF in the inductor. We call this "inductive reactance." Inductive reactance is proportional to frequency, since the counter-EMF is proportional to the rate of change of the magnetic field, which in turn is proportional to frequency. Since DC is zero frequency, there will be zero inductive reactance.

    Capacitors and can be used to "filter" the ripple of rectified voltage. They provide a low impedance path to ground for the AC component of the voltage, but are practically an open circuit for the DC component.

    Some good additional reading may be had here:

    http://www.allaboutcircuits.com/vol_1/chpt_13/index.html

    http://www.allaboutcircuits.com/vol_1/chpt_15/index.html

    http://www.allaboutcircuits.com/vol_2/chpt_3/index.html

    http://www.allaboutcircuits.com/vol_2/chpt_4/index.html
     
  3. DedeHai

    Active Member

    Jan 22, 2009
    39
    0
    I wanna try a more simple approach.

    When you say you use a capacitor in a power supply, which supplies a DC voltage, you connect the Capacitor from + to - (or ground). It is connected parallel to your load, you are supplying with. so it does not block the current, it just serves as a kind of "reservoir" to even out variations in demand of energy. simply put: the current flows in on the same node as it flows out.
    to block DC current, a capacitor has to be connected in series, meaning that the current flows in on one side of the capacitor and out the other end. so as soon as it is "filled with electric charges" the current stops, because the electrons can not actually flow through the capacitor (since it has a gap in the middle).

    With inductors you could say it is the other way around. The magnetic field that builds up when current increases kind of "chokes" the current like it would draw energy from the current into the magnetic field. so as long as you are changing the current (like in AC) the magnetic field changes and "draws energy" and so makes it harder for the current to get through.
    As soon as the current stays the same, the magnetic field also stays the same, no longer "sucking energy out of your current". As a inductor simply is a long piece of wire, wound around an iron core, the resistance of the wire is quite low, so there can flow a lot of current through (almost like a short circuit).
    I hope that helped. It is not really scientifically correct, but should help to understand.
     
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