# Inductor vs a capacitor

I'm a Trainee electrician and pc hardware enthusiast. I was just wondering why a mixture of inductors and capacitors are used on motherboards? Why not just use capacitor? I thought the inductor stores electrical charge but it uses magnetism. What's so special about storing it as magnetism?

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In short, inductors and capacitors have different electrical properties, quite different in fact. One is not a replacement for the other. – Olin Lathrop May 10 '12 at 19:57
@OlinLathrop Not completely true. You can simulate an inductor using a capacitor, but its not going to be perfect. – CyberMen May 10 '12 at 20:40
@Cyber: Nonsense. Inductors and capacitors are fundamentally different circuit elements. In fact you can think of them as mirror images of each other with voltage and current flipped. You can build circuits that simulate one using the other along with other parts, but that doesn't make them somehow alike. – Olin Lathrop May 10 '12 at 23:16
Think OpAmps. Differentiators circuits can simulate inductors. (Im an EE I know they are not the same) – CyberMen May 11 '12 at 13:33
I think the particular circuit being described here is an inductance gyrator – Phil Frost Jan 25 '13 at 21:04

To answer this properly, you should know the properties of a capacitor and an inductor.

Inductors are one of the primary components required by a switching regulator. An capacitor and inductor are similar in the way that a capacitor resists a change of a voltage and an inductor resists a change in current. The way how 'strong' they can resist depends on their value.

Capacitors are widely used to clean up a power supply line, i.e. remove noise or ripple at (higher) frequencies. Inductors are used in switching power supplies where a relatively constant current is passed through an inductor. A switching power supply works in that a switch is opened and closed very quickly. When the switch is closed, the inductor is 'charged'. When the switch is open, the energy is drawn from the inductor into the load. Usually such a power supply is being decoupled with a capacitor to create a stable power supply line.

An inductor is required to make this principle work. If you know a resistor that has an equal resistance for all frequencies of signal, you should view a capacitor as a resistor that will be infinite for DC (0Hz) and 0 for high frequencies. An inductor will be the opposite: it's resistance will be 0 at 0Hz, and infinite at high frequencies. However we don't call this resistance (that's only used for a pure resistor!) but impedance.

A PC motherboard or graphics card is basically not much else than this. They have their main chips and the routing between them, and most other components are power supply or a little bit of interfacing between chips or connectors.

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On pc motherboard inductor are most likely RF choke. – MathieuL Jan 11 at 13:47
"The way how 'strong' they can resist"? I would suggest an edit but I don't know what that was supposed to say! Last sentence of first paragraph. – Aerovistae Jul 3 at 16:22

The basic electrical property of a capacitor is that the voltage across a capacitor cannot change instantaneously, whereas the basic property of inductance is that the current through an inductor cannot change instantaneously. Capacitors preserve voltage by storing energy in an electric field, whereas inductors preserve current by storing energy in a magnetic field.

One result of this is that while capacitors conduct best at higher frequencies, inductors conduct best at lower frequencies. Another result is that if you put an AC current through a capacitor, the voltage will lag behind the current by some phase angle that depends on the capacitance and the frequency - capacitors inhibit changes in voltage. Meanwhile if you put an AC voltage across an inductor, the current will lag behind the voltage by a phase angle that depends on the inductance and the frequency - inductors inhibit changes in current.

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In some situations, inductors and capacitors can substitute for each other. In others, they cannot. Of course, they never directly substitute. What this means is that some circuits can be slightly modified so that an inductor is used instead of a capacitor or vice versa to achieve the same purpose. Some circuits cannot.

An inductor does not store a charge in its magnetic field, but rather energy. When the magnetic field is allowed to collapse, the inductor will spontaneously generate a voltage. The voltage is usually much higher than any voltage which was previously applied to the inductor. A capacitor will never exhibit a voltage which is greater than what was applied to it. So for instance, a capacitor cannot be used to build an ignition coil for a gasoline engine.

A capacitor in series is similar to an inductor in parallel, in some ways. Both approaches can make a filter with the same frequency response. However, the loading effects of these circuits are not the same. A capacitor in series blocks DC, and so to a DC source, it looks like an infinite impedance: the lightest possible load. An inductor in parallel is the exact opposite: a short circuit. The two only look similar from the perspective of the load device: it sees a signal that has been high-pass-filtered, and is free of DC. But the DC is not removed in the same way. Blocking a signal with an open load is not the same as short-circuiting a signal to ground.

Likewise, an inductor in series is similar to a capacitor in parallel, but again, the loading effect is not the same. We can use a capacitor to prevent AC, or AC above certain frequencies, from entering a circuit, by shunting those signals to the return. Sometimes that is acceptable, like when blocking RF noise from entering a device. In some other cases, shunting AC to ground may create an unacceptable load on the source of that signal. An inductor can block AC by creating a high impedance against it.

So even in circuits where we can potentially substitute parallel inductors for series capacitors and vice versa, consideration for the loading differences may require us to choose one or the other.

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This is the question puzzled me for quite a while too, I even did simulation of "step-down" converter without the inductor so now I figured out what is wrong :-).

Basically, if you skip inductor, it will work. But efficiency would be like in linear regulator - voltage drop would be only due to drop in parasitic resistors from 12v supply to output capacitor.

Inductor works like a resistor here, but it does not waste any energy, it rather slowly pump it into capacitor.

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Inductors are put inline to filter electrical noise. Caps are placed in parallel to shunt noise to ground. Both can cause a phase shift between voltage and current, but they do so in opposite directions so the effect cancels out.

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... 'so the effect cancels out' is not generally true. An LC filter may be designed for zero phase change at one frequency. All other frequencies will suffer a phase shift. – transistor Jan 11 at 8:26

As I know, Inductor and capacitor is combined to get the frequency resonance when $X_C =X_L$, Inductor and capacitor also can be used as filter $X_L= 2{\pi}fL$, $X_{C}=\frac{1}{2{\pi}f C}$ , with Band stop or Band pass when $V=0.707V_{max}$

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The core of the answer was given by Jim C. Most didactic materials show a capacitor in series equivalent to an inductor in parallel, and vice versa. That's not completely true, because each'll shift the phase to an opposite direction. So if you don't want the shift, you should combine the inductor and the capacitor. In some circunstances the shift is acceptable in only one direction, so you can use the capacitor or the inductor according to that. Here's a full explanation of the subject.

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