# Capacitors or Inductors?

So I know what they both do. Yet I don't know when you'd choose to use one over the other. There was a similar question on here, but the answer didn't really make sense to me, an electronics newb.

I know caps are often used for smoothing, but then what are inductors mainly used for?

• possible duplicate of Understanding when to use basic Components – endolith Apr 12 '11 at 16:26
• what was the similar question you are talking about? if it is the question endolith linked to then it is probably worth another pass from the community to see if we can explain more/better. – Kellenjb Apr 12 '11 at 16:35
• ^^ That was my question before, but it was more for a general electronics knowledge, This one is specifically for Inductors/Caps – user3073 Apr 12 '11 at 16:35
• electronics.stackexchange.com/questions/9553/… was the similar one – user3073 Apr 12 '11 at 16:39
• Caps are used for voltage smoothing, inductors are used for current smoothing. A switching regulator makes use of current smoothing to produce a relative stable output voltage, however it does have switching noise. Switching regulators are much more efficient than their linear (Like LM317) counterparts. – Hans Apr 12 '11 at 16:53

## 2 Answers

There are many ways to answer this question. I'm going to answer it one way, and you'll just have to keep in mind that you're not getting the whole story. Also, I'm over-simplifying it for the sake of discussion. Here goes.

Impedance is the "effective resistance at a given frequency". The impedance will be measured in ohms, and depending on the device the impedance will change depending on frequency.

Resistor: The "impedance" of the ideal resistor is the same regardless of frequency.

Capacitor: The impedance of the cap goes DOWN as the frequency goes UP.

Inductor: The impedance of the inductor goes UP as the frequency goes UP.

Now, imagine a normal voltage divider made from a couple of resistors. Because the resistor has a constant impedance over frequency it will divide down the voltage evenly for all frequencies.

If you replace the lower resistor with a cap (figure 3 from the web page previously linked to) then you have something different. This will reduce the amplitude of high frequencies more than low frequencies. Effectively making a "low pass filter", which lets the low frequencies pass through, but attenuates the high frequencies.

Now, if you put the cap on top, and the resistor on the bottom, you get a "high pass filter". This will allow the high frequencies to pass through, but block the low frequencies.

The high and low pass filters mentioned are variations of an "RC filter"-- a filter made up from resistors and caps. There are such things as "LC filters", with are made up from inductors and caps. Don't ask me why "L" stands for inductor, but it does. And I'm sure somewhere there is an LR filter that uses inductors and resistors.

The basic concepts of LC and LR filters are similar to the RC filters-- using the various impedance vs. frequency characteristics of the components to create the type of circuit that you want.

Inductors tend to be used instead of resistors in situations where the current of the "pass frequency" is high. But this isn't always the case.

I intentionally ignored one aspect of inductors and caps: They can both be used to store energy. That could be a lesson for a later time, since it can get rather complex.

• Doesn't the 'L' for Inductors stand for 'Loop'? – William Apr 13 '11 at 14:26
• L is for Inductors because I was already used for current. – Lior Bilia Dec 28 '13 at 18:11
• Trivia perhaps, but L is named after Heinrich Lenz – Christian V Sep 13 '16 at 14:30

Inductors are used for a lot, but if I had to pick something I'd say they're used in buck and/or boost converters to help produce the output voltage.

The inductor is an energy storage element - it stores energy in a magnetic field that is produced when current is run through the inductor. Inductors resist changes in the current flowing through them by changing the voltage across themselves. This produces a couple of effects: if there is constant current flowing through an inductor then the voltage across it is zero. If you try to change the current flowing through it (by turning off a power supply for instance) then the voltage across it becomes non-zero.

The second effect is used to produce high voltages: if you put lots of current through an inductor then try to remove it very quickly you get a very high voltage. This is what boost converters do: create an output voltage that's higher than the input voltage. Buck converters do the opposite: produce lower voltage from higher voltage input.