# Fundamentally knowing when to use capacitors / inductors [duplicate]

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I've always stumbled when looking at even simple circuits that use capacitors or inductors and trying to work out why it is there, without using "past experience".

What I mean is, if we need to limit the current we know that we need to use a resistor and we know how to work out which we need. If we need to reduce the voltage to something we again know we can use a resistor.

If we need to stop the flow of current in a specific direction we know that we need to use a diode.

All this can be worked out mathematically and "learnt" as circuits 101.

But I can't seem to find out information how to know when to use a capacitor, for example: We have X and Y and wasn't Z... "Oh so we need a capacitor here".

All the explanations for capacitors I've seen in circuits have been from peoples practical knowledge or past experience... without that, you wouldn't have known that you would have needed a capacitor.

My brother who's doing some electronics in school brought a simple circuit diagram home that they are making and yet there's a capacitor in it.

How do you work out and know that you need a capacitor without relying on past experience "I found if I stick in a capacitor it helps..." how do you learn or know when to use a capacitor from the start / design stage?

## marked as duplicate by placeholder, Gustavo Litovsky, Dave Tweed♦, Scott Seidman, Nick Alexeev♦Oct 10 '13 at 3:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• Im not asking what the difference is, but rather knowing quantitatively when to use them – binarysmacker Oct 9 '13 at 15:00
• You need one any time the manufacturer's datasheet says you need one. – Passerby Oct 9 '13 at 15:08
• BTW: resistors can also be used to increase current (if connected in parallel) and to increase voltage (as shunts). – Curd Oct 9 '13 at 15:22
• "how do you learn or know when to use a capacitor from the start / design stage." Study electrical engineering fundamentals and put that study into practice under the guidance of an experienced designer. – Alfred Centauri Oct 9 '13 at 16:26

## 4 Answers

The best way to visualize, without proper knowledge, is that a capacitor allows high frequency signals to pass through it. An inductor allows low frequency signals through.

Knowing this, you can use it in a circuit in the following ways:

Capacitor:

• If you have unwanted noise (high frequency) on the power line going to an IC, you can put a cap in parallel to the IC. This will "let" high frequency noise go to ground instead of through the IC.

• If you have a part switching on and off quickly, it wants to grab instantaneous current from the power line (high frequency). The power line will dip , which looks like a high frequency blip on the power line. The capacitor stops this by "giving" some if its stored charge to the IC until the power supply can catch up.

• If you have unwanted DC voltage (low frequency), it will block the DC signal and only allow the AC/RF (high frequency) to go through. So, if you have an AC signal, you can put a series capacitor to make sure no DC goes through and hurts the rest of your circuit.

Inductor:

• If you have unwanted noise, you can use an inductor in series in a similar way to a capacitor in parallel (shunt). So, your 5V line is going through a long cable and may have picked up some noise along the way. A series inductor might help.

Anything more specific/complicated than this will require real knowledge of circuits. Any arbitrary placement of capacitors or inductors is likely EMI/ripple/noise mitigation.

So, if you're a hobby electronics sort of person, and your 5VDC line is rippling a little bit, trying out a few capacitors to ground is the type of solution you might try. Again, just a very basic example.

edit: There are many, many uses for these components that are used for complicated reasons. The things I stated are an example of the more arbitrary uses of them. Capacitors/inductors in gain stages, op-amp circuits and filter structures are a different beast and are chosen by analysis, not by "experience".

If we need to stop the flow of current in a specific direction we know that we need to use a diode.

If we need to block DC we use a capacitor.

If we need to block very high frequency AC we use an inductor.

If we need to design a filter we (can) use resistors, capacitors and inductors (and op-amps and transistors etc..)

If we need to design a switch mode power supply we use capacitors and inductors and diodes. If we need to design a better switch mode supply we might replace the diode with a MOSFET.

If we need to reduce ripple voltage on a power supply we use a big capacitor. If we need to reduce ripple some more we might also use an inductor.

If we need to provide isolation between circuits we might use two inductors magnetically coupled to make a transformer.

If we need to convert a squarewave to a higher voltage we might use diodes and capacitors.

If we need to make a Tesla coil we use capacitors and inductors (and stand well clear).

There are several intuitive approaches to the capacitance and inductance elements, depending on what schematic we are thinking about.

1. The frequency dependent resistance - when the schematic is linear, working with sinusoidal or close to sinusoidal signals. This approach is working pretty good. One example is using the capacitors and windings as a filter elements, blocking some or all of AC signals.

2. An energy saving devices - this approach is very useful when the capacitance or inductance work in some pulse schematic, as a time preset element - different RC, LC or RL oscillators, pulse generators. In this case, the proper approach is to think for capacitors and inductors as a energy accumulator. The capacitor stores the energy as a charge/voltage and the inductor stores the energy as a current. As long as the energy needs some time to be accumulated/dissipated, this approach explains why the voltage on the capacitor and the current through the inductor can not be changed instantly.

All these are of course very rough idea of the reality, but it allows quick intuitive analyze of the schematics and understanding how it works in generally. For more precise analysis of course exact equations have to be used.

Capacitors may have many different uses in an electronic circuit. These are some I can think of :

1. as energy tanks on power supply lines. Either to reduce the voltage ripple due to the power supply itself, or to reduce the influence of the changes in power consumption in some part of the circuit (for example, power stages) on the supply voltage.
2. in conjunction with resistors and/or inductors as a part of filters. Filters are designed to provide a specific frequency-response curve. Some eliminate high frequencies (low-pass filters), some allow a specific band of frequency to pass through (band-pass filters. This is what make a radio receiver receive ONE station only), ...
3. as coupling capacitors, for linear transistor amplifiers. A DC bias must be applied to this kind of amplifier to operate properly. The problem is that two consecutive amplifiers do not usually operate at the same voltage bias. You must prevent the DC current from flowing from a stage to an other, whereas the AC signal you want to amplify must go through. This is why you put capacitors.