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How Does A Capacitor Work?

By 124 Comments

Do you often wonder “how does a capacitor work”?

At least I asked myself that many times when I was younger.

I never really liked the “physics explanation”.

It states something like “a capacitor works by storing energy electrostatically in an electric field”.

I don’t know about you, but that sentence didn’t make me any wiser when I was starting out with electronics.

electrolytic-capacitor

I like to answer the question of “how does a capacitor work?” by saying that a capacitor works like a tiny rechargeable battery with very very very low capacity.

The time it takes to discharge a capacitor is usually only a split second. And so is the time to recharge it.

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What Is A Capacitor?

So the capacitor can store charge. And it can release the charge when needed. But how does it do this? How does a capacitor work on a deeper level?

How does a capacitor work? Two plates

A capacitor is made up of two metallic plates. With a dielectric material in between the plates.

When you apply a voltage over the two plates, an electric field is created. Positive charge will collect on one plate and negative charge on the other.

And this is what the physicists mean when they say that “a capacitor works by storing energy electrostatically in an electric field”.

There are many different capacitor types.

What Does A Capacitor Do?

A capacitor is commonly used for filtering. But what is filtering?

Battery Analogy

Let’s look at an example with a battery.

Alarm clock

Many alarm clocks are powered by the electrical outlet on the wall in a house. Sometimes, the power goes down. Most alarm clocks have a backup battery that will take over and power the alarm clock until the power comes back on so that the time is not reset.

Well, in electronic circuits capacitors can be used in the same way.

Decoupling Capacitors

Microcontroller use decoupling capacitors

For example if you have a circuit with a microcontroller running some kind of program. If the voltage for the microcontroller drops for only a split second, the microcontroller restarts. And you don’t want this.

By using a capacitor, the capacitor can supply power for the microcontroller in the split second that the voltage drops so that the microcontroller doesn’t restart. This way it will filter out the “noise” on the power line.

This type of filtering is called “decoupling”. And a capacitor used for that purpose is called a “decoupling capacitor”. It’s also called a “bypass capacitor”.

Using Capacitors For Filters

You can also combine capacitors and resistors to form filters that target specific frequencies. For example in an audio system you can target the high frequencies to remove them (e.g. in a sub-woofer). This is called a low-pass filter.

Low-pass filter using a capacitor and a resistor

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Reader Interactions

Comments

  1. Please explain this sentence( I think this is crucial part) “Since it takes some time to charge the capacitor and it takes a bit of time to discharge the capacitor, the effect becomes that the capacitor will resist alternating currents (AC), but let direct currents (DC) pass.”

    Reply

    • When the capacitor is charging, current will run through it until it is charged. When it is fully charged, no more current will flow in that direction.

      So with DC, a current will flow until the capacitor is charged. Then no more.

      But with AC the capacitor gets charged and discharged all the time, so that the current can flow back and forth through the capacitor.

      Oyvind

      Reply

      • So Capacitor is more like a battery of cellphone. Charging into DC. after fully charge , flow of current stops?

        Battery as AC will continually output of current to the cellphone and discharging current.

        Oh my electronics. haha

        Reply

    • I hope i will be find more $more knowledge from you.please Give me more knowledge about the electronice beacuse i want it.Thanks for all this things.

      Reply

  2. Thanks a lot Mr. Oyvind for these info. May I ask how can radio reception in a solar home system be rectified. The reception is so bad unlike when connected to the grid. Thanks.

    Reply

    • It sounds like the power from the solar system is very noisy. I would suggest creating some kind of filter for it…

      Cheers!
      Oyvind

      Reply

  3. Can you give an example of an audio filter circuit that can be used by anybody for free? Math is not my calling :) I just like constructing pre-made circuits, and would like to know what capacitors to use in an audio filtering circuit, cutting frequencies above 15k and below 50hz, Thanks, ifya would, many Thanks indeed :)

    Reply

  4. Hey, Hi ! I just want to ask how to select the values of resistor & capacitor to make a low pass, high pass frequency or a band pass filter….i have a enough knowledge of electronics components individually but how to club them together to make a circuit of interest….when to connect which component ?? these questions always get a place in my mind every time….

    Reply

    • Hey!

      If you want to make a super simple and basic filter you only need one resistor and one capacitor for either a low-pass or high-pass filter.

      For a band-pass filter you can connect a low pass and a high-pass filter in cascade.

      To select values for the resistor and the capacitor you use the formula: Cut-off Frequency = 1 / (2 * 3.14 * R * C)

      So if you have decided on the cut-off frequency and the resistor value, you can find the capacitor value you need by re-arranging the formula above:

      C = 1 / (2 * 3.14 * R * frequency)

      Does it make sense?

      Cheers!
      Oyvind

      Reply

  6. thanks for your videos.

    i am interested in energy storage. What are the best capacitors able to store and for how long?

    Reply

    • At the time of writing, it’s possible to get my ebook “Creating Cheap Circuit Boards” for free if you sign up for my newsletter ;)

      Oyvind

      Reply

  8. Dear honorable Dr. Oyvind,

    High regards to your good self and commendable efforts in imparting knowledge in others. First, kindly send me your e-book “Creating Cheap Circuit Boards” as a subscriber of your newsletter.

    Second, is it possible to have the following books purchased in e-book format; either pdf or other format.

    Electronics Concepts and Theory
    #1 Getting Started in Electronics by Forrest.M.Mims

    #2 Make Electronics – Learning by Discovery by Charles Platt

    #3 All New Electronics – Self Teaching Guide by Harry Kybett & Earl Boysen

    #4 Practical Electronics for Inventors by Paul Scherz

    Best regards.

    Nurudeen

    Reply

    • Hey, the free ebook is in the first email you recieve after confirming your subscription. Check your spam folder if you can’t find it. The books you are looking for – I recommend you go to amazon.com and see if they have them. I don’t.

      Oyvind

      Reply

    • So there is a sote called humblebundle, that where i got most of my e-books from they offer them once in awhile. So i received like 20 ebooks for only 15 bucks which were mostly all from Make: electronics

      Reply

  9. Hello Mr Oyfind
    I hope that you can help me with my problem say i got a 380 volt or A 220 Volt
    generator and this is feeding 380 or 220volt motors
    Is there a way that i can use capasitors to cut out volt drop to the motors [in series or parr]?

    NB I am not a pro on this please help me

    theuns

    Reply

    • Hey Theuns

      In theory that should work, but you would probably need some really big capacitors. I haven’t worked with those kinds of things, so I would advice you to talk to someone who has done it before.

      Cheers!
      Oyvind

      Reply

  10. I have a quick couple of question here. Why a capacitor is used in a ceiling fan? And what is the source to charge a capacitor? Is it AC or DC source?

    Reply

    • 1. There are so many ways a capacitor could be used in a ceiling fan, so it’s hard ti answer that.

      2. Both. But with AC you will also discharge it

      Oyvind

      Reply

  11. Excellent explanation!

    The way I think of a capacitor or an inductor is that I think of them as tanks in a water system. A capacitor is a ‘tank’ for voltage. So, if there is less or no voltage in the circuit the capacitor ‘tank’ will supply it until it is completely drained. Similarly, an inductor is a ‘tank’ for current. So, if there is less or no current in the circuit the inductor ‘tank’ will supply it until it is completely drained.

    After a water stoppage in a water system, if you are connected directly to the pipe from the water company, you will have water pouring out immediately from the faucet when the water supply is resumed. On the other hand, if you have a tank you will not get water from the faucet until the tank is filled first. The presence of a tank will cause a delay in getting the water out of the faucet after a water stoppage. The addition of a tank may have the advantage of continuous supply in the case of water stoppage but it will also carry with it the phenomenon of delay.

    If you carry the example of the tank in the previous paragraph to electric components, the capacitor (which is a voltage ‘tank’) will cause a delay in the presence of voltage in a circuit. The volt will lag as a result and the current will lead. Similarly, the inductor (which is a current ‘tank’) will cause a delay in the presence of current in a circuit. The current will lag as a result and the voltage will lead.

    Reply

  12. I’m working on a project, what is the difference between a Ceramic Capacitor and Electrolytic Capacitor. Pros and Cons of both?

    This article was really helpful.

    Thank you.

    Reply

  13. hi…!!! frd..i could nt how to make any circuit, and cocept..pls help how to understand…still i dnt get clear concept about circuit and devices…can give any idea???

    Reply

  14. In my own contribution, I think the answer to the question should have been explain in a plain not by further telling us the meaning of capacitor… course we needed to hw a capacitor works so I think the answer shud have been on how a capacitor works… thank you

    Reply

  15. If i have a constant supply of say 1V…can i use a capacitor to build up to 9v, then discharge and repeat.i.e. in the analogy of the water tank, a small inlet fills up the tank, which then unloads its volume, then repeats the process. Is there a simple circuit to achieve this

    thanks
    dingo

    Reply

  16. Hi Mr. Oyvind, I really like how you explain those stuff. But I need your help. How am I going to explain capacitor with my classmates using cellphone as an example? I need to explain to them in a very simple language I can. I hope you can help me. Thank you

    Reply

    • 1. Learn how the capacitor works (by reading this article).
      2. Figure out some use case in the mobile phone where a capacitor is used, and explain it without using complex words.

      Good luck!
      Oyvind

      Reply

  20. How does the capacitor work?
    It is possible to discharge when charging in progress?
    Or it will only discharge when fully charge?

    Reply

  21. Hello Dr. Oyvind, and thank you so much for sharing your knowledge with us.
    I have a question about capacitors.
    I am a DIY audio effects builder, mainly to use in my guitar playing. Up to now I simply copied circuits on the net, but I’d like to go a bit further in understanding. My first obstacle is … a capacitor!
    I found out that, in the effects I build, the input signal (from my guitar) goes to a capacitor, and from there to the rest of the circuit. Shouldn’t this be insulating the input? What’s the point in putting a capacitor in series with the input, before going to transistors, op amp’s etc.?
    Sorry if my question is dumb, a good concept of infinity is my ignorance…
    Thank you in advance!

    Reply

    • Hey Benagro,

      A capacitor in series on the input will only let ac signals through. So if you for example connected a battery directly to the input, it would be ignored by the rest of the circuit.

      Cheers!
      Oyvind

      Reply

  22. Hi

    I want to know that in the complex circuit -full of component- the capacitor can be used to store charge from man DC battery, then capacitor supply another part of circuit permanently?
    i mean when capacitor charged by main DC power can simultaneously supply another circuit or another part?

    thanks

    Reply

  23. Hi ,nice explanation for capacitors but i don’t understand how the capacitor discharges ,in mostof the circuits we connect the connect capacitor in between the ckt then, how will the electrons collected in positive plate will reach the negative plate when there is no dielectric in between.

    Reply

    • For example, yu can connect a battery directly to the capacitor and it will charge pretty fast. To discharge it, connect for example a resistor between the two pins of the capacitor and the electrons will flow through the resistor (or any other electronic circuit).

      Cheers!
      Oyvind

      Reply

  25. what is the uses of capacitor in circuit of heartpace maker , radiation detecting device . electronic filter?
    Please

    Reply

  26. Hi, please could you tell me if there are different types of capacitor that allow for different lengths of time for the discharge of the energy, what I trying very poorly to ask is, can you get some capacitors that will discharge in milliseconds and others that discharge in seconds and if so what is the maximum length of time they can they take to discharge.

    Thanks in advance from a very uneducated man

    Reply

  28. Hi Oyvid

    I have had a look and a read of your posts, i am unable to find what im looking for, I have a 47000uf 40 vdc capacitor that is fed from a 24v dc supply, it is used as a smoothing capacitor i believe, it is then in parallel to a set of batteries that are being charged from it and is used to drive a 24v dc motor back and forth, the problem i have is that the capacitor has gone faulty and has increased the output to 36 volts. i need to change the capacitor but unsure if i need the same size or could i use a smaller uf rating?
    I take it that the 40v dc is its max working voltage?
    Regards
    Dave

    Reply

  29. Yeh! man I have learnt a lot from the video clip “How to work a capacitor” .
    Thank you,
    But I have also a curiosity about the question, “Why a capacitor is connected through A.C. supply?”
    If your answer is only for smoothing the circuit or about high pass , low pass; please I want to know about high pass and low pass circuit.

    If you send your reply to-
    [email protected]

    I shall by highly grateful to you.

    Reply

  30. why does the capacitor charges and discharges ”exponentially”
    please answer this question
    thanks in advance

    Reply

  32. Hi

    I have a dc motor that does not start up sometimes. It is a bidirectional motor that runs in one direction, then stops and then runs in the opposite direction. Sometimes, not always, it wont start running, I have to tap it or give it a turn. I want to put a cap on as a Kick Starter but am not sure of the values. Please assist me:-)

    Reply

    • I would be making an educated guess at why caps are used in a ceiling fan and I think it has to do with the use of a dimmer switch on the fan motor / light fixture. Dropping the voltage via a dimmer switch, on light bulbs makes them dim. This is called a potentiometer. If you do the same thing to the fan motor and drop the available voltage, this causes current to go up (voltage and current are inversely proportionate to each other). This in turn will cause the fan motor to hum very loudly and would eventually burn out the fan motors windings. You can install capacitors and resistors into the fan itsef to turn the potntiometer effect into a rheostat effect. Now, the current goes down instead of the voltage and the fan slows silently in cadance with the dimming light kit on the ceiling fan.

      Reply

  38. i have a question.
    if a dielectric material is in between the two plates of a capacitor than how current passes through it?

    Reply

  39. Hi Oyvind,

    I wish to create an AC to DC converter, 100~240VAC input, 24VDC output & 5A. From electronic book, it mentioned converting from AC to DC by using Full Wave Rectifier, but that cannot fit to what I need. Let say I design a circuit according to input 110VAC and I can get 24VDC output, but when I change the input to 230VAC, the output VDC also vary. Please can you guide me how to create a constant 24VDC output with 5A, regardless of the input supply?
    Thanks and appreciated

    Ervin Yu

    Reply

  41. Hai sir
    I have one silly question on capacitor why we use capacitors in AC circuits because with out ac cannot work any ac devices
    Clarify me sir plz

    Reply

  44. why are we putting screens with capacitor in a current transformers? how does capacitor divide the voltage or current in a 132kv transformers?

    Reply

  46. heyy oyvind. I tried to put capacitor in my circuit just as u did in ur video but the led still gets off without a delay. Tried it in series and Parallel, can u tell what goes wrong?? The arrangement of components are same as urs in the video!.. On it. Is written 63 v and 1microfarad

    Reply

    • Hey Savidh,

      1 microfarad is too little to see the delay. In the video I used 2200 microfarad.

      Best,
      Oyvind

      Reply

  47. Hi Sir

    What If we use a conductive dielectric material in the capacitor for AC supply?

    The current that will through it will be the displacement current??

    Reply

  48. This is such a great explanation on the basics of capacitors.

    I really like your video example showing how a capacitor acts as a “local storage” element that will charge up in the presence of a voltage, and will discharge when that voltage is removed. What a great way to illustrate that with a LED.

    And of course, given how a capacitor works, it makes total sense to use them for decoupling and filtering.

    Thanks for putting this together.

    Mike

    Reply

  49. Great Video, thanks. I am an amature model rocketer getting back into the hobby after a 40 year hiatus and want to build a firing circuit that amplifies the 12v dc energy into a momentary surge or pulse that will ignite multiple engine clusters (if you’re familiar with the hobby) and send the model rocket skyward. Basicly you are heating up nichrome wire with resistance to generate a spark to ignite the propellant in solid fuel model rocket motors. With multiple motors you have a significant voltage drop across the circuit so the possibility of a misfire is high. I figure that if the amperage is boosted with the help of a capacitor this will solve the problem Thanks!

    Reply

  52. You did an excellent job of teaching what a capacitor is and how it works. You physical explanation was great and understandable for those of us who are visual learners. It was basic, easy to understand, engaging, with a flawless explanation. You are a good teacher.

    Reply

    • If the input is DC, it will behave like a wire without resistance at first (for a few micro- or nanoseconds). Then after a few milliseconds or seconds, it will behave like one side has no connection to the other side.

      Best,
      Oyvind

      Reply

  56. Sir, i started engg. In elecctronics.but u don’t know basics. Please send me basic electronics pdf or ebooks by mail.

    Reply

  57. Dear Sir,
    Very good article, I want to learn hardware design and software for arm processor to develop a project need guidance, please let me know how I can contact you.
    Thank you

    Reply

  64. Thanks Dear,I appreciate your efforts in educating us.
    Sir,I believe students learn easily when difficult problems are presented graphically.
    Sir, please can you explain how a capacitor work as a filter and as a Decoupler using circuit diagrams the way you did in transistor and LDR circuit explanations.
    Thanks sir as I wait for your response.

    Reply

  65. This is a great article. It’s just so well-written. This should be mandatory for everyone to read as core-education.

    Reply

  67. We have problem with AC voltage for microswitch, I preferred provide 0.022 Micro Farads non-electrolytic capacitor parallel to that switch to avoid leakage current in it. when we are measuring the voltage with respect to ground i am getting 170 VAC one pole at micro switch and nearly 50 VAC at other pole, so I am planning to provide capacitor in parallel to filter the voltage.
    can you tell me my idea is correct or not?

    Reply

  70. Pls, I need more explanation about; how a capacitor works when a DC current passes through it in series connection & in parallel connection. Also, when AC current does the same.

    Reply

  71. Can a capacitor on an Air conditioner work some of the time and not work some of the time? On my RV, one A/C works all the time but the other 2 work some of the time. Their fans blow but the air is not cold.

    Thank you.

    Reply

  72. How actually is the capacitor work means how the charges flow in the circuit through the air medium. What is the principle of working

    Reply

  73. Sir what happened to capacitor if we remove it from circuit at that instant?
    either it Discharges or remain constant

    Reply

  74. I understand the electrons will build up on one side. But, what about the other side – why don’t electrons build up there too?

    Reply

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