Voltage vs Capacitance in Super Capacitors

Discussion in 'General Electronics Chat' started by Sparky49, Mar 27, 2015.

  1. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Hi all,

    I have been looking at supercapacitors recently, and it seems that you can either get 'low' voltage, high capacitance, or high voltage low capacitance versions. However, what is the differences between these two types? What mechanically is different about them, and in what applications might one be better over another?

    Many regards,

    Sparky
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    High capacitance and high voltage will be expensive.
    What F and V are you looking for ?
     
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  3. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Hi R!f@@ hope you are well. :D

    I'm not really looking at getting either - just curious as to why one might chose a high V, low C capacitor over a low V, high C capacitor or vice versa, as well as the differences in their construction.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Me doing just fine..thank you. :)
    I dunno much about construction on how super caps are made. :(
    But higher voltage requires higher dielectric strength to withstand the voltage hence it get's larger and expensive.
    High Farad requires larger foils (dielectrics)or what ever they are made off thus high price.
    if both F & V are high then more money..!

    So high capacitance is made to suite for the intended purpose like memory caps or back up caps where there are no voltage variations. Unlike for smoothing caps where the working voltage should be 10% higher than the nominal voltage.
    Super caps are rated very close to the working voltage.

    This is what I believe.
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    It is a matter of compromise. As R!f@@ says, high capacitance and high voltage will be physically large and expensive.
    So you go for one and forgo the other.
    Putting aside differences in type of dielectric, high capacitance requires larger area and smaller separation.
    High voltage would require higher separation.
     
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  6. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    So really the primary factors for choosing is the physical form and separation of components? Neither has a tendency for a longer discharge or esr, for example?

    Many thanks for your replies.
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Not quite.
     
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  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the only thing that affects the discharge time is capacitance and load. when trying to use capacitors for running things, you have to remember a plain old battery would have been a better choice.
     
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  9. #12

    Expert

    Nov 30, 2010
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    Just look at the definition of capacitance: greater with area, lesser with separation distance.
    Plastic films improve the ability to get electrical separation in a very small thickness but the rules remain. More voltage survival requires more separation. More capacitance requires more area.
    You want microfarads at killovolts? Look on the power poles when you travel. They exist, but you can't fit them in your Ipad!
     
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  10. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Thanks guys. Just to clarify, I'm not actually using them for anything, just curious if were just as simple as area/separation.

    Many thanks.
     
  11. #12

    Expert

    Nov 30, 2010
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    Yes. Some things never change. We merely get better at it.
     
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  12. MrChips

    Moderator

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    Just to be certain you understand what is meant by separation.
    We mean the separation between the plates in the capacitor, not separation of PCB components.
    Separation is built in the capacitor in order for it to withstand higher voltages. There is nothing you can do about it except make sure you do no exceed manufacturers specs (minus a safety margin).
     
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  13. crutschow

    Expert

    Mar 14, 2008
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    One consideration in using super capacitors is the amount of energy stored in the capacitor (= 1/2 CV^2).
    The energy stored is directly proportional to the capacitance, but proportional the the square of the voltage.
    So doubling the capacitance doubles the stored energy, but doubling the voltage increases the energy stored by a factor of 4.
    So if you are interested in storing the most energy you need to look at the cost/size trade-off between the higher voltage capacitors and the higher capacitance capacitors.
     
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  14. Roderick Young

    Member

    Feb 22, 2015
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    My information is a little dated, so if someone wants to correct me, feel free to jump in.

    My understanding was that all supercapacitors of the economically viable type natively had a working voltage of about 2.5 volts. Might be 2.7 or 3.2 volts, but about in that range. (I think there were some slightly higher voltage types, but those were so expensive, only the military could afford them. Kind of like multilayer photovoltaic cells - no one puts those on their roof.) This wikipedia article explains their construction somewhat.

    So when you see a supercapacitor sold with a working voltage of 5.5 volts, it's really two capacitors in series, possibly with some simple circuitry included to balance the voltage across the two. Or if you see a supercapacitor with a working voltage even higher, it's almost certainly a whole bunch of capacitors in series, with voltage equalizing circuitry included.

    I've been saying that if you want to be the next billionaire, invent a supercapacitor that can natively withstand 100 volts (while supplying the same or better energy density).

    When capacitors are put in series, the total capacitance goes down, so it makes sense that when you have high working voltage, you have low capacitance, and vice-versa for a given physical size. That said, supercaps have orders of magnitude better energy storage per cubic centimeter than other kinds of capacitors. Still not as good as batteries, though.
     
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