The uni-versatility of capacitors

Discussion in 'The Projects Forum' started by ajhalls, May 19, 2016.

  1. ajhalls

    Thread Starter New Member

    May 12, 2016
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    I am jumping into some projects where I will be using some capacitors and wondering how "universal" they are. By that I mean: does it matter if it is AC or DC, if I have a 1KV 1F Capacitor, can I use it for 1V and 1KF?

    While I understand a low voltage capacitor may not give enough shielding to prevent a higher voltage from jumping across and shorting out, does the high voltage capacitor have too much separation to be used at low voltage?

    Can I think of them as simply storing a certain amount of watt hours, and then depending on the voltage I am supplying it will fill up to full, but only at that voltage - as in I will put 1V in, and it will stay at 1V, but develop a massive collection of that? Obviously if I am using a 250V capacitor on a 110V line, it isn't storing it at 250V, so I would assume my assumption is correct, the question becomes is there a limit to how low the voltage it will store actually is?

    Say on a motor starting capacitor, if it calls for a 250V 5uF, but I supply a 5KV 1F, it shouldn't do anything bad since the motor is just going to use what it needs right? I realize that using too small a capacitor would result in it not working as desired, but kind of like plugging in a D cell battery to a tiny LED, if the voltage is right, it doesn't matter what the ability of the battery is to deliver more is.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    No! No! No!!!

    V and F are not transmutable, i.e. you cannot exchange one for the other.

    Farads is the capacity of the capacitance. It gives the amount of charge (coulombs) Q you can store. Q equals to C x V.
    The voltage rating is the maximum voltage that the capacitor is designed for. Exceed that at your own peril. Usually it blows up in your face rather dramatically.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    You have a rather shotgun of somewhat disjointed questions there so I'll try to sort it out.

    Yes, whether the signal is AC or DC is important.
    Standard polarized electrolytic capacitors, for example, cannot be used with AC.
    Other types of capacitors can generally be used with AC or DC as long as the peak AC voltage doesn't exceed the capacitor voltage rating.

    You can use a higher voltage rated capacitor in place of a lower rated one with the same performance. It will generally just be physically larger.
    The voltage rating has only a small effect on how the capacitor works otherwise
    All capacitors work down to zero volts, independent of their voltage rating.

    Motor starting capacitors need to be close to the designed value.
    The motor will not operate properly with a much larger or smaller capacitor.
    It does not use "just what it needs", it uses the whole capacitance.

    Incidentally a 1kF capacitor would likely fill your house. :rolleyes:
     
  4. ajhalls

    Thread Starter New Member

    May 12, 2016
    7
    0
    Thank you for helping me understand, you too crutschow. I was certainly never looking to exceed the recommended voltages, simply trying to figure out as I take so many things apart like TV's, microwaves, copiers and keep a lot of the things I find along the way what I can use them for. In what I am doing, typically price and size isn't a real consideration, so it is merely compatibility with the purpose.

    I have kinda been thinking of capacitors like water bladder tanks[​IMG]

    Where when inserted inline on a circuit it will draw power till it is full, then deliver only what it needs to in order to satisfy the maximum required power that may be momentarily higher than the "pump" or in this case incoming power is supplying. In that case, the amount of water (or to my understanding Farad) the tank can hold is independent of the PSI (or in this case voltage), it has a fixed size, and will hold what it will hold, but to get a higher voltage, you need to apply that extra energy to get it in.
     
    Last edited: May 19, 2016
  5. recklessrog

    Member

    May 23, 2013
    338
    102
    One thing about electrolytic capacitors is that depending on the manufacturer/electrolyte used, and several other chemical actions, (forming the electrolyte) it is best to operate them at around at least a third of their rated voltage ie a 10volt capacitor should be operated between 3.3v to within 10-15% of max but preferably around 2/3rds to 3/4 and a 100v capacitor should be used at the same ratio. Some actually need up to 50% of the rated voltage before they "form" properly. Generally, the ESR (equivalent series resistance) is higher the greater the voltage rating of a given value of a capacitor, so a 10 mfd 50volt capacitor has a higher esr than a 10mfd 10volt capacitor. Ripple rating would therefore be different. There is a whole article by a major manufacturer that explains the ins and out in great detail, and if I can track it down, I will post a link on here.
     
    Last edited: May 19, 2016
  6. #12

    Expert

    Nov 30, 2010
    16,343
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    I have found something similar to what Rog said about capacitor voltages, but it's about oil filled run capacitors. They don't measure anything near their rating with a 1 volt capacitor checker, but if you put them in series with the power line voltage, they will pass current correctly compared to their labeled size and the power line frequency. Metal film and ceramic capacitors do not have this massive discrepancy but they do change capacitance a little bit as the applied DC voltage changes.
     
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