Capacitor Capacitance and Voltage relation

Discussion in 'Archived Threads 2001-2004' started by Aslam Imran, Jul 17, 2002.

  1. Aslam Imran

    Aslam Imran Second Unit

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    What's the relationship between capacitance and voltage in a capacitor. I have seen amps that are rated higher but with lower total capacitance (but higher voltage on their capacitors) compared to amps with higher total capacitance(but lower voltage) rated lower. Does anyone know why would that be?
     
  2. Saurav

    Saurav Cinematographer

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    I'm not sure what exactly your question is. A capacitor is rated for two things - its capacitance, and the voltage it can handle.

    The first value (capacitance) tells you how much charge it can store for a given amount of voltage across it. The higher the capacitance, the more charge (i.e. electrical energy) it can store for the same voltage.

    The second value, voltage, tells you the maximum voltage you can put across that cap before it gets damaged. This does not mean that the amp's design actually puts that much voltage across the cap, because no one will design an amp that is always on the verge of blowing up. Usually, you would use a cap that's rated to at least 2x the maximum voltage you expect to have across that cap. So, the voltage rating of a cap is more of a safety rating, and doesn't directly impact the audio performance of an amplifier.

    Here's an example - let's say you have a cap at a point in a circuit where it could charge up to 50V. You have a 2uF (or whatever) / 100V cap there. If you replace it with a 2uF/200V cap, will it make any difference? No. (This assumes that the caps are otherwise identical in design and construction). If you replace it with a cap that says 2uF/25V, will it make a difference? Yes, that cap will probably fail pretty quickly, it could explode the moment you power the amp on. But as long as it's working, it won't make a difference to the sound.

    If you replace it with a cap that says 1uF/100V or 3uF/100V, will it make a difference? Yes, now things may sound different. Whether they will or not depends on where that cap is, what it's doing, etc.

    As a side note, it is not a good idea to blindly increase the capacitance value of power supply caps. Bad things can happen. Increasing their voltage rating is usually harmless.

    I hope that answers some of your questions. Like I said, I didn't totally understand what you were asking, so if this raises more questions, feel free to post them. And if anything in what I said is incorrect, I'm sure some of the more knowledgable members will correct me.
     
  3. Aslam Imran

    Aslam Imran Second Unit

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    See if this makes sense. For example the B&K 5125 (125 x 5) uses four capacitors of 25,000 mF at 50V, giving it a total capacitance of 20,000 mF per channel. The B&K ref2220 (220 x 2) uses four 9000 mF @ 75V, giving it a total capacitance of 18,000 mF per channel. So my question is why does the B&K 2220 have lesser capacitance/channel even though it is rated at almost twice the output capability of the 5125. Hope this makes sense.
     
  4. Saurav

    Saurav Cinematographer

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    Ah, so you're trying to find a correlation between the amplifier's power rating and it's power supply capacitance. I think the issue here is your basic assumption that those two are directly related. The capacitance dictates how much power is held in reserve, how much the amp can output for short durations over and above the basic capacity of the power supply. The capacitance doesn't dictate how much steady-state power the amp produces, it has more of an effect on transient reserve power above that steady-state level.
    So, in your case, the second amp probably has a more powerful PS, and it has lesser reserve capacitance.
    Does that make more sense? [​IMG]
     
  5. Eric T

    Eric T Second Unit

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    Capacitance is a measure of charge stored at a given voltage. It varies depending on the voltage and the contruction of the capacitor. The ratings given for the amps show the capacitance (in mF? probably uF, or micro-Farads, not milli-Farads), and at which voltage that capacitance was measured.

    I don't think you can use these figures to determine capacitance per channel. Most likely the capacitors are tied to a common power source (which is why you only see 4 capacitors on a 5-channel amp).
     
  6. Aslam Imran

    Aslam Imran Second Unit

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  7. Larry B

    Larry B Screenwriter

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    Aslam"
     
  8. Saurav

    Saurav Cinematographer

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  9. Aslam Imran

    Aslam Imran Second Unit

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    Thanks Saurav, I know what you mean.

    Larry by 'lower shelf' you mean a shelf that sits lower in the rack or do you mean a shelf in which the lower plate sits lower than the lower plate of the other shelf assuming that the upper plates on both the shelves are at the same height?
     
  10. Blake R

    Blake R Stunt Coordinator

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    Aslam - I did not see this posted so here's the actual engineering relationship.

    V=Q/C

    V = voltage
    Q = charge in coulombs
    C = capacitance in farads
     
  11. Saurav

    Saurav Cinematographer

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  12. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    The factors relating voltage and capacitance of a capacitor are size and cost. A capacitor capable of storing a given charge at voltage x can be made smaller and cheaper than a capacitor of identical capacitance and construction rated to a higher voltage y.

    A capacitor is basically two plates with a gap in between. The capacitance is a function of the area of the plates (bigger is better) and the gap (smaller is better). The material filling the gap is known as the dielectric, and must provide excellent insulation. However, to prevent breakdown, the gap must be made larger to support higher voltages.

    In a component capacitor, the plates are rolled up tightly, hence the cylindircal shape.

    Aslam - in your example (third post of the thread), the voltages shown are the lowest voltage rating available that is higher than the amps' supply voltage. The amp with higher output per channel probably has a higher supply voltage to give it a higher output voltage limit to achieve the higher output. Note that the 125Wx5 amp's total output is 625W compared with 440W for the 220Wx2.
     
  13. Blake R

    Blake R Stunt Coordinator

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    The dielectric material is also a determining factor in the energy density storage limit of the capacitor. If you choose for example tantalum, it will give you greater storage capacity per unit area of plate than say mylar, or paper. The selection of dielectric properties can allow you to reduce plate area, plate separation, or both, in order to get more capacitance into a smaller space.
     
  14. Larry B

    Larry B Screenwriter

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    Saurav:

     
  15. Aslam Imran

    Aslam Imran Second Unit

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