Easy question

Discussion in 'General Electronics Chat' started by jose2, Feb 28, 2011.

  1. jose2

    Thread Starter New Member

    Feb 28, 2011
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    Hi all and thanks in advance. I have a subject that is not related with my studies and I have to solve a small question.

    Which advantages or when must I use series capacitors?
    Which advantages or when must I use parallel capacitors?

    Than you very much.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Capacitors should never be connected in series but my 32" Sony TV had some in series because higher voltage ones were not available.

    Capacitors are frequently used in parallel. A high value one filters low frequencies and a low value one filters high frequencies.
     
  3. jose2

    Thread Starter New Member

    Feb 28, 2011
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    I have read with series capacitors, you can increase voltage with a lower capacitance, but this answer is very short and I don´t now the reasons very well...

    Thanks again
     
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    For example if you needed a 100pF capacitor, and all you have are 200pF on hand, you could place two of the 200pF in series and get your equivalent 100pF. Also, two polarized capacitors of equal value can be placed in series to form a non-polarized version (like ends together), at half the value and twice the voltage rating. Note that in using two parts, tolerances may add up.
     
  5. jose2

    Thread Starter New Member

    Feb 28, 2011
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    But in which situations is more suitable one than another?

    Thanks
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    I think it is stupid to have two capacitors in series. Buy one with the proper high voltage rating or the proper value. If you need a non-polarized capacitor then use one because they are available and are inexpensive.
     
  7. jose2

    Thread Starter New Member

    Feb 28, 2011
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    I posted this topic as a theorical question...

    Thanks anyway, really.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Parallel connection of capacitors is far more common, either to obtain a larger total value (the capacitance simply adds) or to obtain a particular value which may not be available otherwise.

    The use of a large number of capacitors in parallel may allow a large discharge of energy to be obtained with a high peak current, as in the case of a capacitor bank used to power a pulsed laser.

    As has already been mentioned, series connection of capacitors to obtain a greater maximum working voltage is problematic. Leakage currents in the capacitors will tend to unbalance the capacitor voltages, and can lead to failure unless the leakage can be swamped (e.g.) by parallel resistors.

    There are a number of circuit applications involving capacitors connected in series, such as in the capacitive voltage divider, the Colpitts oscillator, or diode/capacitor voltage multipliers, but in these cases the junctions of the capacitors are required to allow the circuits to function. The aim is not simply to obtain a single equivalent capacitor.
     
  9. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Another example of capacitors in series are relatively low voltage, high capacitance, "super caps" that are used instead of batteries in a "hold-up" circuit. As previously mentioned, in such cases the capacitors must also be in parallel with an appropriate resistor chain to equalize distribution/charging and protect the capacitors. For instance, using four 6.3V super caps in series to support 20V for a specified period after the main 20V supply goes off.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The voltage of a discharging capacitor drops quickly like a rock.
    The voltage of a discharging rechargeable battery stays high until near the end.
    A huge difference.
     
  11. JMac3108

    Active Member

    Aug 16, 2010
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    Capacitors are placed in series commonly. Here are two very common examples.

    (1) Power supplies where you need high capacitance and low ESR (equivalent series resistance of the cap - remember that caps are not ideal and have a small amount of resistance inside them). You place several caps in parallel and the capacitances add up, and the ESR (resistance) is reduced (remember parallel resistors formula).

    (2) Bypass caps in digital circuits. Its common to place several values of ceramic capacitors in parallel to bypass a chip. For example a 1uf, .1uf, and 100pf. Each one targets higher frequencies with the smallest 100pf cap for the highest frequencies.

    Capacitors are not often placed in series. Theoretically you can put them in series and the voltage ratings add up. For example two 100uf 50V caps in series result in a total of 50uf and 100V rating. However, the problem is that the caps will not always divide the voltage evenly. In other words, in my example above with the two 100uf 50V caps in series, if you put 100V across them, one might take 60V and the other 40V. Sometimes designers are forced to put caps in series for reasons usually related to physical space constraints and high voltage requirements (they can't fit the high voltage cap in the space they have available so they use two smaller caps in series). When this is done, the usual scheme is to put a voltage divider (resistors) in parallel with the caps in the hope that the resistors help to divide the voltage to the caps evenly.

    Hope this helps.
     
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