Capacitor question

Discussion in 'General Electronics Chat' started by Guinness1759, Jan 26, 2011.

  1. Guinness1759

    Thread Starter Member

    Dec 10, 2010
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    Hi,
    I'm looking for the a word or term that refers to the following. When you have a capacitor that has charge stored on it and some of that charge is drawn away quickly, for example in the case of a transmitter, the voltage drops way down but comes back up after the transmitter is turned off. The capacitor is by itself without external voltage source supplying it power, so it's like it charges itself back up, not quite to the original level obviously. This is happening with large capacitors, such as 5F or 10F.
    I'm noticing this happening and I need to write about it, but can't think of the word that describes this behavior, or if there is one. Thoughts?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Rebound?

    It happens in many systems and can result from taking your measurements before equilibrium/steady state is reached.

    John
     
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  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Rebound has to do with internal parasitics of the capacitor. The bulk of the capacitors charge can be "dumped" quickly, leaving one to believe it is discharged. With large value caps and/or high voltage, there is more charge "deeper" in the structure of the capacitor that cannot be drained as easily.

    Once the majority of the current is dumped, if the terminals are left with no load (disconnected), these charges will migrate to the areas where they can be quickly dumped, giving the appearance of the capacitor "recharging itself". This charge can build up a few times, though weaker each time.

    The only analog I can think of is when you run an Alkaline battery dead, if you turn the item off for a ten minutes, you can usually get more use from the device from the "dead" batteries due to a similar chemistry effect.

    Caps taken out of high voltage/high capacity circiuts should have a 100k or so resistor shorted across the terminals for continual equalization of the plates.
     
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  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    How about if you have a hot date and the next morning, she decides she hates you. Does that count as another example?

    John
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    We need a "smite" button. ;)
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You are thinking of NiCd batteries.
     
  7. magnet18

    Senior Member

    Dec 22, 2010
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    Unless you try to get back together after this happens, but you slip up in the get-together phase just once, and she blows up in your face and is gone forever. Then its a LiPO battery.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This type of behaviour can be a serious safety hazard with some types of high voltage capacitors. Even if they have been discharged after being used or tested, it is possible for a dangerous voltage to build up across them some time later.

    Because of this, it is usual to store such components with their terminals short-circuited.
     
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