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Discharging capacitor without resistor

  1. Jun 29, 2013 #1
    Hi all,
    So I created this account specifically to get some clarification on capacitors. I've been reading alot, but can't seem to find any info on whether a resistor is essential in series to discharge a capacitor. Think of the simplest circuit possible that switches between a battery to charge and a resistor in series to discharge. I have seen that later this concept is applied to slow the rate of discharge, but is a resistor actually required, or will the capacitor discharge just fine without one? I get the sense it may have to do with too much current possibly causing dielectric breakdown, but I'm not sure. Any clarification would be greatly appreciated.
     
  2. jcsd
  3. Jun 29, 2013 #2

    tiny-tim

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    welcome to pf!

    hi psilocybin! welcome to pf! :smile:
    there's no such thing as a circuit without any resistance!

    the resistor R that you see in the equations is the sum of all the resistances in the circuit (including the internal resistance of any battery)

    so if you put a capacitor C into a circuit, there'll be an R anyway

    the value of R affects the half-life of the discharge …

    the time constant for the discharge (1/the decay constant) will be CR …

    V = Voe-t/CR
     
  4. Jun 29, 2013 #3
    Disregard that guess about dielectric breakdown, I can see why that wouldn't make sense. I'm grasping for straws here, I just get the idea something is problematic about not including extra resistance.
     
  5. Jun 29, 2013 #4
    As indicated by tiny-tim the greater the resistance in the circuit the longer the time it takes for the capacitor to discharge. If the resistance is very small the discharge time will be small also and I think that can result in a spark that, depending on the size of the capacitance etc may be damaging.
     
  6. Jun 29, 2013 #5
    For very large capacitors, the discharge current is high enough to damage the capacitors internal connections. It's always a good idea to use a resistor to discharge a large capacitor.
    A resistor may be required to limit the capacitor discharge current to less than the discharge device's current rating.
     
  7. Jun 29, 2013 #6
    What, exactly, do you mean by the term 'damaging depending on the size of the capacitance'....do you mean damaging to people, components,....what?
    Can you give numerical examples?
    Not being rude and not seeking infractions but would appreciate these terms specified.
     
    Last edited: Jun 29, 2013
  8. Jun 29, 2013 #7
    Thank you guys for some of the clarification, my question was mostly assuming negligible resistance due to battery and wire. Mostly, what happens when a silver/copper (or some other wire of low resistance) wire is attached to both ends of the capacitor. Is the discharge so fast it will damage the capacitor or even overheat the wire?
     
  9. Jun 29, 2013 #8

    NascentOxygen

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    Certainly, a high capacity condenser can be damaged by rapid discharge. The metal foil which form the capacitor plates is thin, and of considerable length, so has significant resistance. A high current generates heat, and this is confined. If a short develops inside the capacitor, all remaining stored energy may be dissipated in that tiny region. The dielectric destruction is likely to generate vapour, hence pressure! Bang!
     
  10. Jun 29, 2013 #9

    Integral

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    Not so sure about new materials.

    In the old days the best way to work on circuits with large caps was to start by using a screwdriver to short the caps. That is the ONLY way to ensure that it is fully discharged.

    It would be my guess that if a modern dialectic could be damaged by shorting the leads, then there would be a resistance built in that would prevent a direct short.

    So the simple answer to the OP is, No you do not need a resistance in series with a Cap.
     
  11. Jun 29, 2013 #10
    Given the size of the capacitance(C) the voltage to which it is charged(V) and the effective resistance(R) of the disharge circuit you can calculate the following

    Energy stored=0.5CV squared
    Initial(maximum)discharge current=V/R
    I (and V)fall exponentially with time with a time constant RC. For a small R a lot of energy can be dumped in a short time.
    Currents can be high enough to melt parts of the circuitry particularly around any tapered metal parts.
    Before any short circuits are actually made the field strength in the gap between the connectors increase as they approach and when it reaches a value of about 3.6 million Volts per metre(I think that's about the right value ) a spark can be generated which can vapourise parts of the circuitry.
    There is also the danger of shock. It's better to be safe than sorry so always treat capacitors with respect.
     
  12. Jul 1, 2013 #11

    sophiecentaur

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    I think the manufacturers specify a maximum discharge rate for high power capacitors, along with a lot of other data which we often just ignore - once we have the right Capacitance and working voltage. Choose a particular Capacitor and 'dig around' the data sheet.
    'Ripple' is another important factor for some applications.
     
  13. Jul 1, 2013 #12
    As I have often referred to this case - the capacitor stores ENERGY - to discharge the capacitor, you must get rid (dissipate, transfer ) the energy.

    If you close a switch across a charged capacitor - with no intentional added resistance, the current will probably be higher then the rating of the switch - this may word for a while, but generally degrade the switch over time.

    And.... I have to counter point Integral - A screwdriver is not the best way to discharge a large cap. Actually - an alligator lead with a built in series resistor is better, then once discharged, the terminals of the cap can actually be wired together - shorted. On large utility grade caps - you would not want to weld your screwdriver to the terminals...
     
  14. Jul 7, 2013 #13

    Baluncore

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    A capacitor has an ESR specification, the effective series resistance. If the energy stored in the capacitor was dumped in the ESR of the capacitor by a short circuit, it might cook the dielectric. If so, rather than a short, you will need to use external resistance to remove the energy from the capacitor component.

    Capacitors also have a maximum surge current rating. That will be a little less than the current that will fuse the very thin conductor foil used.

    You could in theory use an inductor to discharge a capacitor, but oscillation may reverse the voltage and so destroy a polarised dielectric such as a tantalum capacitor.

    So avoid shorting capacitors, use a resistor when possible. If there is a safety issue, then use a screwdriver only to verify your foolish conviction that the capacitor was discharged.
     
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