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cap self discharge

Discussion in 'General Electronics Discussion' started by ratstar, Feb 17, 2021.

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  1. ratstar

    ratstar

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    Aug 20, 2018
    Ok. So myth busters time.

    When people show cap self discharge, you may get a mental picture like this.

    [​IMG]

    The blue circuit, is it false or factual - it means the energy has bypassed the circuit to the other side of the capacitor without touching the circuit - or does self discharge mean the capacitors are constantly letting go but its actually into your provided circuit.

    It makes a big difference, if its at least making it to the circuit you might still be able to use the energy, otherwise its a useless waste completely.

    But I just thought of something, it means that if a capacitor isnt in a loop, it'll never leak current - and that might not be true!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Self discharge happens internally to the capacitor (blue resistor).
    If the capacitor were to discharge into the red circuit, this wouldn't be called self-discharge.
     
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  3. Martaine2005

    Martaine2005

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    May 12, 2015
    The circuit ‘myth’ is in fact very true.
    Without the resistor, the capacitor would eventually discharge through the circuit. Or at least hold its charge.
    The resistor discharges the cap when the supply is interrupted. The resistor also keeps the cap discharged when no supply is available.
    A typical example is a bleed resistor.

    So, think about your TV stand by LED. When you unplug the TV, the LED fades away rather than instantly off.

    Martin
     
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  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Martin, I think @ratstar wasn't referring to physical external bleeder capacitors. Self discharge happens when the unbalanced charge distribution (charged capacitor) between the two electrodes of a capacitor equalizes with time due to leakage through the non-ideal isolating material separating the two electrodes.
     
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  5. Martaine2005

    Martaine2005

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    May 12, 2015
    Ah yes, good point.
    In that case a simple experiment can provide the required answer.
    Charge a cap out of circuit. Measure it’s voltage. Let it sit all alone on the bench for 10 minutes, measure the voltage again. Keep doing this and you’ll see that it’s self discharging over time.
    Obviously the time to self discharge will depend on the cap value.

    Martin
     
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  6. ratstar

    ratstar

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    Aug 20, 2018
    Yep thanks for that, I understand now. So theres energy being lost that you cant get back, I wonder how much of the battery even enters the circuit, theres 25% just disappears without a trace, maybe more...
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Not as self discharge within capacitors. That rate is typically rather small.
    But if you wait long enough the loss will be 100 %.
    Again I have no idea what you're up to.
    @ratstar : before jumoing to premature conclusions you need to read a bit...
     
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