Capacitor theory question

Discussion in 'Homework Help' started by c1rcu1ts, Nov 21, 2013.

  1. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
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    what is the difference between a polarized capacitor and a non polarized capacitor?

    what would make you use a polarized capacitor over a non-polarized one?

    When i used a non-polarized capacitor as the output capacitor for my common emitter BJT (H-bias design), i observed that the waveform appeared to be ''slanted'' instead of being horizontal when clipped (on the oscilloscope). When i replaced the capacitor with a polarized one, the waveform acquired the horizontal part when clipped. Why?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    A polarized capacitor uses construction techniques that result in a very high capacitance per size or weight. The techniques are such that if voltage is applied in the opposite direction it breaks down the capacitor and it starts conducting current.

    Off the top of my head, I would use a polarized capacitor when I need high capacitance, know that it will never be reverse polarized, can tolerate the type of leakage currents that many polarized capacitors exhibit, and don't need to rely on accurate, stable performance.

    To answer this, we need to see the circuit and some shots of the waveforms you are seeing. It might be a result of ESR, self-resonance, or leakage currents.
     
    c1rcu1ts likes this.
  3. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
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    Thanks but i do not have pictures.
     
  4. WBahn

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    Mar 31, 2012
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    The use Paint to sketch the circuit and the waveforms. We aren't mind readers.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are examples where electrolytic's are used on AC where large values are required without resorting to what would be excessive physically large packages.
    An example is in start capacitors for AC split phase motors, In this case the capacitor is made bi-polar by virtue of the manufacturing process including two capacitors back to back in the same case, but the ESR is still very low, so the application should ensure that they are not in circuit for any lengthy period, otherwise they can destruct.
    Max.
     
  6. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Are you saying that, given two otherwise identical start capacitors, but with one having a high ESR and one having a low ESR, the one with the low ESR will be more likely to be damaged if left in circuit for a lengthy period?
     
  7. WBahn

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    Mar 31, 2012
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    Is it the ESR that is the problem, or the leakage current? Or does ESR, in this case, stand for Effective Shunt Resistance instead of Effective Series Resistance? I hope not, but I've seen wackier contradictions.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes, basically.
    ESR, Equivalent Series resistance on AC.
    In a typical split phase motor with a start capacitor and a run capacitor, the run cap is usually a bi-polar AC rated high ESR capacitor, therefore can be left in circuit continuously, often construction is oil filled paper etc..
    The start capacitor is generally a larger capacitor value so in order to keep the physical size down, the case contains two electrolytic capacitor connected back to back, but being both low ESR can only be allowed in circuit on AC for a short period otherwise overheating and destruction results.
    Max.
     
    Last edited: Nov 22, 2013
  9. WBahn

    Moderator

    Mar 31, 2012
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    I'm not following this. You say that the run cap is low ESR and can therefore be left in continuously. But then you are say that the start cap can't be left in because it is low ESR. So why is low ESR desirable in the run cap but not the start cap?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Sorry, miss-type, it should be high ESR, I will correct it.
    Max.
     
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