What is the problem with my electrolytic capacitors

Discussion in 'General Electronics Chat' started by Jacky Lin, Jun 30, 2016.

  1. Jacky Lin

    Thread Starter New Member

    Jun 27, 2016
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    In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal. A capacitor is much simpler than a battery, as it can't produce new electrons -- it only stores them.
    Electrolytic capacitors are well known to pass a small DC current. How much they pass is a complex function of temperature, capacitance, the age of the capacitor, and the DC voltage across it.
    Although I search a lot of related information, I still have a quesition and hope someone can tell me the answer.
    Why is that when I use electrolytic capacitors (the cylindrical ones) and measure the end with respect to ground, there is some DC voltage appearing (where in fact it should be 0 volts since capacitor passes only AC signal but blocks DC signal), but when I substituted a non electrolytic capacitor, like tantalum capacitors, they work well. In a simulation program, electrolytic and tantalum shows no difference and they block DC, but in real life, electrolytic fails to function correctly. What's happening?
     
    Last edited: Jun 30, 2016
  2. crutschow

    Expert

    Mar 14, 2008
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    The problem is, we only speak English here. :oops:
     
  3. Jacky Lin

    Thread Starter New Member

    Jun 27, 2016
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    I am sorry. I have corrected into English.
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    I'm still not clear what you mean but:
    Tantalum capacitors are electrolytic they just use different chemicals to achieve a similar effect to aluminium electrolytics.
    Batteries don't produce new electrons either, they just 'suck' them in one side 'blow' them out the other side.
    As regards the remaining voltage, I am not sure just what you are measuring. Could this be leakage through the capacitor or dielectric absorption (https://en.wikipedia.org/wiki/Dielectric_absorption).
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    1,789
    If you look at the differential equation for an RC circuit, you will notice that with finite component values it takes an infinite amount of time to charge a capacitor to 100% of the source voltage. Similarly it takes an infinite amount of time to discharge one to 0% of the source voltage (aka GND) as well. A voltmeter is a high impedance device so any residual charge on the capacitor can be measured and will discharge very slowly.
     
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  6. EM Fields

    Member

    Jun 8, 2016
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    Can you post a schematic showing the capacitor in the circuit in which you measured the voltage and describe how you made the measurement, please?
     
  7. RichardO

    Well-Known Member

    May 4, 2013
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    If I understand your description correct then I think the electrolytic capacitor has more leakage current than the tantalum capacitor. Look at the properties for the capacitors you are using in the simulator. You may find that the equivalent parallel resistance specified is not realistic.
     
  8. Techno Tronix

    Member

    Jan 10, 2015
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    I agree. Even bad electrolytic capacitors cause frequent failures of switch mode power supplies.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,100
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    The first post reads to me like the TS is seeing a DC voltage on electrolytic capacitors but never on others. This is completely normal and a result of the much greater capacity of the electrolytics. The other capacitors are very likely less than 1µF and thus do not contain enough charge to power the impedance of the multimeter long enough to get a reading. They discharge the instant they are touched. An electrolytic might be 1000µF and has plenty of charge to provide a reading. You can watch it slowly discharge through the meter's impedance, which is probably 1-10MΩ.

    The notion that a capacitor cannot pass a steady state DC voltage, except a tiny leakage, is true. But a large capacitor may take a long time – minutes or more – to achieve steady state. While it is not in steady state, the voltage will look a lot like DC.
     
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