Polarized Capacitor in reverse direction??

Discussion in 'General Electronics Chat' started by rrrchandu, Sep 22, 2012.

  1. rrrchandu

    Thread Starter New Member

    Aug 9, 2010
    I am very much confused with the polarized capacitor's response when it is charged with reverse polarities, i.e. its negative terminal connected to positive terminal of battery and its positive is to negative of battery without any resistor in between.
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Back at my first job we used to do just that for fun, take an electrolytic cap, put it on some stiff leads, slide it along the floor under someone else's bench, and turn it on with reverse voltage from a lab power supply.

    They rather quickly exploded. Loud bang. White smoke. Confetti too.

    Pretty cool. Kids, don't try this at home.
  3. JDT

    Well-Known Member

    Feb 12, 2009
    This will normally cause very high leakage current. The capacitor will be damaged. Sealed tantalum capacitors will quite possibly explode!
  4. ramancini8


    Jul 18, 2012
    Reverse polarity reverses the chemical process in the capacitor (depending on type) causing a gas buildup that sometimes explodes. Other types have a reverse reaction with less gas buildup, but the quality of the capacitor (leakage current) is degraded. A parallel diode is required if voltage is reversed momentarily, but using a nonpolarized capacitor is better practice.
  5. rrrchandu

    Thread Starter New Member

    Aug 9, 2010
    thank you all.
    usually a polarized cap is used with bjt amplifier as input cap, but in reverse direction.
    what's the reason there? i am not understanding
  6. panic mode

    Senior Member

    Oct 10, 2011
    what do you mean in reverse direction? can you post circuit?
  7. Wendy


    Mar 24, 2008
    Tantalum tend to simply catch fire like a match head in my experience.

    You can not attach a electrolytic backwards without damaging it. There is, however, one exception to this rule. You can make a non polarized cap with two electrolytic caps, it is an old trick. The forward biased cap takes the strain.


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  8. MrChips


    Oct 2, 2009
    A BJT amplifier uses a capacitor to pass the AC signal while it blocks the DC voltage. If you allowed the DC to get to the input of the BJT it would mess up the critical base bias of the amplifier.

    Ideally, you would use a non-polarized capacitor but any cap over 1μF is expensive, hard to find and physically large. A capacitance less than 1μF is too low in value and the amplifier would have poor response at low frequencies.

    Hence an electrolytic capacitor greater than 1μF is substituted, usually 10μF-47μF for the input side depending on impedances and the frequency response desired.

    The output capacitor to an 8Ω speaker would have to be 2200μF-4700μF if you want to achieve decent performance below 100Hz.

    Which way should the electrolytic capacitor be connected?

    "Reverse direction" is a misunderstanding.

    Simply, the capacitor should be connected with the +ve lead towards the more positive node of the circuit. That is, you want to maintain the correct voltage bias on the electrolytic. It should never be reversed biased.

    Hence one has to examine and analyze the two nodes of the circuit where the capacitor is to be installed. Determine which node is the more positive and install the capacitor accordingly.