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schematic

simulate this circuit – Schematic created using CircuitLab

I have a 470ohm resistor, 22uF capacitor and a 5mm LED in series (same sequence as described), connected to a 9V DC battery. I was expecting the LED to fade away but it didn't, it continues to glow with full power.

I am not able to understand why, your inputs will provide clarity.

Edit:

I realized that the polarity of capacitor was reversed. I corrected it and am now getting the expected result. However, I'm still confused as to why the LED was lit when the capacitor was reversed.

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    \$\begingroup\$ If the capacitor is polarised have you got it the correct way round? \$\endgroup\$ – Steve G Mar 7 '16 at 16:15
  • \$\begingroup\$ There is a schematic diagram editor you can use if you click "edit" under your question. Draw your circuit with it. Diagrams make it easier to discuss circuits. \$\endgroup\$ – JRE Mar 7 '16 at 16:17
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    \$\begingroup\$ @trickyal: because leakage current is a thing \$\endgroup\$ – PlasmaHH Mar 7 '16 at 16:19
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    \$\begingroup\$ Don't close it. Let someone answer it, then accept the answer. This is a good question that could help someone else. All it needs is a good answer posted as an answer instead of a comment. \$\endgroup\$ – JRE Mar 7 '16 at 16:30
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    \$\begingroup\$ @SteveG, you should take this one and answer it! \$\endgroup\$ – user65586 Mar 7 '16 at 16:31
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The electrolytic cap in your circuit is reverse biased. The reverse biasing of the capacitor removes the isolating oxide layer, so it allows current to pass. If you connect the capacitor the right way after mistreating it this way, the electrochemical process that dissolved the oxide layer is reversed and the capacitor recovers. You have to be careful to limit the current through the capacitor, because high currents caused by leaks in the oxide layer can permanently damage the capacitor. The dropper resistor in your LED circuit is likely enough to limit the current to safe values, but do not connect an electrolytic capacitor that got reversed directly to a battery or lab supply without setting a current limit.

The isolating oxide layer can not only be damaged by reverse polarity, but also dissolve over time if no voltage is applied to the cap. As soon as forward voltage is applied, the leakage current cures the oxide layer by electrolysis, so the cap heals itself. This is why it is sometimes recommended to slowly ramp up the voltage on vintage tube radios that were unpowered for dozens of years, although in some configuration under-heating a tube is bad for the cathode. The process of creating or restoring the oxide layer is called forming.

So as you likely understood, you can abuse an electrolytic cap as rectifier, because it conducts current when reverse biased, but blocks current when polarized the correct way. People actually did use a similar configuration of electrodes and electrolyte as you find in an electrolytic cap as rectifiers before semiconductor rectifiers were invented. They were called electrolytic rectifiers.

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  • \$\begingroup\$ I was under the impression--probably from watching too many YouTube videos--that if you connect an electrolytic capacitor backwards, it will explode. Is that only true for high voltages or currents? What is safe? \$\endgroup\$ – Willis Blackburn Mar 12 '16 at 5:29
  • \$\begingroup\$ Capacitors can "explode" if the electrolyte gets gaseous. This could happen either caused by temperature (through resistive heating) or by electrolysis of the electrolyte (it contains water, so you can split it into oxygen and hydrogen). For thermal destruction, its just the current that is important, it might either be AC or DC. The amount of gas produced by electrolysis also depends on the current, but in this case it has to be DC. To get DC current through a capacitor, you have to destroy the isolating oxide either with reverse voltage or over voltage. \$\endgroup\$ – Michael Karcher Mar 12 '16 at 6:26
  • \$\begingroup\$ @Willis Electrolytic caps are not specified to operate conductively with reverse voltage applied, so there us no definitely safe current. A rule of thumb is that typical caps are not destroyed by applying line-frequency AC if it does not exceed around 3% of the rated DC voltage. I expect this rule to be false for high-capacitance low ESR caps, because even low voltages mean a lot of charge. \$\endgroup\$ – Michael Karcher Mar 12 '16 at 6:30

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