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I'm trying to debug a friend's AMP6 audio amp kit. It seems that an electrolytic smoothing cap on the output of a 5 V regulator has been soldered in backwards. This seems bad, but the component is going to be a little tricky to desolder because of its physical location. Because of this, I'm trying to be certain that this is the problem before attempting the job.

Before it explodes, how does a backwards cap behave? Like a 100 ohm resistor?

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I have actually intentionally blown up an electrolytic capacitor in order to demonstrate to students the concern of putting them in properly.

When I applied 5v to it, my power supply was current limiting at over 5amps. Clearly the effective resistance is much lower then 1 ohm.

However, when using a DMM to measure resistance, the DMM will only be applying a very small amount of current, small enough that you wont hurt the capacitor. As for the actual resistance shown, I am not sure of this and expect it will depend on many aspects including how the DMM is measuring and the fab process of the cap.

If you have a DMM with a diode tester, I would try to use this to determine the polarity. This will put more current through the capacitor, but it shouldn't be large enough to cause it to go pop.

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I would be more concerned with the risk of explosion/destruction of the capacitor, than I would with the resistance.

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Most electrolytic capacitors are polarized and require one of the electrodes to be positive relative to the other; they may catastrophically fail if voltage is reversed. This is because a reverse-bias voltage above 1 to 1.5 V[1][2][3] will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst. This is because, if the aluminium foil with a layer of aluminium oxide on it is made negative the oxide ion will get reduced and will convert into oxygen gas generating a high pressure and hence may burst open the capacitor.[citation needed] This is same as the electrochemical principle in an electrolyte with 2 electrodes.

http://en.wikipedia.org/wiki/Electrolytic_capacitor

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That would depend upon the type/construction/driver characteristics for this particular instance.

I have had a number of instances of faulty manufacture resulting in reversed electrolytic capacitors. The results were

  • Aluminium foil - A roomfull of shredded aluminium confetti. Very pretty but a pain to clear up as it got everywhere.
  • Solid Tantalum (cylindrical) - a molten blob like a very large lump of solder between the component leads. In this case the short circuit current was limited by a series resistance in the supply line.
  • Chip Tantalum - Smoke and flame from the burning plastic body of the components.

In all cases accompanied by the "lovely" aroma of burning plastic.

I never had a chance to find out what they looked like as a circuit element in the few seconds before the catastrophic failure.

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  • \$\begingroup\$ I'd like to add a 'burning smell, and definite pop sound' to the Aluminum foil description. \$\endgroup\$ – ArielP Nov 1 '10 at 14:29
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    \$\begingroup\$ I hate the smell of a pop'ed electrolytic. \$\endgroup\$ – Kortuk Nov 1 '10 at 14:36
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We had a couple .1uF electrolytic go on backwards on a PCB due to the solderer not realizing they were polarized caps(we did not mean to order them).

This caused very odd problems only solved by adding large decoupling capacitors to power(10uF). It was very odd but only had around 20 extra mA being pulled at 5V. We found out we had put them on backwards, swapped them and there was no need for the extra caps on power.

My long and short of it, I am not sure how I would have told in the same situation that this happened.

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