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I'm into model railroading, and I was modifying one of the cars to have extra LEDs.

There are 2 wires going through this car that connects to the tracks to receive power. I cut the red and black wires, connected everything correctly to the right polarity for the LED and the capacitor. So I apply full power to the car, 18 volts. The capacitor can take up to 50V (220uf).

All seemed fine, I cut the power to the train and the LED stayed lit for a few seconds as it's supposed to. So I decided to give it full power again and leave it so it can charge the capacitor. All of sudden I started hearing noises then BOOM!!!! The capacitor exploded around my face and burst into flames nearly killing me and destroying my house.

Here's how it looks like.

The film is completely out. I asked one of my friends and he said it's because the power going through the wires is AC and it can feed negative polarity to the positive wire.

Regardless, next time I must use a diode to keep the flow only "+" to the "+" leg. So my question is Why has this happened?

I'm now traumatized of ever using capacitors again.

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Was this capacitor a component you added to the car, or was it already in there? –  whatsisname May 4 at 16:33
    
Is your rail power DC or DCC? With DCC, the catastrophic failure of the electrocap would be a pretty sure thing :-) With DC, when you reverse the loco, same result, boom! Call it a prototypical rail accident. –  Anindo Ghosh May 4 at 16:53
    
Radial has nothing to do with it. Electrolytic has everything. –  mickeyf May 5 at 1:07
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2 Answers 2

Electrolytic caps are polarized, they can indeed blow up if you get polarization wrong or apply a higher than rated voltage to them. Same goes for tantalum capacitors, these can be even more dangerous.

If you look at the capacitor, there is a + and a - sign on it, they're there for a reason.

Depending on what you are trying to accomplish, a series diode or a diode bridge may be a solution, but it is hard to tell without a full circuit diagram of your set up.

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Polarized caps cannot be used with AC without a DC bias.

Polarized, or electrolytic capacitors, are used because despite that restriction, they provide a good capacitance per volume, and so are useful in situations where that limitation is not a problem.

What happens when you reverse the voltage, is the electrolyte breaks down due to electro-chemical reactions, and ultimately creates a short circuit in the capacitor. This short circuit can boil the electrolyte, causing the capacitor to catastrophically discharge, as you saw.

For your train situation, because you reverse the voltage on the tracks to back up the train, you cannot use a polarized capacitor. I'm doubtful that a diode would resolve the situation, for I suspect that will just create a short circuit when you reverse the train. Instead, use a non-polarized capacitor. We need to see your entire circuit to be sure.

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How would a diode cause anything other than to open the circuit, in response to a current against its bias? Adding a diode would indeed protect the capacitor, though at the (maybe significant, maybe not) cost of the LED circuit being entirely de-powered in reverse. If this is a problem, then simply replace the single diode in series with the capacitor, with a bridge rectifier of 4 diodes across the track, connecting the LED-cap circuit to the rectifier's output instead of to the track. –  Matthew Najmon May 4 at 16:39
    
@MatthewNajmon: I'm not exactly sure, but that's just my spidey sense because the OP hasn't posted his circuit, and I am assuming it would have errors. –  whatsisname May 4 at 16:48
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