polarized caps

[ericbirecki]

I would like to understand why electrolytic caps and some others are unidirectional.

One of my instructors tried the scare tactic that they are almost the same size as m-80s and will blow with equivalent force. I know that's not true, they simply heat up, heat causes the electrolyte to vaporize causing pressure that will vent. But, what causes them to heat when there is no heat generated in other caps.

As I am writing this, I am assuming that some of the energy is stored in the electrolyte, but that doesn't sound right either because a cap can deliver an instantaneous burst, and I don't think that would be possible with a chemical reaction. And, the electrolyte must be touching both terminals within the cap, which would cause a short.

Lastly, what is the difference in the AC cycle that affects the direction of the cap. More I think about this topic, the more I wonder.

Eric B.

 

[Oznog]

It will conduct current if you give them a significant reverse voltage. This is what causes the reaction in the electrolyte as long as that voltage persists, and boom boom results. It can be a pretty decent pop with electrolytic cans, tantalums burst into flame. Don't ask me how I know.

 

[Noggin]

Yeah I fucked up with a tantalum SMT. I mounted it backwards on a board and plugged it in. Came back the next day, cap was black and the pads lifted off the board. 16v tant if I remember correctly and -5v was applied to it.

 

[zachtheterrible]

hey noggin, i hope that avatar isnt a picture of yourself :lol: thats prety funny

 

[Dean Huster]

Eric, the electrolyte and applied voltage work together to form a thin insulating layer on one of the plates. It's this insulating layer that is the dielectric and it's the electrostatic field in a dielectric where energy is stored in a capacitor. The electrolyte not only helps to form the dielectric, it is also in the most intimate contact that you can imagine with the dielectric, allowing the distance between the plates to be closer than in any other type of non-electrolytic capacitor. That's why the capacitance values are so high in electrolytics.

I've put together a video of exploding capacitors for my students in years past. We even had them sent to the local TV station to have them transfer to their big equipment to give us a slo-mo of the action. A standard electrolytic, put under reverse voltage conditions with a power supply capable of a lot of power will explode. BANG!! The aluminum jacket will rupture, the contents will confetti everywhere and if it weren't for the fact that the leads are soldered to a PCB in real life, pieces other than the contents would be ejected forcefully. They are not playthings and because of this possiblity, my students were always required to wear eye protection when breadboarding circuits using electrolytics. Our op amps course always had a couple of 10µF radial-lead electrolytics installed on the solderless breadboard power rails. If a student installed one of these backwards, it could put an eye out. A radial electrolytic, when it catastrophically "explodes" shoots the "can" off with the force of an air rifle.

Dean

 

[lavenatti]

Ah yes, I remember raiding my friends fathers capacitor stash for those big old electrolytics. We'd plug them into an extension cord outside, hide in his basement and plug the extension cord in. The lights would dim and kaboom! :twisted:

Come to think of it, that's probably what got me interested in electronics in the first place.


The newer caps are vented and not as dangerous, but still....

 

[samcheetah]

The newer caps are vented and not as dangerous, but still....

yes, the newer caps have a X on top of them. when you apply a reverse voltage the cap will explode but now the pressure will just burst the top of the cap. the whole can wont shoot off. but still then everything that comes out of the top is dangerous and can still do damage to the eyes. so its always better to wear glasses as Dean has suggested.

 

[Styx]

What is worse is new caps (ie tants) are marking the POSITIVE (SMT type) with a line as oppose to the standard (diodes and electroytic) with markings at the negative terminal.

 

[zachtheterrible]

Ah yes, I remember raiding my friends fathers capacitor stash for those big old electrolytics. We'd plug them into an extension cord outside, hide in his basement and plug the extension cord in. The lights would dim and kaboom!

thats AC though. i take it that the caps weren't rated for 120v? ill hav 2 try that sometime :twisted:

 

[ericbirecki]

Wow, thanks for the responses. I am surprised at how dangerous they sound. I won't be so cavalier about them in the future.

But, I would still like to know what the difference is in the AC waveform that a cap can tell which way it is plugged in?? I thought that AC was simple push then pull, ie positive voltage then negative voltage. If both the positive and the negative voltages are exactly the same why (how) does it matter to a component??

I don't mean to be dense, but this sounds fundamental, I feel like I'm missing something.

Eric B.