SparkFun Forums

[roach]

I understand that there are (at least) two different varieties of caps, polarized and non-. Beyond the obvious issue of polarity, what reason would I have to ever use polarized capacitors? Do they have a faster discharge rate or something? I've seen plenty of schematics where a polarized cap pops up in the middle of nowhere, for no apparent reason, and can't help but think I'm missing something here...

Any help is appreciated,

[jasonharper]

A polarized electrolytic capacitor is much, much smaller than a non-polarized part with the same capacitance and voltage rating. The majority of applications for large capacitors only have one polarity applied, so why waste space for a capability that isn't needed? Polarized caps are cheaper, too, although it's hard to say how much of that is due to simpler design vs. larger market.

[upand_at_them]

It's not about using a polarized versus non-polarized capacitor. You choose the type of cap to fit the application...amount of capicitance needed, temp range, voltage, etc.

Some caps, like electrolytics, are polarized because of how they're constructed.

Mike

[transcendentnb2]

The real down and dirty reasons for using different capacitors is a) size and b) reactance at specific freqencies. Capacitors will dissipate different amounts of energy at different frequencies (and hence can potentially be wasteful). For instance, electrolytic caps are only good at lower frequencies, though I don't have a specific range off the top of my head.

It all depends on the dielectric inside and how quickly the dielectric can change polarity. Electrolytic caps use a wet dielectric that runs along a grooved film (the grooves give significantly more surface area and hence greater capacitance). The type of material inside actually forms a mock PN junction (which is why it has a polarity)! But, the wet capacitors can dry out eventually and change their capacitance value. Takes a while, but it happens.

[wiml]

I don't think modern electrolytics have a problem with drying out. In decades past, capacitors had that problem.

Like other people have said, there's no electrical advantage to a polarized capacitor, but if you don't need a non-polarized cap, you can sometimes get a smaller or cheaper component by going with a polarized one.

In an electrolytic capacitor, the actual dielectric --- the insulating separator that provides the capacitance --- is chemically formed on the surface of one strip of foil, and goes betwen the foil and the liquid. So really one electrode is foil and the other electrode is liquid (and then there's another strip of foil that gives you an electrical connection to the liquid). The thing is, the reaction that forms the insulating film is electrochemical, and if you reverse-bias the capacitor, the reaction runs in reverse and destroys the dielectric layer. Oops.

Besides capacitance and high-frequency impedance, some other things to consider when choosing a capacitor are voltage rating and ripple current and temperature raing. The filter capacitors on switching power supplies have to handle really high ripple currents, and a number of products lately have had failures because their capacitors couldn't handle it, overheated, and popped. (The Apple Airport base station is the one I'm familiar with.)

[transcendentnb2]

Arg.... oops, brain fart. Forgot that it was the conductor that was the liquid. The electrolyte is the liquid. Maybe I should have mentioned that caps from the 70s would maybe just *now* be going bad too, so it's not really something to worry about anyway. Forgot to mention that you can get solid electrolyte caps too (tantalum). Enough rambling from me...

[SOI_Sentinel]

No, modern electrolytics don't dry out. They form an oxide layer inside ove r the course of a year or two in storage, possibly shorter. Especially in higher voltage applications (220V, 480V, etc) this can cause the cap to rupture catastrophically if full voltage and load are applied suddenly. This is also why sometimes if you plug in a piece of equipment that hasn't been run in a few years it'll die suddenly.

They can be reformed, however, back to operation.

FYI: The capacitors I deal with are behind a bridge rectifier (AC variable frequency drives), so we start with a low voltage (40V) from our 3 phase variac and slowly raise the voltage by 20-40V every 10 minutes, usually up to the overvoltage trip point of the drive (close to the max voltage rating of the caps). There's no load and the drive is just idle during this time. The home equivalent would be using a variac or a light dimmer, through a bridge rectifier, into your capacitors (with load resistor so you have an RC circuit going). Start low and raise it slowly to max over the course of an hour (step every 10 minutes). Do NOT do this with your house electronics, though, good chance you might fry something else that doesn't like brownout conditions.

[roach]

Wow. This is more information than I expected. So, what I'm hearing is that there's no real reason to go with one over the other, except size and price. I'm mostly working with 0805 SMT packages here, so I'll probably stick with non-polarized, to avoid accidentally putting them in backwards (hard to read polarity markings at that small size).

Thanks for all the info!