Common to all types
Higher voltage capacitors are bulkier for the same capacitance as their lower voltage counterparts, which means that the goal most of the time is actually to obtain the lowest safe voltage rating for the application.
Most capacitors are rated for the maximum DC voltage, which must be treated as the maximum peak voltage (in the best case) if there is an AC component. Ceramic capacitors excel at AC handling, while other types are less impressive, and polarised types (electrolytic, tantalum, and conductive polymer) can fail if AC is passed through them without a DC bias of the correct polarity (or if DC of the wrong polarity is applied). However there are applications that do use AC-rated caps (the rating being frequency-dependent, as higher frequency results in more current flow at the same voltage), notably EMI filtering, power factor correction of inductive loads, and starting so-called "split-phase" induction motors.
Aluminium electrolytic
They are characterised by low cost and good volumetric efficiency (the capacitance you can get into a given space, with a given voltage rating), but those are their only advantages...though the cost one is very important much of the time (as with tantalum or ceramic capacitors it would often become prohibitive). The use of liquid electrolyte means they have a limited lifespan, and the manufacturing quality is absolutely critical to ensuring they last as long as they should. That last one is a huge problem as there are only a small number of good manufacturers of them (based mostly in Japan) and a huge number of very bad manufacturers (from Taiwan among other regions).
They cannot be used at the nominal temperature limit for any appreciable time, as their usual lifespan at that limit is a very low value - on the order of 2000 hours while still maintaining "acceptable" performance is commonly given (and no-one wants a PC that only lasts as long as an incandescent lightbulb). Reducing the temperature extends the lifetime in accordance with Arrhenius' Law (which is usually approximated as "double the lifetime for every 10°C cooler").
As their design is such a significant compromise, there are probably more variations on them than for any other type of capacitor. Those you are likely to see:
- General purpose: Designed for high volumetric efficiency at low cost, suited to linear PSUs, audio coupling, and similar applications. They come in two temperature ratings, 85°C and 105°C, but the latter have lower ripple current ratings, so the choice between the two should be based on whether most of the heating is environmental (in which case the higher rating is indeed better) - as is usually the case where these caps are used in-PC - or internal (in which case you may actually be better off with the lower temperature rating). They have ratings of 6.3~450VDC, though a distinction is made between "low" (up to 100VDC) and "high" (160VDC and up) voltage types.
- Low ESR: These are used in switching supplies and regulators, which are used throughout the PC for their efficiency and compactness. They are almost always rated for 105°C and have higher endurance ratings than general-purpose types. As the name indicates, they have lower losses and can handle more ripple current than the above type, but they are bulkier for the same capacitance and voltage. They are available with aqueous and non-aqueous electrolytes. Non-aqueous electrolytes are less temperature dependent (and (to my knowledge) electrically quieter, in case you choose to use them in a coupling application) while aqueous types can achieve lower impedances (but there is a cross-over point where modern non-aqueous types meet primitive aqueous types). These usually have ratings of 6.3~100VDC though 63V and 100V types are rather uncommon. There was a sub-set of aqueous capacitors that sacrificed the endurance rating to achieve the lowest impedance possible, which was previously common on motherboards but has now been made obsolete by polymer types. Those only went up to 16VDC. This is probably the most commonly failed category of capacitors when low quality brands are used.
- High voltage high reliability: The counterpart to the above for 160VDC and up. Again they have a standard rating of 105°C, and they last longer, handle more ripple, and are bulkier than general-purpose types.
It is worth noting that Panasonic TS-ED series (a snap-in type; its radial counterparts are A-ED and A-EE) handles about the same ripple current as the 85°C general-purpose types, making them ideal replacements. - Bi-polar: These have dielectric layers formed on both electrodes, allowing them to handle DC of either polarity (for a time - maintain the same polarity for too long and the capacitor "polarises" due to reforming) without damage - but as a result, they are bulkier than polarised types. They are mostly used for audio coupling. Large units are sometimes used in passive cross-overs, but are actually ill-suited to the application, where they have inadequate stability and are put under too much stress - resulting in the treble getting weaker and weaker as they dry out. In a pinch, you can "make" one by connecting two identical polarised units of double the value back-to-back (with their negative terminals joined together and their positive terminals to the circuit - or the other way around also works).
In appearance, they are cylindrical, with the negative terminal marked. They come in five main case styles:
- Radial: The most common type, which have diameters usually of 4~18mm, occasionally 22mm (but I would never recommend designing around the latter).
- Snap-in: Used for larger caps (≥22mm diameter) to make installation easier. Although it is said that their pins do not need to be trimmed, I recommend doing so anyway in a PC PSU due to the relatively small spacing (≈6mm) between the unit's PCB and casing.
- SMD: These usually have a coated case rather than the sleeves common on larger types. The capacitor itself is attached to a plastic base that assists in mounting to the board.
- Axial: An obsolete type that had one lead exiting through each end, with the negative lead visibly connected to the case.
- Screw-mount: These are absolutely monstrous both in capacitance and physically and are never used anywhere in a PC. They are mounted using clamps. This is the only type where the sleeve has an insulation rating.
I haven't found strong proof of a tangible difference in reliability between aqueous and non-aqueous capacitors, though. After all, there is, as far as I can see, no shortage of bad non-aqueous caps any more than there is of bad aqueous types (see CapXon GF and GL for example).
For what it's worth, a thing I've observed about Panasonic's capacitors is that they aren't in any rush to put out new series. This presumably means they use the extra time to perform more rigorous testing before anyone else can get their hands on them. Considering that they are often rated as the most reliable of the brands, I think it's entirely worth it.
Conductive polymer
They were once expensive and rare but are now increasingly popular. They have ESR and ESL lower than "wet" electrolytics, but not as low as ceramic capacitors, while physically they are smaller than electrolytics, but larger than ceramic capacitors, so in computing they essentially serve as a "middle ground" of sorts. They are only suitable for low voltages. While generally more reliable than their "wet" electrolytic counterparts, catastrophic failures have been reported, notably of those made by APAQ and Lelon.
They come in two physical forms. Small capacitors of this type come in a rectangular form similar to SMD tantalums, but with (on some versions) a + sign added to the positive terminal. Larger types come in cylindrical form like electrolytics. You can use the latter to replace some electrolytic capacitors, but will likely have to compromise on the value and note that there are (mostly older) capacitors of this type with higher ESR than some electrolytics. (Contrary to popular belief, you cannot take the absence of a sleeve as confirmation that it's a polymer type. There are polymers with sleeves (such as OS-CON SP (now discontinued)), as well as ordinary electrolytics without (including those time-bombs known as Sacon FZ series).)
Ceramic
Together with plastic film, these are among the most reliable of the capacitor types. Most of them are physically small. They have the lowest ESR and ESL of the common types. There are three common physical forms:
- Through-hole disc - a single circular (usually) ceramic layer with an electrode on each side, to which the connecting leads are bonded and the assembled capacitor is then coated for protection. The oldest of the three, it is still used for high voltage, low value parts (such as Y2 safety class capacitors - see below).
- Through-hole multi-layer - fairly self-explanatory; multiple thinner layers with alternating electrodes are stacked together to provide a much higher capacitance part for (relatively) lower operating voltage. The leads are attached and the part coated as with the disc type. You may be able to substitute lower-voltage disc types with multilayer types that have better stability (see below).
- Multi-layer ceramic chip - one of the most prominent components of the "infrastructure" of technology as we know it. They are small, rectangular, and usually brown in colour aside from the two terminals, which are soldered to rectangular pads on the board. They are available in sizes with a length of 1mm or even shorter - though I have my reservations about putting the smallest units out in the open. However, most of them are unlabelled. These are the most susceptible of the three types to mechanical damage, but their overall reliability is still light-years ahead of certain parts of the system (BGAs anyone?).
They have the widest range of voltage ratings of all the types, from <10VDC into the kilovolts.
Solid tantalum
They were once common but are no longer popular. You will most likely find them on hard disk drives from 2003 and earlier. Many (but not all) of them are yellow in colour. They are polarised, but unlike aluminium electrolytics, tantalums have the positive terminal marked.
As they have a solid "electrolyte", they can last several decades in normal use. However, they are fragile in that they are vulnerable to current spikes, which stress the dielectric layer - and if the dielectric layer does fail, the capacitor can burn or explode if current is not explicitly limited. Even the charging current at turn-on can damage these, so if it is not explicitly limited to a level safe for the capacitor, it should be operated at a third or less of its nominal voltage rating. Installing the capacitor backwards will cause it to blow after a highly unpredictable length of time (which could be in seconds or years). They are also very expensive nowadays due to a diminishing supply of raw materials. Combining those drawbacks with the progressive availability of lower voltage (and therefore higher value) ceramic capacitors, in most new designs the choice is decisive.
In summary, I don't hate them, but nor do I see an incentive to continue using them (outside of high-reliability, long-life applications where liquid electrolytes are not an option but ceramic capacitors are too small).
There was also a "wet" type of tantalum capacitor, which fortunately was not used in PCs. Those contained sulfuric* acid, so are best stayed well away from. (The electrolytes in aluminium electrolytic capacitors are not as dangerous.)
*I know it's technically spelled "sulphuric" here in Aus., but I dislike that spelling as it's harder to read.
Plastic film
In-PC these are used primarily as X2 safety class capacitors on the PSU input. They come in larger sizes than ceramic capacitors. They are more linear than anything else except class 1 ceramic capacitors, but with physical bulk to match. They are seldom used elsewhere in the PC so I don't feel the need to elaborate further.
X and Y safety classes
These are designed for continuous operation on mains power. They include X1, X2, Y1, and Y2. X-class is for use where failure cannot present a shock hazard (i.e. live to neutral), while Y-class is for use where failure could result in shock if the earth connection is lost. The "1" variants are intended for heavy-duty industrial use (or for Y1 class, across double insulation (although two Y2 caps in series can also be used)), with the "2" versions for more (relatively) graceful environments. They are tested to withstand impulses of the following strength without failure:
X1: 4kV
X2: 2.5kV
Y1: 8kV
Y2: 5kV
X-class capacitors are (almost) always film types, often polypropylene for its low losses, while Y-class capacitors are available in film but are usually ceramic discs in small supplies. The maximum value of Y-class capacitors is restricted by what is considered an "acceptable" amount of current to "leak" into earth; in small supplies they are usually ≤4.7nF each. I consider the use of Y-class capacitors (as Y1 class, or 2 * Y2 class in series) in class II ("double insulated") equipment silly as it allows you to get a tingle just by touching the device and the ground simultaneously, and can damage signal circuits if "hot-plugged". But it's a very common practice so don't be surprised.
These are not cheap (relatively at least), and low-quality PSUs often don't bother with them and use ordinary 400VDC polyester caps across the line and 1kVDC ceramic types coupling to earth - which are fire and electrocution hazards, respectively.
Metallised paper dielectric was promoted as providing better self-healing capability than plastic films (with plastic film the insulation resistance degrades after breakdown; with paper it actually improves), but those capacitors unfortunately succumb to cracks in their brittle encapsulation, so should be replaced with plastic film.
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If you need more details, look up the manual.
v0.1 - I know it's a bit bland without pictures of what the capacitors actually look like. But I'm confident that the quality of the writing itself is a good deal better than what a certain forum has to offer.
v0.2 - Some refinements. It turns out that X1 and X2-class ceramic capacitors technically exist, although only barely (Digi-Key stocks a few). It's debatable whether it was even worth mentioning in the first place, though...(I suppose you could say the same for paper dielectric)
v0.3 - Okay, so one Y2 cap is not enough for double insulation - for that you need a Y1 cap, or two Y2 caps of double the value in series. With the earth connection, though, it's considered acceptable.
v0.4 - Corrected the line on MP capacitors.
