Voltage Ratings
A capacitor’s voltage rating is an indication of the maximum voltage that should be applied to the device. The context of the rating is significant; in some instances it may indicate a maximum safe working voltage, in others it may be more akin to a semiconductor’s “absolute maximum” rating, to which an appropriate de-rating factor should be applied.
Tolerance
A capacitor’s tolerance describes the limits of deviation from nominal capacitance value that a device should be expected to exhibit under specified test conditions, particularly AC test voltage and frequency. Quoted tolerance figures include the steady-state deviation from nominal value due to variability in manufacturing, and may (on rare occasions) also include temperature-induced variation in capacitance value over the stated operating temperature range. It should be noted that test conditions (temperature, frequency, amplitude, and DC bias value of test voltage among others) frequently have a strong influence on observed device parameters.
Safety Ratings
Capacitors designed for use in applications where failure may pose a risk to the safety of persons or property (typically those involving AC line voltages) are designated with an alphanumeric safety rating, such as X1, X2, Y1, Y2, etc. according to regulatory standards. “X” rated devices are certified for applications where failure is not expected to pose a shock hazard, such as “line to line” applications, while “Y” rated devices are certified for applications where failure would pose a shock hazard, such as “line to ground” applications. The number in the designation indicates a level of tolerance to surge voltages, as specified in applicable regulatory standards such as IEC 60384-14. Devices may also carry multiple safety ratings, indicating their certification for use in different circumstances; for example a capacitor with an X1Y2 safety rating may be used in applications requiring an X1 rating as well as those requiring a Y2 rating.
Dielectric/Electrode Type
Capacitors are distinguished by the materials used in their construction, and to some extent by their operating mechanism. “Ceramic” capacitors for example use ceramic materials as a dielectric; “aluminum electrolytic” capacitors are formed using aluminum electrodes and an electrolyte solution, etc. Further specification of dielectric characteristics (and hence device performance characteristics) within a general capacitor type are often made, particularly among ceramic capacitor types.
One common distinction to note is that between electrolytic and non-electrolytic capacitor types. Electrolytic capacitors use a dielectric material which is formed in-place electrochemically, usually by oxidizing the surface of the electrode material, whereas non-electrolytic (often called “electrostatic” capacitors) use dielectric materials that are generally formed through various mechanical processes and are not a chemical derivative of the electrode material itself.
This distinction is useful, because the two device categories share general traits within themselves, allowing one to roughly predict a given device’s qualities and application suitability simply by identifying whether or not it is an electrolytic type. Generally speaking, electrolytic capacitors offer high capacitance per unit volume, are polarized, low cost, high-loss, and exhibit lousy parameter stability. Non-electrolytic device types in contrast, tend to be bulky for their ratings, are non-polar, relatively expensive, low-loss and with a handful of notable exceptions, exhibit fair to excellent parameter stability.
Operating Temperature Range
A capacitor’s (operating) temperature range indicates the range of temperatures over which a device has been qualified for use. When specified separately, a storage temperature range is that range of temperature across which storage in a non-active state will not cause damage to the device or result in irreversible parameter shifts upon operation within the normal temperature range. For unassembled devices, further (more stringent) environmental specifications may be made regarding storage in order to assure that lead finish materials do not degrade to a point that would inhibit proper assembly.
Unlike most other qualifying parameters, operation outside of a device’s specified temperature range (particularly at lower temperatures) is often comfortably possible, provided that provisions are made to account for parameter shifts that occur as a result, and that the temperature excursions do not result in mechanical damage to the device. Operation at temperatures above a device’s rated limit is more perilous due to the presence of temperature-related wear and failure mechanisms, but is often possible in circumstances where device longevity is not a significant concern. Such out-of-spec operation is, however, at the designer’s risk and requires due care in device qualification.
Ripple Current Ratings
A capacitor’s ripple current rating indicates the maximum AC current that should be allowed to pass through the capacitor. Because current flow through a capacitor results in self-heating due to ohmic and dielectric losses, the amount of current flow a given device can tolerate is finite, and is influenced by environmental conditions.
Lifetime
Many capacitors, aluminum types in particular, possess strong wear mechanisms that limit their service longevity. A lifetime specification is an indication of a device’s expected service life under specified operating conditions. Note that definitions of service life may vary; one common definition is the length of service under specified conditions (which usually are near rated maximum values) within which 50% of fielded devices can be expected to fail. Some specifications are more stringent, others may be more lenient.
Military/High Reliability/Established Reliability
For applications that are ill-tolerant of device failure, capacitors are available which are produced and tested according to defined protocols in order to provide a statistical assurance of device reliability. Particularly sensitive applications commonly require that components be procured through documented channels, which allow a given component’s origins to be traced back through the production processes, in order to assure device integrity and facilitate root cause analysis in the case of failure. MIL-HDBK-217F Notice 2 is, at the time of writing, the most widely used guide for predicting reliability of electronic equipment, though the procedures established by Telcordia have also been widely used, particularly in the telecommunications industry.
Packaging and Mounting Types
Like most electronic components, capacitors are available in a variety of package and mounting types. Device characteristics and common application constraints influence the available options, which may include surface mount devices, axial- and radial-leaded through-hole types, and chassis-mount types.