Capacitors

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Introduction

At the time of writing, a conservative estimate showed upwards of 37,000 unique manufacturer part numbers for capacitors in stock at Digi-Key (114k including all packaging options). Simply maintaining that kind of SKU count involves an awful lot of paperwork, a lot of forklift mileage, and a lot of cash tied up in physical inventory. Being able to re-package those products to order and have orders on the loading dock ready for shipment in as little as 15 minutes after receiving an order? That’s a small miracle of modern industry.

Why spend so much effort on simple, two terminal components? The combinatorics of voltage and current ratings are definitely a factor, but the more subtle reason is that capacitors aren’t really that simple. The schematic symbols we use to represent them are lies by omission for convenience, and obscure details that are often a) rather important and b) not well treated in academia. For example, some capacitors have anger management issues and will try to set your house on fire or poison you if you provoke them. Others grow comfortable with the idea of being unemployed and if abruptly called upon to work again, will throw a hissy fit and quit in short order. Some capacitors are from Venus and exhibit wide parametric variations with environmental and application variables, while others are from Mars and are more or less oblivious.

Making informed design choices requires awareness and consideration of these varying qualities, and the objective of this work-in-progress is to offer the reader a guide to capacitor technology in an easy-to-swallow capsule with a (hopefully) non-drowsy formula.

Capacitor Basics

Capacitors are devices which store electrical energy in the form of an electric field. The process is quite similar to the way mechanical springs store energy in the form of elastic material deformation, to the extent that the math describing both is quite similar, save for the variables used. The similarity may in fact be part the reason that students of electrical or mechanical engineering often find the others’ studies arcane and inscrutable; “v” means “voltage” to an EE, but “velocity” to an…

Non-Ideal Properties of Capacitors

For many purposes, real capacitors can be represented using a relatively simple lumped element model, consisting of an ideal capacitor with several additional components. [image] [image] ESR Equivalent series resistance (represented by Resr­ in the model) describes losses associated with moving charge through a capacitor. The resistance of the electrode and lead materials is a contributing factor, and losses occurring within the dielectric material itself also occur and are often dominant. Th…

Failure Mechanisms of Capacitors

Capacitors (like all other human contrivances) eventually fail, either parametrically or catastrophically. Parametric failures are those in which a device continues to function, but has slowly degraded to a point where it no longer meets performance specifications. Catastrophic failures on the other hand, are characterized by abrupt, dramatic shifts in device characteristics that result in out-of-spec behavior, which might include self-disassembly, combustion, incandescence, etc. Dielectric br…

General Capacitor Specification

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 expec…

Aluminum Capacitors

Aluminum Capacitors Aluminum capacitors are a family of devices that fall under the umbrella of “electrolytic” capacitors. As such, they offer high capacitance values in small packages at relatively low cost. In trade for these desirable qualities, their electrical properties and service lives tend to be relatively dismal. Though ill-suited for all but the most barbaric of signal-related applications, aluminum capacitors are a staple for DC power-related functions. Three distinct types are avai…

Capacitor Quick Reference Guide

The table below provides a brief summary of different capacitor types and their relative merits, arranged approximately in terms of decreasing quantity (or increasing quality) of capacitance offered by each type. Capacitor type Sub-type/variant Approx. value range C*V product range Advantages Disadvantages/things to beware of Good for Lousy for C V Electric double layer, Supercaps 6.8mF to 4000F 2.1 to 75v .03 to 10,000 Exceptionally high C/V ratio. Possible alternative to…

Ceramic Capacitors

Ceramic Capacitors Ceramic capacitors are electrostatic devices characterized by their use of various ceramic dielectric materials, which are commonly based on barium titanate (BaTiO­3). They are non-polarized with characteristics covering much of the quantity-quality spectrum, with perhaps a slight bias toward quality. Many variations in construction and dielectric properties are available to address diverse application needs, and this broad applicability together with relatively low cost stru…

Electric double layer capacitors

Electric double layer, Supercaps: Device Construction & Distinguishing Traits: Electric double layer capacitors (EDLCs) and supercapacitors are a group of electrolytic-like devices characterized by extremely high capacitance per volume and low voltage ratings, typically no more than a few volts. Construction types and operating principles among these devices differ and are topics of ongoing R&D efforts, but common themes found among them are the use of electrode materials that offer extremely …

Film Capacitors

Device construction Devices in the Film Capacitors category are electrostatic in nature, and made using dielectric materials such as paper or various polymers that are formed into thin sheets or “films” and interleaved with electrode materials to form a capacitor. The term “film capacitor” generically refers to any device made using this sort of process, and the term “film” is in reference to the nature of the dielectric material used. When the term “metal” is used as a qualifier for “film” as …

Mica/PTFE Capacitors

Device construction Mica is a naturally occurring group of minerals characterized by an ability to split readily into flat, thin films, with the specific type known as “muscovite” mica being preferred for capacitor applications. As a dielectric, mica offers excellent stability over time and applied voltage, a low temperature coefficient, high temperature tolerance, very good dielectric strength, and low loss characteristics over a wide frequency range. Aside from being excellent dielectric mate…

Tantalum Capacitors

Device construction & distinguishing traits Tantalum capacitors are electrolytic devices primarily used where a compact, durable device with relatively stable parameters is needed, and modest capacitance and voltage ratings are sufficient. Traditionally, tantalums’ advantages over aluminum electrolytics have been found in terms of capacitance per volume, parameter stability over temperature, and longevity; tantalums in general do not suffer from dry-out problems or issues of dielectric degrada…

Niobium Oxide Capacitors

Device construction & distinguishing traits Niobium oxide capacitors are similar in construction to tantalum and manganese dioxide (Ta/MnO2) devices, using sintered niobium oxide (NbO) in lieu of tantalum metal as the anode material. Produced chiefly by AVX as an alternative to Ta/MnO2 capacitors that doesn’t have the nasty inclination to deflagrate upon failure and also having the potential for improved raw material supply logistics, niobium oxide capacitors are in competition with tantalum po…

Silicon & Thin Film Capacitors

Device construction & distinguishing traits Silicon and thin film capacitors are a relatively new crop of devices produced using tools, methods, and materials borrowed from the semiconductor industry. The precise control over structure and materials that these techniques provide allows production of near-ideal capacitors with excellent parameter stability, minimal ESR & ESL, wide service temperature capabilities, and comparable to better capacitance per volume compared to the Class 1 ceramic t…

Trimmer & Variable Capacitors

Device construction & Distinguishing traits Trimmer and variable capacitors are devices that provide a capacitance which is variable within some range, the difference between the two terms being mostly one of design intent; a “trimmer” capacitor is usually intended to be adjusted only a handful of times over its service life, while a “variable” capacitor anticipates routine adjustment. Numerous different construction types are used, but with few to zero exceptions, they are of the electrostati…