Capacitor - Wikiparency

# Capacitor A **capacitor** is a two-terminal component that stores energy in the [[Electric_field|electric field]] between two conductors separated by an insulator. Its defining property, [[Capacitance|capacitance]], is the charge it holds per volt applied. In circuit theory that single number describes it completely; in practice a capacitor also has series resistance and series inductance, and above a frequency set by those parasitics it stops behaving as a capacitor at all. The microsim on this page sweeps frequency across a real capacitor so the reader can find that point and see what lies beyond it. The consequence is the central fact of decoupling and filtering: a capacitor is chosen not for its capacitance alone but for where its impedance minimum falls and how low that minimum is. Two parts of the same nominal value in different packages can differ by orders of magnitude in their usefulness at a given frequency. ## History The earliest capacitor was the [[Leyden_jar|Leyden jar]] of the 1740s, a glass vessel with conducting foil inside and out, developed independently by Ewald Georg von Kleist and Pieter van Musschenbroek. It stored enough charge to deliver a painful shock and was the first device able to hold static electricity in quantity, which made controlled experiments on electricity possible at all. [[Benjamin_Franklin|Benjamin Franklin]]'s work with such jars established that the charge resided in the glass rather than in the foil, and the flat form that followed — sheets of conductor separated by a dielectric — is the form a capacitor still takes. The name reflects a later understanding: early writers spoke of condensers, on the mistaken picture that charge was being condensed, and the modern term follows from capacitance as the measured quantity. Industrial development followed the needs of telegraphy and then of power distribution and radio. Mica and paper dielectrics dominated the first half of the twentieth century; aluminium electrolytics made large values practical; and the multilayer ceramic chip capacitor, which stacks many thin dielectric layers in parallel inside one body, made large capacitance available in a package small enough for [[Surface-mount_technology|surface mounting]]. ## Theory of operation Charge accumulates on the two conductors in equal and opposite amounts, and the voltage between them is proportional to that charge. For the parallel-plate geometry the capacitance rises with the facing area and with the [[Permittivity|permittivity]] of the dielectric, and falls with the separation between the plates. Every construction is an attempt to make the area large and the separation small without the [[Dielectric|dielectric]] [[Breakdown_voltage|breaking down]]. The energy stored is proportional to the capacitance and to the square of the voltage. Because current is the rate of change of charge, a capacitor passes current only when its voltage is changing: it is an open circuit to a steady voltage and a decreasing impedance to a rising frequency. This frequency dependence is [[Electrical_reactance|reactance]], and it falls inversely with frequency, which is the behaviour the ideal asymptote in the microsim shows. In series with a resistance the capacitor gives an exponential approach to the applied voltage with a [[Time_constant|time constant]] equal to the product of the two, the basis of timing circuits and of the simplest [[Low-pass_filter|low-pass filter]]. Capacitors in parallel add their capacitances; in series the reciprocals add, so a series string has less capacitance than its smallest member but divides the applied voltage between its members. ## Non-ideal behavior Three parasitics matter, and the microsim is built around them. Equivalent series resistance, ESR, is the lumped resistance of the plates, leads and dielectric loss; it sets the floor of the impedance curve and determines how much heat the part makes when carrying ripple current. Equivalent series inductance, ESL, comes from the geometry of the current path through the part and its connections. Leakage appears as a resistance in parallel, slowly discharging a capacitor left standing. Together the capacitance and the series inductance form a series resonant circuit. Below its resonant frequency the part is capacitive and its impedance falls; above it the part is inductive and its impedance rises; at resonance the two reactances cancel exactly and the impedance equals the ESR. This self-resonance is the sharpest real limit on a capacitor's usefulness: past it, adding capacitance changes nothing, and only a lower ESL — a smaller package, a shorter connection — helps. Capacitance is also less constant than the marking suggests. Class 2 ceramic dielectrics lose capacitance as the applied DC voltage rises, an effect large enough that a part may retain well under half its nominal value at its rated voltage, and they vary with temperature by design class. Electrolytics age and dry out, their ESR rising as they do. None of this is visible on a schematic. ## Capacitor types Types are classified by dielectric material, and the choice is a trade between capacitance density, stability and loss. [[Ceramic_capacitor|Ceramic capacitors]] use a ceramic dielectric: Class 1 formulations such as C0G are stable and low-loss but limited in value, while Class 2 formulations such as X7R achieve far more capacitance in the same volume at the cost of voltage and temperature dependence. [[Film_capacitor|Film capacitors]] use a polymer dielectric, are stable and tolerate ripple well, and are the usual choice in audio and power electronics where accuracy and low loss matter more than size. [[Electrolytic_capacitor|Electrolytic capacitors]] obtain large capacitance from a very thin oxide layer grown on an etched metal foil. Aluminium electrolytics are the cheapest route to large values and are polarised, failing destructively if reversed; [[Tantalum_capacitor|tantalum types]] are smaller and more stable but less tolerant of surge. [[Supercapacitor|Supercapacitors]] store charge in the double layer at an electrode surface rather than across a conventional dielectric, reaching capacitances measured in farads at low voltage, and occupy the ground between capacitors and the [[Electric_battery|battery]]. ## Capacitor markings Large parts are marked directly with capacitance, tolerance, rated voltage and, for polarised types, which terminal is negative. Electrolytics carry a stripe or a moulded indicator for polarity, and increasingly a date or lot code. Small parts use a three-digit code in which the last digit is a decimal multiplier and the unit is implicitly the picofarad, so 104 denotes 100,000 pF, which is 100 nF. A letter suffix gives tolerance. Ceramic chip capacitors are frequently unmarked altogether, being too small to print on, which is why they are handled in labelled reels and why a loose one is effectively unidentifiable without measurement. ## Applications Four roles cover most uses. In decoupling, a capacitor placed close to an [[Integrated_circuit|integrated circuit]] supplies the short current pulses the chip demands faster than the power distribution can, holding the local supply voltage steady; here self-resonance decides everything, and several values in parallel are used to cover a wider band. In filtering, the frequency-dependent impedance separates signals, smoothing the output of a [[Rectifier|rectifier]] or setting the corner of a filter. In coupling, a capacitor passes a changing signal from one stage to the next while blocking the steady bias voltage that each stage needs at a different level. In timing and energy storage, the charge and discharge of a capacitor through a resistance sets an interval, or a bank of capacitors delivers a large pulse of energy far faster than its source could. Capacitors also serve as sensors, since anything that changes the geometry or the dielectric between two conductors changes the capacitance: this is the mechanism of [[Capacitive_sensing|capacitive touch sensing]], of many microphones, and of pressure and humidity transducers. ## Hazards and safety A charged capacitor remains charged after the supply is removed, and a large one at high voltage holds enough energy to injure or kill. Equipment containing such capacitors carries bleeder resistances to discharge them, but these can fail, and a part may also recover some voltage after being shorted as charge held in the dielectric redistributes. The safe practice is to measure rather than assume, and to discharge deliberately through a resistance rather than by shorting, which damages the part. Polarised types fail violently if reversed or overvolted, venting hot [[Electrolyte|electrolyte]], which is the reason for the vent scoring on the case of an aluminium electrolytic. Class 2 ceramic parts can crack under mechanical or thermal stress and then fail short, and a cracked ceramic capacitor across a supply is a fire risk. Tantalum types are particularly intolerant of surge current and are usually derated in voltage well below their rating. ## Microsim The microsim plots the impedance magnitude of a real capacitor across eight decades of frequency on logarithmic axes, from the series combination of capacitance, ESR and ESL. The reader sets each of the three and moves a frequency marker along the curve. The shape to look for is a V. On the left the curve follows the ideal capacitive asymptote and falls inversely with frequency. It bottoms out at the ESR floor. On the right it follows the inductive asymptote and rises. The notch sits at the self-resonant frequency, and the readout confirms that the impedance there equals the ESR exactly. Three things are worth trying. Raise the capacitance and watch the notch move *down* in frequency, not up — more capacitance buys a lower impedance at low frequency and a worse one at high frequency. Then lower the ESL, which in practice means a smaller package and a shorter return path, and watch the notch move up. Finally, put the marker well above resonance and watch the real-to-ideal readout climb into the tens: this is the quantitative form of the statement that above self-resonance the part is no longer a capacitor.  **Microsim — three.js (Wikitube framework):** *The capacitor: impedance, ESR and self-resonance* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/electronics/Capacitor.html" data-title="Capacitor"></div> *Built from `MICROSIM_GUIDE/specs/sims/Capacitor.json`; part of the [[Electronics]] set ([[PORTAL_Electronics]]).*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Capacitor) : [Wikitube](https://en.wikitube.io/wiki/Capacitor) Skeleton mirrored at revision 1374397199. Prose, emphasis and the microsim are Wikitube's own. ## See also - [[Capacitance]] - [[Resistor]] - [[Inductor]] - [[RLC_circuit]] - [[Electric_battery]] - [[Rectifier]] ## Notes The impedance model used by the microsim is the standard series resistance-inductance-capacitance lumped model. It does not represent dielectric loss varying across frequency, nor the voltage and temperature dependence of Class 2 ceramic capacitance, both of which are described in prose above but are outside what a single series model can show. This is stated on the sim's own notes line. ## References The electrostatics and circuit theory here — capacitance from geometry and permittivity, stored energy, reactance falling inversely with frequency, series and parallel combination, the resistance-capacitance time constant, and series self-resonance with impedance equal to ESR at resonance — are standard textbook material and are not separately footnoted, per the Wikitube style guide §6.1. *Citation needed:* the eighteenth-century history in the first section, the named dielectric classes (C0G, X7R) and their behaviour, and the typical ESR and ESL ranges quoted on the microsim's notes line are given as classes and as generally reported history rather than from a pinned source. Pinning these to primary history of science sources and to the relevant IEC dielectric-class standard is queued for the next pass; no source is asserted for them here. ## External links - IEC dielectric class definitions and manufacturer impedance-versus-frequency data, to be pinned with the citations above.