How To Find The Capacitance Of The Capacitor

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ITG SUBJECT: CAPACITOR

The purpose of this ITG is to acquaint the investigator with the capacitor. Only the nuts will be discussed, since it is beyond the scope of this ITG to go into great detail. It is stressed that there is no unmarried capacitor that out performs all others, as each capacitor is designed to perform a specific task. This ITG volition explicate capacitor operation theory, the diverse types of capacitors, concrete and electrical specifications of capacitors, the failure modes of the various types, pattern considerations, and environmental effects.

Theory

Electrically, "capacitance" is present between whatever two adjacent conductors. A capacitor consists of two conductors, usually parallel metal plates, separated by a dielectric cloth or vacuum so equally to shop a big electric charge in a small-scale book. Depending on proposed awarding the dielectric can be air, gas, newspaper, organic motion-picture show, mica, glass, or ceramic. The operation of a capacitor is similar to blowing upwardly a airship and releasing the air from it. Imagine blowing up a airship, pinching the air nozzle for a few seconds, and then releasing the air nozzle so that air may flow out. Similarly, a capacitor is charged (blown up) to some voltage (air force per unit area) by an AC or DC voltage source (air blower). Once the voltage source is removed the capacitor will hold the voltage for some time (pinching the air nozzle) and and then it will begin to rid itself of the electricity (releasing the air nozzle). The rate at which the capacitor discharges is dependent on how much resistance the discharging electric current meets. The more resistance you have the slower the electric current will belch from the capacitor. Thinking in terms of balloons, we can say that the tighter you lot pinch the air nozzle (resistance) the slower the air will catamenia out (electric current discharge). If a big piece of metallic is put across the ii capacitor terminals, the capacitor will instantaneously discharge and sparks will occur. This is due to the sudden period of discharge current thru a neglible resistance. This phenomenon is similar to popping a balloon where the unresisted flow of air through the pinhole is and then great that the balloon explodes.

The basic equations governing the functioning of a capacitor are:

(1) Capacitance (C) = Accuse (Q)    = ke A                                               -----------      --o-                                                          Voltage (V) d

Where C is in units of farads (f), Q is in coulombs (C), and 5 is in volts (V). A capacitor possesses one farad of capacitance if its potential is raised one volt when it receives a charge of one coulomb. On the right hand side of the equation, k is the dielectric constant (no units), east o is the premittivity of air (8.85 10 10 -1 two f/cm), A is the surface area of one of the capacitor plates (cm ii), and d is the separation distance between the 2 plates (cm). Capacitance is most commonly expressed in 10 6 subdivisions called microfarads (uf).

(2) Energy (J) = 1/2 Capacitance (c) x Voltage 2 (V) = QV                                                           --                                                            ii

where J is in units of watt-seconds or Joules.

Equation (i) shows that capacitance tin be increased in several means; by decreasing the voltage, obtaining a dielectric with higher k, increasing the capacitor plate area, or decreasing the distance between the capacitor plates. Equation (2) shows that the energy experiences its largest increase if the voltage is increased.

Capacitors are mainly used as energy storage devices; that is, they store electrical energy until the energy is required to enter the excursion which is using the capacitor. Capacitors are now widely used for keeping DC current from inbound a part of a circuit (blocking), ridding a circuit of unwanted noise or distortion (filtering), combining desired frequencies to resonate in a circuit (coupling), and excluding certain frequencies from resonating in a circuit (bypassing).

Types

Capacitors generally come in two types; fixed and variable. Fixed capacitors are manufactured to possess a specific capacitance which cannot be changed and variable capacitors are manufactured to allow capacitance to be varied over a wide range.

Capacitors are also classified into two generic categories; electrostatic and electrolytic. Electrostatic capacitors are filled with dielectrics composed of a gas, liquid, solid, or combination of these. Electrolytic capacitors are characterized by a very sparse metallic oxide dielectric moving picture formed on the surface of one or more electrodes.

A. Fixed Capacitors

Ceramic Capacitors - These are a unique family of capacitors with dielectric constants ranging from 6-10,000. They can be easily manufactured to desired concrete and electrical characteristics by applying ceramic chemical science. Ceramic capacitors are so widely used that they come in three classes. Class I ceramics are used for resonant circuits and loftier-frequency bypass and coupling. These capacitors take a wider temperature range compared to Class Ii and Grade III capacitors. Class II ceramics are used where miniaturization is required for bypassing at radio frequencies, filtering, and interstage coupling. Class Iii ceramics are used where low-voltage coupling and bypassing in transistor circuits are necessary.

Vacuum Capacitors - These capacitors have the everyman possible dielectric abiding and are limited to capacitances of x three pf (10- 3 uf), tin can range up to 50 kv (50x10 iii volts), and can carry huge currents up to 100 amperes. Vacuum capacitors, are extremely useful because their lifetime, barring whatever particle contagion in the vacuum sleeping accommodation, is indefinite.

Mica Capacitors - These capacitors detect their utilise in such applications as high-frequency filtering, bypassing, blocking, buffering, coupling, and fixed tuning.

Metalized Paper and Film Dielectric Capacitors - The employ of this class of capacitors is platonic where great amounts of heat volition be present in a circuit. These capacitors possess a unique belongings called cocky-healing whereby they eliminate momentary short circuits induced in their dielectrics acquired past surrounding circuit elements. Once the capacitor becomes as well hot, the localized heat generated is sufficient to vaporize the thin electrode in the surface area of the possible breakdown. The power to cocky-heal permits these capacitors to take college voltage ratings for a given thickness.

Radio Frequency Interference (RFI) Capacitors - RFI capacitors are platonic for suppressing unwanted noise from electronic circuits. This minimizes the amount of noise passing from one phase of the circuit to another, thus improving overall circuit performance.

Film Capacitors - These capacitors are widely used where circuits volition experience exposure to moisture. Their resistance to moisture penetration is, by far, superior. Film capacitors are applied in circuits requiring blocking, buffering, bypassing, coupling, tuning, and timing.

Electrolytic Capacitors - Electrolytic capacitors are very different from those previously mentioned in that electrolytics are commonly polarized. This means that the polarity of the applied voltage must match the polarity of the capacitor or intense heating will occur and the capacitor will burn out. Electrolytics run into blueprint needs for depression-frequency filtering, long-term timing, coupling and decoupling, and certain bypass applications requiring high capacitance values and pocket-sized volumes.

Other capacitors commonly used every bit stock-still capacitors are air, glass, and paper types. These are the primeval capacitors to be used and they nonetheless find usage in full general purpose cases.

B. Variable Capacitors

Variable capacitors, also called trimmers, are invaluable in the design of electronic equipment. Variable capacitors are generally employed to provide a range of capacitance and are usually used in applications where exact capacitance values cannot be obtained using normal blueprint procedures. These capacitors are ordinarily constructed such that varying the capacitance is accomplished by adjusting the metal plates in the capacitor. Screws on these capacitors increment or subtract effective plate expanse thereby causing an increase or decrease in capacitance. (Inspection of Equation (1) shows this.) The near widely used trimmers are ceramic, glass, air, plastic and mica.

C. Special Capacitors

Feed-through Capacitors - These capacitors are used in cases where conventional capacitors are not constructive for filtering at high radio frequencies. Feed-through capacitors are 3 terminal devices that practise not exhibit the series-resonant characteristic of the conventional capacitor. This enables them to suppress radio-frequency interference over a wide range of frequencies and they are specially valuable in filtering power-supply and control-circuit wiring in shielded high-frequency equipment.

High-free energy Storage Capacitors - These capacitors are synthetic with oil-impregnated paper and/or motion-picture show dielectrics. Their primary utilize is for pulse forming networks which employ voltages greater than yard volts. For slightly lower voltages special electrolytic capacitors can be used. Commutation Capacitors - These are constructed from oil-impregnated newspaper and pic dielectrics. They are mainly used in triggering circuits since they are characterized past fast rising times (time information technology takes capacitor to rising from x% to 90% of its maximum voltage) and high current transients and height voltages associated with switching.

Packaging - Capacitors come up in a wide multifariousness of packaging styles. The most common styles are molded, drinking glass-encased, chip, potted, coated, and Dual-In-Line Packaging (DIP). Molded capacitors are rectangular-fleck capacitors that can be molded into radial or axial-atomic number 82 rectangular packages or axial-atomic number 82 cylindrical packages. Glass-encased capacitors can be single or multi-layered fries with axial leads attached sealed into a glass tube. These expect a lot like molded capacitors. Fleck capacitors are sparse, flat rectangular capacitors without leads or trunk encasement and then that they may be put into microelectronic circuits. Potted capacitors, in many ways, are synonymous with molded capacitors. The just difference is that potted capacitors are oven cured. Coated capacitors, more than ordinarily known as dipped capacitors, come in rectangular and disk styles with radial leads and are dipped in liquid resin. Coated capacitors find smashing usage where exact dimensions can be compromised. DIP capacitors are single or multi-layered capacitors candy into integrated-circuit-type packages. Mica chips come in button styles. This package is composed of a stack of silvered-mica disks connected in parallel.

Figures 1, ii, and 3 show a few of the diverse types and packaging styles of capacitors. Figure 1A (image size 29KB) shows dipped- radial-lead capacitors (acme row) and molded-axial-lead capacitors (lesser group); Figure 1B (prototype size 29KB) shows glass-encased- axial-lead capacitors (A), chip capacitors (B & C), molded-radial- pb capacitors (D), molded-centric-lead capacitors, and dipped-radial- lead capacitors (F); Figure 1C (image size 29KB) shows the diverse styles of feed-through capacitors; and Effigy 1D (paradigm size 29KB) shows dipped-radial-atomic number 82 capacitors (tiptop and bottom left), molded- axial-pb capacitors (lesser right group), button capacitors (Middle- center grouping), and stock-still terminal capacitors (top eye and peak right). Figure 2A-C (prototype size 13KB) shows various types of trimmer capacitors. Figure three (image size 7KB) (Figure shows (a) mica; (b) drinking glass; (c) ceramic; (d) general-purpose ceramic; (e) solid electrolyte tantalum; (f) foil tantalum; (g) feed-through button mica and ceramic; (h) full general-purpose plastic film; and (i) full general- purpose paper.

Physical and Electrical Specifications

In that location are numerous criteria which the designer uses to choose the capacitor that will best perform a specific task. Listed here are some of the virtually of import specifications used in evaluating capacitor performance.

Dissipation Cistron (DF) - This is a mensurate of loss in a capacitor. Sometimes this is interchanged with a measure of loss called the power factor (PF). Losses in large Air conditioning coil and newspaper capacitors are DF's while losses in most capacitors used in DC or low-level Air conditioning capacitors are PF's. Ideally electric current should lead voltage by 90 in a capacitor but due to manufacturing processes the current leads the voltage by some angle A. The DF = tan(90 -A) and PF = sin(90 -A). The lower the DF, the better the capacitor.

Equivalent Series Resistance (ESR) - In capacitors, this is defined as the AC resistance (R) of a capacitor expressing loss at a given frequency (f). The ESR is related to the PF by the relation:

          R =  PF   x ten 6                                            ---                                            2 fc

in units of ohms.

Insulation Resistance (IR) - This is the resistance beyond the terminals of a capacitor. IR is inversely proportional to capacitance and temperature so as capacitance (or temperature) increases the IR will decrease.

Dielectric Force - This corresponds to the maximum voltage that a dielectric material can withstand without rupturing. Electrostatic capacitors are often specified by their dielectric withstanding voltage (DWV) and this is synonymous with dielectric forcefulness. Dielectric strength is commonly specified in volts per mil at constant temperature.

Dielectric Assimilation - This is the property of an imperfect dielectric where all electrical charges inside the body of the material caused past an electric field are not returned to that field. Dielectric assimilation is measured by determining the "reappearing voltage" which appears across a capacitor at some point in time afterwards the capacitor has been fully discharged under short circuit conditions. It is expressed as the ratio of reappearing voltage to charging voltage.

Volumetric Efficiency - This is achieved by getting the most capacitance out of the smallest volume possible. The volume is a role of dielectric cloth used and the method of structure. Capacitors with high volumetric efficiency are the most applicative in about of the new integrated-excursion electronic-equipment designs.

Temperature Coefficient (TC) - TC is the alter in capacitance per degree change in temperature. It may be positive, negative, or even naught and is expressed in parts per 1000000 per degree Celsius (ppm/ 0C). The equation that determines the TC is:

TC = C1-C 2 x 10 6               ------             (T i-T ii)C 1

where C i and C ii are the initial and last capacitances and T 1 and T 2 are the initial and final temperatures.

Voltage Ratings - There are two types of voltage ratings to consider when evaluating capacitor performance; DC and surge voltage and Air conditioning voltage. In the case of DC and surge voltage ratings, the thickness of the dielectric determines the maximum surge and DC voltages that may be applied. Air-conditioning voltage ratings are commonly specified for ceramic capacitors. This rating corresponds to the Ac voltage required to make the sum of the given DC voltage and Air conditioning voltage less than the rated DC voltage.

In add-on to these ratings there are certain types of electrolytic capacitors in which the practical voltage is of principal concern. Electrolytic capacitors are sensitive to the effects of voltage because they are highly polarized devices. Even if the applied voltage is less than the maximum voltage specified, the voltage drop across the ESR of the capacitor will shorten the capacitor's life expectancy through an accelerated effect of internal heating.

Current Ratings - Current ratings to consider are the leakage and ripple currents. Leakage current is the stray DC current of relatively small value which flows through the capacitor when voltage is applied across the terminals. Ripple current is the Ac component of a unidirectional current. For electrolytic capacitors, there is as well a maximum allowable accuse and discharge electric current rating.

Frequency - Since there is an internal inductance in a capacitor there will be a resonant frequency. Depending on capacitor blazon, this frequency may or may non fall in a range that is a problem for the designer. This problem would arise because the designer would desire the capacitor to cake or minimize DC current, and at resonance the internal impedance is a minimum which causes maximum DC electric current.

Failure Modes

Electrolytic Capacitors - Nearly failures in electrolytic capacitors result from two cases; either the breakdown of the dielectric film due to depression IR or the leakage of the electrolyte due to high IR. Dielectric breakdown is an electrochemical failure that is caused past improper chemical composition of dielectric material used in their manufacture. The addition of contaminants such every bit chloride is also a predominant gene in dielectric breakdown. Electrolyte leakage is a mechanical failure and is most commonly caused by insufficient compression seal, leakage at the weld on the bottom of the cylinder (in centric-pb devices), and leakage around the aluminum or tantalum terminals in plastic (molded) headers or seals. Other failure modes exist in the course of poor welds or pressure connections becoming open-circuits afterward a short shelf life or operating life.

Ceramic Capacitors - Most failures in ceramic capacitors are caused by encasement materials used to protect the capacitor and lead associates from external environments. Other failures include electric degradation and intermittent failures. Electrical degradation is caused by thermal expansion of encapsulants and moisture betwixt the coating and capacitor section. Intermittent or open failures are acquired by poor soldering techniques and terminal design that outcome in loose or detached leads.

Paper and Film Capacitors - Newspaper and movie capacitors are subject to the same failure modes as electrolytic capacitors with the exception of electrolyte leakage. Seal leakage is common in poorly fabricated oil-impregnated capacitors. Mechanical failures are caused by fracture of the electrode tab at the point of zipper to the electrode or to the external pb. Rough edges on foil electrodes cause early shorting, especially if the lower plate is thicker than the upper.

Pattern Considerations

The reliability of a capacitor is dependent upon the degree of success achieved in housing the capacitor element in a mechanically and environmentally secure enclosure. Capacitors with internal lead construction must exist mechanically and electrically sound before the encasement is applied. Encapsulated dipped, or molded capacitors can not withstand dynamic environments such as high levels of shock and vibration. For mechanical integrity, metallurgical bonds and reinforcing materials should be used.

When because which capacitor best performs a specific circuit task there are several options available. These options depend on the cost of the capacitor and the capacitor'south concrete and electrical properties with respect to the task information technology is almost to perform. If precision is a must, then information technology is advised that mica, glass, ceramic and film (polystyrene) capacitors be employed. These capacitors possess exceptional capacitance stability with respect to temperature, voltage, frequency, and life. Circuits that will settle for semiprecision can employ paper/plastic film capacitors (with foil or metalized dielectric) since they presently constitute a large portion of applications. If precision is of no importance any, then general purpose capacitors are recommended. These are the least expensive capacitors and they have proficient functioning ratings. Where suppression of radio-frequency interference is required, RFI and feed-through capacitors are the best equipped. For heavy currents (60-40 Hz power supplies), newspaper or film dielectric capacitors should be used for suppression, and ceramic and button-mica high-frequency mode capacitors are recommended for low currents. Ceramic bit capacitors are highest on the list for use in microelectronic circuits. These capacitors are electrically and physically the best suited for such purposes. If a capacitor needs to be used equally a transmitter, then information technology is advised that gas, vacuum, or ceramic capacitors be employed. These capacitors possess the necessary high radio frequency (rf) power-treatment adequacy, loftier rf current and voltage rating, low loss, low internal inductance, and very low ESR.

Environmental Effects

The constructive operation of a capacitor is greatly dependent on the physical environment that volition be surrounding it. Out of these many possible effects, those of primary concern with respect to medical devices are temperature, humidity, dynamics, pressure, and radiation.

Temperature - The maximum operating ambient temperature surrounding a capacitor in an application is of critical importance. Equally the ambient temperature changes, the dielectric constant and capacitance of most capacitors change. The useful service life of a capacitor decreases if it is subjected to high temperatures for cracking amounts of fourth dimension. Equally the temperature of the environment which surrounds the capacitor rises, the capacitor should receive less than the rated practical peak voltage.

On the other terminate of the spectrum, common cold temperatures tin can present issues every bit well. Electrolytic capacitors modify their capacitance immensely inside a few degrees once they are exposed to temperatures below 25 C. Aluminum electrolytics lose their capacitance at -55 C and tantalum loses virtually xx%. Equipment at low temperatures should exist given time for the capacitance to rising in one case the equipment has been powered up.

Humidity (Wet) - An of import consideration in the application of a capacitor is making certain that no moisture penetrates the sealing of the capacitor case. The effects of humidity are parametric changes (especially IR), reduced service life, and serious failure due to gross wet penetration. Most sensitive to wet are the paper-dielectric nonhermetically-sealed capacitors. Wet can easily penetrate into paper and can be trapped during manufacture, penetrate the capacitor during service life, or penetrate the capacitor once exposed to a moist environment.

Dynamic Environments - Dynamic environments tin mechanically damage or destroy a capacitor. The primary dynamic environments are in the form of shock, vibration, and acceleration. The movement of a capacitor associates inside a case can cause capacitance fluctuations, electrode attachment failures, and dielectric and insulation failures. A capacitor's susceptibility to dynamic environments is dependent on its physical construction; the larger the complex elements in the capacitor, the lower the frequency of response of the elements.

Barometric Pressure - The pressure dictates the altitude at which a hermetically sealed capacitor can safely operate. This altitude is dependent on the design of the stop-seal instance-wall, the voltage at which the capacitor volition be operated, and the type of impregnant used in the dielectric material. Equally the distance increases, the dielectric strength beyond the stop-seal will decrease. If the altitude is increased with barometric pressure reduced, then the force per unit area inside the capacitor will increase the mechanical stress on the instance and seal until failure occurs.

Radiation - Radiation particles tin can degrade the electric performance of capacitors. The principle cause of radiation-induced capacitor defects is dimensional changes in the interelectrode spacing. This change is due to gas evolution and swelling. Changes due to radiation are more pronounced in organic-dielectric capacitors. Capacitors using organic materials like polystyrene, polyethylene terephthalate, and polyethylene are less satisfactory in a radiation surround by well-nigh a factor of x than those capacitors employing inorganic dielectrics. The electrolytic capacitors (aluminum and tantalum) are capable of extended radiation exposure with tantalum being more than radiation resistant. Another defect from radiation occurs when the dielectric in the capacitor experiences a noticeable increase in its conductivity in an ionizing-radiation surround. This results in the very unsafe discharging of a charged capacitor.

References

Chute, George Grand., Electronics in Industry. New York: McGraw-Hill Book Company, 1971.
Fink, Donald G., ed., Electronics Engineers Handbook. New York: McGraw-Hill Book Company, 1975.
Fink, Donald G., ed., Standard Handbook for Electric Engineers. New York: McGraw-Hill Book Visitor, 1960.
Fugiel, Max, Modernistic Microelectronics. New York: Research and Didactics Clan, 1972.
Harper, Charles A., ed., Handbook of Components for Electronics. New York: McGraw-Hill Volume Company, 1977.

FIGURE 1 (1A, 1B, !C, !D) are TYPICAL CERAMIC (A-C) AND

MICA (D) CAPACITORS

TYPICAL CERAMIC (A-C) AND MICA (D) CAPACITORS