Capacitor

Description

The capacitor or the electrical condenseris a device for storing an electrical charge. In its simplest form a capacitor consists of two metal plates separated by a nonconducting layer called the dielectric.

fig.1- Capacitors

When one plate is charged with electricity from a direct-current or electrostatic source, the other plate will have induced in it a charge of the opposite sign; that is, positive if the original charge is negative and negative if the charge is positive. The Leyden jar is a simple form of capacitor in which the two conducting plates are metal-foil coatings on the inside and outside of a glass bottle or jar that serves as the dielectric.

The electrical size of a capacitor is its capacitance, the amount of electric charge it can hold.

Capacitors are limited in the amount of electric charge they can absorb; they can conduct direct current for only an instant but function well as conductors in alternating-current circuits. This property makes them useful when direct current must be prevented from entering some part of an electric circuit.

What can a capacitor do ?

Fixed-capacity and variable-capacity capacitors are used in conjunction with coils as resonant circuits in radios and other electronic equipment. Large capacitors are also employed in power lines to resonate the load on the line and make it possible for the line to transmit more power.

Capacitor Types

Capacitors are produced in a wide variety of forms. Air, mica, ceramics, paper, oil, and vacuums are used as dielectrics, depending on the purpose for which the device is intended.

Fixed-value or variable capacitors

fig.2 - Symbols

A fixed-value capacitor has a series of coloured bands around its body, which signifies the capacitor's value. Variable capacitors are ones whose capacitance can be varied between an upper and lower limit. They may be adjusted by rotary controls and are often used on radio sets to change frequency.

Capacitor Identification

There are no international agreements in place to standardise capacitor identification. Most capacitors have printed values and are normally in microfarads or if the symbol is n, nanofarads. Working voltage is easily identified. Tolerances are upper case letters: M = 20%, K = 10%, J = 5%, H = 2.5% and F = ± 1pF.

0.22 microFarads with Tolerance of 10% and working voltage of 250 V

Another example …

A capacitor coded 474 K 63 means:
The unit is picofarads and the third number (4) is a multiplier. So we have 47 followed by (4) zeros .That means 47 × 10000 pF which is equivalent to 470000 pF or 0.47 microfarads. K indicates 10% tolerance and 63 is the working voltage.

Ceramic disk capacitors have many marking schemes. Capacitance, tolerance, working voltage and temperature coefficient may be found. Capacitance values are given as number without any identification as to units. (uF, nF, pF) Whole numbers usually indicate pF and decimal numbers such as 0.1 or 0.47 are microfarads. Odd looking numbers such as 473 is the previously explained system and means 47 nF.

Remember the multiplication factors

Mili (m) = 0.001 
Micro (m) = 0.000 001 
Nano (n) = 0.000 000 001 
Pico (p) = 0.000 000 000 001

Connections

Capacitors can be connected in series or in parallel.

fig.3 - Connections

EC=Equivalent capacitance

Connection in series

Connection in parallel
EC=C1 + C2 + C3

More information about capacitors

Q or Quality Factor 

The Q of a capacitor is important in tuned circuits because they are more damped and have a broader tuning point as the Q goes down. Q = 1/RXC where XC is the capacitive reactance (XC = 2pFC) and R is the soon to be defined term of ESR. Q is proportional to the inverse of the amount of energy dissipated in the capacitor. Thus, ESR rating of a capacitor is inversely related to its quality. 

Dissipation Factor

The inverse of Q is the dissipation factor (d). Thus, d = ESR/XC and the higher the ESR the more losses in the capacitor and the more power we dissipate. If too much energy is dissipated in the capacitor, it heats up to the point that values change (causing drift in operation) or failure of the capacitor. 

Ripple Current Rating 

The ripple current is sometimes rated for a capacitor in RMS current. Remembering that P = I2R where R in this case is ESR it is plain to see that this is a power dispassion rating. 

Dielectric Absorption 

This is the phenomenon where after a capacitor has been charged for some time, and then discharged, some stored charge will migrate out of the dielectric over time, thus changing the voltage value of the capacitor. This is extremely important in sample and hold circuit applications. The typical method of observing Dielectric Absorption is to charge up a cap to some known DC voltage for a given time, then discharge the capacitor through a 2 ohm resistor for one second, then watch the voltage on a high-input-impedance voltmeter. The ratio of recovered voltage (expressed in percent) is the usual term for Dielectric absorption. 

ESL 

ESL (Equivalent Series Inductance) is pretty much caused by the inductance of the electrodes and leads. The ESL of a capacitor sets the limiting factor of how well (or fast) a capacitor can de-couple noise off a power buss. 

ESR Defined 

ESR is the sum of in-phase AC resistance. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency. ESR acts like a resistor in series with a capacitor (thus the name Effective Series Resistance). This resister can cause circuits to fail that look just fine on paper and is often the failure mode of capacitors.

Useful Links

Capacitance | Media gallery (capacitor)