CAPACITORS

                    

                         

Basic definition of CAPACITOR: 

A capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed.

Physics of the CAPACITOR:

Typical designs consist of two electrodes or plates, each of which stores an opposite charge. These two plates are conductive and are separated by an insulator or dielectric. The charge is stored at the surface of the plates, at the boundary with the dielectric. Because each plate stores an equal but     opposite charge, the total charge in the device is always zero.

Click here for the animation of working of a capacitor

CAPACITANCE: 

The capacitor's capacitance (C) is a measure of the potential difference or voltage (V) which appears across the plates for a given amount of charge (Q) stored on each plate.

                             C=Q/V

In SI units, a capacitor has a capacitance of one farad when one coulomb of charge causes a potential difference of one volt across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarad (µF), nanofarad (nF) or picofarad (pF).

The capacitance is proportional to the surface area of the conducting plate and inversely proportional to the distance between the plates. It is also proportional to the permittivity of the dielectric (that is, non-conducting) substance that separates the plates.

Properties of CAPACITORS:

•  Important properties of capacitors, apart from the capacitance, are the maximum working voltage (potential, measured in volt) and the amount of energy lost in the dielectric. For high-power or high-speed capacitors, the maximum ripple current and equivalent series resistance (ESR) are further considerations. A typical ESR for most capacitors is between 0.0001 and 0.01 ohm, low values being preferred for high-current, or long term integration applications.

 CAUTION:  Since capacitors have such low ESRs, they have the capacity to deliver huge currents into short circuits, which can be dangerous. For safety purposes, all large capacitors should be discharged before handling. For board-level capacitors, this is done by placing a high-power 1 to 10 ohm resistor across the terminals.

•  ESL (equivalent series inductance) is also important for signal capacitors. For any real-world capacitor, there is a frequency above DC at which it ceases to behave as a pure capacitance. This is called the (first) resonant frequency. . Large capacitors tend to have much higher ESL than small ones. As a result, instrumentation electronics will frequently use multiple bypass capacitors - a small, 0.1uF for high frequencies, a large electrolytic for low frequencies, and occasionally, an intermediate.

•  In the construction of long-time-constant integrators, it is important that the capacitor does not retain a residual charge when shorted. This phenomenon is called dielectric absorption or soakage, and it creates a memory effect in the capacitor. This is a non-linear phenomenon, and is also important when building very low distortion filters.

•  Capacitors will also have leakage -some level of parasitic resistance across the terminals. This fundamentally limits how long capacitors can store charge.

• Capacitors can also be fabricated in semiconductor integrated circuit devices using metal lines and insulators on a substrate. Such capacitors are used to store analogue signals in switched-capacitor filters, and to store digital data in dynamic random-access memory (DRAM).

Basic relations in CAPACITORS:

•    Energy storage:

      

  
 

•   Current-voltage:

                        

CAPACITOR networks:

             For capacitors in parallel:

 

Capacitors in a parallel configuration each have the same applied voltage. Their capacitance add up. Charge is apportioned among them by size. Using the schematic diagram to visualize parallel plates, it is apparent that each capacitor contributes to the total surface.

     For capacitors in series: 

 

 

Connected in series, the schematic diagram reveals that the separation distance, not the plate area, adds up. The capacitors each store instantaneous charge build-up equal to that of every other capacitor in the series. The total voltage difference from end to end is apportioned to each capacitor according to the inverse of its capacitance. The entire series acts as a capacitor smaller than any of its components.

Practical capacitors:
                            
Capacitors are often classified according to the material used as the dielectric.

•  Ceramic:

The main differences between ceramic dielectric types are the temperature coefficient of capacitance, and the dielectric loss. Ceramic capacitors tend to have low inductance because of their small size.

Ceramic chip: 1% accurate, values up to about 1 μF, typically made from Lead zirconate titanate (PZT) ferroelectric ceramic.

• Paper:

                                                        
Common in antique radio equipment, paper dielectric and aluminum foil layers rolled into a cylinder and sealed with wax. Low values up to a few μF, working voltage up to several hundred volts, oil-impregnated bathtub types to 5,000 V used for motor starting and high-voltage power supplies.

• Tantalum:

Compact, low-voltage devices up to about 100 μF, lower energy density and more accurate than aluminum electrolytic, but less accurate and higher energy density than signal capacitors. Since these capacitors rely on an electrolyte, they are polarized, meaning that they can only support a potential in one direction and are suitable only for DC applications.

• Aluminum:

Compact but lossy, in the 1 μF to 1,000,000 μF range, up to several hundred volts. The dielectric is a thin oxide layer. Like tantalum capacitors, these are polarized. They contain corrosive liquid and can burst if the device is connected backwards. Over a long time the liquid can dry out, causing the capacitor to fail. Bipolar electrolytic contain two capacitors connected in series opposition and are used for coupling AC signals.




Applications:

Capacitors find wide range of applications in electronic and electrical systems. It is very rare that we see a electrical component or a product that does not include at least one for some purpose.

Some of the applications of capacitors are listed below:

• Energy storage:

A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed which prevents loss of information in volatile memory.

Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy density, while capacitors using developing technologies can provide more than 2.52 kilojoules per kilogram.

The best example for this application is car audio systems wherein large capacitors store energy for the amplifier.

•   Power factor correction:

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (VAr). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive.

•  Noise filters and snubbers:

When an inductive circuit is opened, the current through the inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If the inductance is large enough, the energy will generate a spark, causing the contact points to oxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch. A snubber capacitor across the newly opened circuit creates a path for this impulse to bypass the contact points, thereby preserving their life; these were commonly found in contact breaker ignition systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage the switch but will still radiate undesirable radio frequency interference (RFI), which a filter capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single package.

Capacitors are also used in parallel to interrupt units of a high-voltage circuit breaker in order to equally distribute the voltage between these units. In this case they are called grading capacitors.

• Signal processing:

The energy stored in a capacitor can be used to represent information, either in binary form, as in DRAMs, or in analogue form, as in analog sampled filters and CCDs. Capacitors can be used in analog circuits as components of integrators or more complex filters and in negative feedback loop stabilization. Signal processing circuits also use capacitors to integrate a current signal.

Hazards and safety:

Capacitors may retain a charge long after power is removed from a circuit; this charge can cause dangerous or even potentially fatal shocks or damage connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easily capable of delivering a shock.

Capacitors may also have built-in discharge resistors to dissipate stored energy to a safe level within a few seconds after power is removed. High-voltage capacitors are stored with the terminals shorted, as protection from potentially dangerous voltages due to dielectric absorption.

Some old, large oil-filled capacitors contain polychlorinated biphenyls (PCBs). It is known that waste PCBs can leak into groundwater under landfills. Capacitors containing PCB were labeled as containing "Askarel" and several other trade names. PCB-filled capacitors are found in very old (pre 1975) fluorescent lamp ballasts, and other applications.