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Introduction of Capacitors Work
A capacitor is an electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric. Also called condenser.
The dielectric can be air, paper, plastic or anything else that does not conduct electricity and keeps the plates from touching each other. You can easily make a capacitor from two pieces of aluminum foil and a piece of paper. The capacitor won't be a particularly good capacitor in terms of its storage capacity, but the capacitor will work.
The Basic Principle of a Capacitor
A capacitor cannot produce new electrons, it only stores them.
Once the capacitor's charged, the capacitor has the same voltage as the battery (1.5 volts on the battery means 1.5 volts on the capacitor). For a small capacitor, the capacity is small. But large capacitors can hold quite a bit of charge. You can find capacitors as big as soda cans, for example, that hold enough charge to light a flashlight bulb for a minute or more. When you see lightning in the sky, what you are seeing is a huge capacitor where one plate is the cloud and the other plate is the ground, and the lightning is the charge releasing between these two "plates." Obviously, in a capacitor that large, you can hold a huge amount of charge!
If the capacitor is pretty big, what you would notice is that, when you connected the battery, the light bulb would light up as current flows from the battery to the capacitor to charge it up. The bulb would get progressively dimmer and finally go out once the capacitor reached its capacity. Then you could remove the battery and replace it with a wire. Current would flow from one plate of the capacitor to the other. The light bulb would light and then get dimmer and dimmer, finally going out once the capacitor had completely discharged (the same number of electrons on both plates).
Farads - The Unit of Capacitance
The unit of capacitance is a farad. A 1-farad capacitor can store one coulomb (coo-lomb) of charge at 1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One amp represents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.
A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle. So you typically see capacitors measured in microfarads (millionths of a farad).
If it takes something the size of a can of tuna to hold a farad, then 10,080 farads is going to take up a LOT more space than a single AA battery! Obviously, it is impractical to use capacitors to store any significant amount of power unless you do it at a high voltage.
The Capacitor Applications
The applications includes Filters, condenser microphone, ludspeaker, crossover, networks.
A variety of filter circuits can be made from combinations of capacitors, inductors, and resistors.
Most loudspeakers use multiple drivers and employ crossover networks to route the appropriate frequency ranges to the different drivers.
The difference between a capacitor and a battery is that a capacitor can dump its entire charge in a tiny fraction of a second, where a battery would take minutes to completely discharge itself. That's why the electronic flash on a camera uses a capacitor -- the battery charges up the flash's capacitor over several seconds, and then the capacitor dumps the full charge into the flash tube almost instantly. This can make a large, charged capacitor extremely dangerous -- flash units and TVs have warnings about opening them up for this reason. They contain big capacitors that can, potentially, kill you with the charge they contain.
Capacitors are used in several different ways in electronic circuits:
Sometimes, capacitors are used to store charge for high-speed use. That's what a flash does. Big lasers use this technique as well to get very bright, instantaneous flashes.
Capacitors can also eliminate ripples. If a line carrying DC voltage has ripples or spikes in it, a big capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges (which is instantaneous if the capacitor is small). However, any alternating current (AC) signal flows through a capacitor unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing.
One big use of capacitors is to team them up with inductors to create oscillators. See How Oscillators Work for details.
Charge on Series Capacitors
Since charge cannot be added or taken away from the conductor between series capacitors, the net charge there remains zero. That constrains the charge on the two capacitors to be the same in a DC situation. This charge is the charge you get by calculating the equivalent capacitance of the series combination and multiplying it by the applied voltage V.
You store less charge on series capacitors than you would on either one of them alone with the same voltage!
Does it ever make sense to put capacitors in series? You get less capacitance and less charge storage than with either alone. It is sometimes done in electronics practice because capacitors have maximum working voltages, and with two "600 volt maximum" capacitors in series, you can increase the working voltage to 1200 volts.
Condenser (Capacitor) Microphones's Principle
Sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. The plates are charged to a total charge.
Advantages:
Best overall frequency response makes this the microphone of choice for many recording applications.
Disadvantages:
Expensive
May pop and crack when close miked
Requires a battery or external power supply to bias the plates.
A change in plate spacing will cause a change in charge Q and force a current through resistance R. This current "images" the sound pressure, making this a "pressure" microphone.
Capacitor Types
Metal film: Made from high quality polymer foil (usually polycarbonate, polystyrene, polypropylene, polyester (Mylar), and for high quality capacitors polysulfone), with a layer of metal deposited on surface. They have good quality and stability, and are suitable for timer circuits. Suitable for high frequencies. The popular items are CBB60 motor running capacitor, CBB61 polypropylene film capacitor, CBB65 explosion-proof capacitor, CBB80 lamp capacitors, and so on.
Mica: Similar to metal film. Often high voltage. Suitable for high frequencies. Expensive. Paper: Used for high voltages.
Glass: Used for high voltages. Expensive. Stable temperature coefficient in a wide range of temperatures.
Ceramic: Chips of altering layers of metal and ceramic. Depending on their dielectric, whether Class 1 or Class 2, their degree of temperature/capacity dependence varies. They often have (especially the class 2) high dissipation factor, high frequency coefficient of dissipation, their capacity depends on applied voltage, and their capacity changes with aging. However they find massive use in common low-precision coupling and filtering applications. Suitable for high frequencies.
Electrolytic: Polarized. Constructionally similar to metal film, but the electrodes are made of aluminium etched to acquire much higher surfaces, and the dielectric is soaked with liquid electrolyte. They suffer from high tolerances, high instability, gradual loss of capacity especially when subjected to heat, and high leakage. Special types with low equivalent series resistance are available. Tend to lose capacity in low temperatures. Can achieve high capacities.
Tantalum: Like electrolytic. Polarized. Better performance with higher frequencies. High dielectric absorption. High leakage. Have much better performance in low temperatures.
Supercapacitors: Made from carbon aerogel, carbon nanotubes, or highly porous electrode materials. Extremely high capacity. Can be used in some applications instead of rechargeable batteries.
This article is a brief introduction of capacitors work.
Keywords
Capacitors, Film Capacitor, Lighting Capacitor, Motor Capacitor
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