The Capacitor

Capacitors are among the most commonly used components in electronics. Their construction is fairly simple, two metal plates and a dielectric layer separating them. Capacitors are very similar to batteries since they store electrical charge. However, capacitors must be charged with electricity, unlike batteries which produce their own using chemicals.

The capacitor's charge capacity depends upon the size of the metal plates. The larger the plates, the higher the charge and vice versa. The dielectric can be anything that disallows the plates from touching each other and discharging, but still allows the electric force to pass through. When charged, a capacitor gains the same voltage as the power source that was used to charge it.

The storage rating of a capacitor is based on the Farad unit. A capacitor with a capacitance rating of one Farad is capable of storing one coulomb of charge (6.25 x 10 ^ 18 electrons) at 1 volt. Although that many electrons seems like a lot, it can only power an average incandescent light bulb for about a minute.

The reason capacitors are used is often because of their quick discharge ability. A chemical reaction in a battery takes time, while the capacitor requires no chemical reaction to discharge electricity. This makes the capacitor a lot faster when it comes to discharging. That is why capacitors are used in cameras and lasers to create a bright flash, rather than batteries.

Capacitors are also used to make DC voltage constant. In power supplies, the voltage can vary. With a capacitor included, it makes up for a lack of voltage and absorbs the excessive voltage. This is necessary in sensitive electronic devices that require constant voltage supplies.

Capacitors are also used to block direct current. Since a capacitor connected in series with a power source is essentially a broken circuit, current cannot flow, once the capacitor is charged. However, alternating current can still flow when connected to a capacitor, since the voltage shifts and the capacitor charges and discharges. When capacitors are connected in parallel the total capacitance in the network is the sum of all the capacitance, Ct = C1+C2?Cn. For example if C1 was 10uF and C2 is 47uF the total capacitance is 57uF.




Two capacitors in parallel



When capacitors are connected in series the capacitance is given by 1/Ct = 1/C1+1/C2芒鈧?1/Cn.




Two capacitors in series



Capacitors are usually connected in series to increase the total voltage that can be connected between them; this is common with Tesla Coil Circuits as finding a capacitor with the exact capacitance and voltage would be almost impossible to find.

Special care must be taken with high voltage capacitors, such as capacitors where mains voltages (110-120v and 220-240) or the capacitors used in microwaves and TV sets and they can store enough charge to kill. Capacitors can store a charge for years after the power supply has been disconnected and the terminals should be shorted to remove the charge, some high voltage capacitors have leed resistors in them to drain the charge when the power is disconnected.

The different types of capacitors are generally named by the dielectric used in them, and have different purposes.

Aluminium electrolytic capacitors consist of one plate that is a chemical electrolyte and a dielectric that is an oxide on one side of the other metal plate. Aluminium electrolytic capacitors store the most charge in the smallest space with respect to other types of capacitors due to the oxide dielectric's amazing properties as an insulator. There are two main types of capacitor structural designs that you will run into when working with electronics. The two types are radial and axial. The radial design has both leads coming out of the same side of the capacitor. The axial design has one lead coming out of the center of each side, creating an axis.




An axial capacitor



Electrolytic capacitors are polarised, they can only be connected one way around. The polarity is indicated on the case of the capacitor, in most cases it will have an arrow pointing to the negative lead, but there are capacitors with arrows pointing to the positive as well. In the picture above the polarity arrow can be seen and is pointing to the negative terminal. The negative lead will also be shorter than the positive lead.




A radial capacitor



These capacitors are used in power supplies to smooth the voltage and anything that requires large energy storage, their capacity can range from as little as 0.22uF for filtering in audio circuits and they can have capacities beyond 10,000uF and even 100,000uF for filtering power supplies. Its impractical to use anything beyond 10,000uF capacitors in most cases as they are quite large and heavy. Almost all power supply circuits work satisfactorily with 2200uF.




The 100,000uF capacitor dwarfs the 33uF capacitor



Care must be taken to ensure electrolytic capacitors are not connected in reverse polarity, if they are the dielectric dissolves which allows high current to pass though the electrolyte which will vaporise and the built up pressure will be released with the capacitor bursting open with a loud bang if the capacitor is relatively small to the sound of an explosive detonating for large filter capacitors (3300uF or so). In some situations where reverse polarity will occur a special Bi-Polar electrolytic capacitor is used. They can be identified by having no polarity markings and have the letters printed on the case.

A variation on the electrolytic capacitor is the Tantalum capacitor, which uses tantalum film instead of aluminium. Tantalums contain electrolyte in dry form and are more resistant to reverse polarity than electrolytic but the polarity must still be correct.




A Tantalum capacitor



Ceramic capacitors also known as disc capacitors as they look like small discs offer small capacitances, the lowest being 1pF which is an extremely small storage capacity. They are used in bypassing and filtering circuits.

Polyester capacitors, also known as reencaps because of their appearance are the most common general purpose capacitor. Their values range from 10nF to 0.33uF or green caps and up to 10uF for MKT polyester capacitors.




Left: A polyester greencap capacitor; Right: Two ceramic disc capacitors



Article written by Mojo'D