Capacitors are devices, those are store electric charge. They are commonly used to set the time in timing circuits, smooth curves and filter signals. Capacitance is measured with farads. (F)
The capacitance of a capacitor is determined by four factors:
1. Area of the plate
2. Distance between the plates
3. Type of dielectric material
Types of capacitors
Many different types and styles of capacitors are manufactured to satisfy the needs of the electronic industry. Capacitors may be name to indicate their dielectric material, their enclosure, the process used in their construction, or their intended use.
Polarized capacitors are not like unpolarized capacitors as they have two poles. There are two polarized capacitors.
Electrolytic capacitors provide more capacitance for their size and weight than any other types of capacitors. This is their primary advantage over other capacitors.
A common electrolytic capacitor consists of two aluminium foil plates separated by layer of fine gauze or other absorbent material. The foil plates and separators are long, narrow strips. These strips are rolled up and inserted in an aluminium container. One plate (the negative plate) is usually electrically connected to the aluminium container. Electrical terminals from the foil plates are brought out one end of the container then the container is sealed.
One of the aluminium plates is oxidized. Since aluminium oxide is an insulator, it is used for the dielectric. The separators are saturated with a chemical solution (such as borax), which is called the electrolyte. The electrolytes having relatively good conductivity serves as part of one of the plate of the capacitor. It is in direct contact with both the dielectric and the pure aluminium foil plate. The electrolyte is also necessary in forming and maintaining the oxide on one of the plate.
The method used to produce electrolytic capacitors result in the plates being polarized. When using these capacitors always keep the same voltage polarity as was used in manufacturing them. This polarity is always marked on the body of capacitor. Again the aluminium container is usually connected to the negative plate. It is never connected to positive plate.
Reverse polarity on an electrolytic capacitor cause excessively high current in the capacitor. It causes the capacitor to heat up, and it can cause the capacitor to explode. Thus, the common electrolytic capacitors are limited to use in DC circuits. (Note: - special electrolytic capacitors are manufactured for use in AC circuits, using special techniques)
Tantalum Bead Capacitors
These capacitors also behave like electrolytic, but the only difference is the size. They are smaller. So they are used where a high capacitance and high voltage needed but less space is available.
Some of these capacitors use a colour code to express the values. The upper two colours tell the significant number of the value and the spot tells the multiplier. Other colour is used to express the voltage. Standard colour code is used here, but for the spot, grey is used to mean × 0.01 and white means × 0.1. In the voltage band, the colours are used as follows.
Blue 2 0V
Pink 3 5V
There are several types of unpolarized capacitors. They are small in size as well as small in the voltage handling. The lifetime is short when comparing to the electrolytic capacitors.
Film and paper capacitors
Paper capacitors and film capacitors are constructed using a rolled foil techniques described in previous section. The leads for the capacitor are then connected to the ends of completed role. After the leads are attached, the complete assembly is covered with a protective coating of insulation. Some times the insulation is molded around the assembly to produce molded capacitors. Sometimes it is dipped in plastic insulation material producing dip capacitors. Sometimes it is placed in an insulated tube and the end of the tube is then sealed. This is preferred to as tubular capacitors.
The paper-dielectric capacitor is usually bulkier; les stable, and has less dielectric resistance than the film capacitors. The paper dielectric may absorb moisture from the atmosphere if there is the slightest flow in the end seals. As changing the moisture content and the insulation resistance of the paper, as a dielectric material, it is rapidly being replaced by plastic film material. Such material as polystyrene, polyester and polycarbonate are popular replacements.
Many of the plastic film capacitors use metallised plates with these metallised film capacitors, the metal plates is deposited directly on the film this keeps the distance between plates as small as possible and produces a small compact capacitors.
Film and paper capacitors used in industrial application can be as large as several 100MF. They are often rated in terms of VA as well as DC WV. These large values are usually enclosed in a metal container, which is filled with a special insulation coil. How ever in most electronic circuits, paper and film capacitors are less than 1MF in value.
For low value capacitors, (less than 0.1MF) ceramic is a popular electric material. The most common style of ceramic capacitor is the disc ceramic.
They are used in a wide variety of application where low values of capacitance are needed.
Mica capacitors are limited to even lower values than are ceramic capacitors. This is because mica has a lower dielectric constant than ceramic. However it is easier to control production tolerances with mica dialectics, and mica has good high temperature characteristics.
It takes less space to provide a given value of capacitance with ceramic than with mica. Also it is easier to construct a ceramic capacitor therefore mica capacitors are not as common as ceramic capacitors.
A variable capacitor is constructed in such manner that its value of capacitance can be varied. A typical variable capacitor (adjustable capacitor) is the rotor-stator type. There is another one called Trimmer capacitor.
It consists of two sets of metal plates arranged so that the rotor plates move between the stator plates. Air is the dielectric. As the position of the rotor is changed, the capacitance value is likewise changed. This type of capacitor is used for tuning most radio receivers.
This capacitor consists of two plates separated by a sheet of mica. A screw adjustment is used to vary the distance between the plates, thereby changing the capacitance. This is also used as a preset. Set the values when a circuit is made.