Symbols for capacitors

Metalized Polyester Film capacitors

No polarity, dielectric made of Polyester or DuPont trade name "Mylar". Good quality, low drift, temperature stable. Because the electrodes are thin they can be made very very small. Good all-round capacitor.Care is necessary, because the component lead easily breaks off from these capacitors. Once lead has come off, there is no way to fix it. It must be discarded.

Polystyrene capacitors

No polarity, is used as a dielectric. Constructed like a coil inside so not suitable for high frequency applications. Well used in filter circuits or timing applications using a couple hundred KHz or less. Electrodes may be reddish of color because of copper leaf used or silver when aluminum foil is used for electrodes.

Excellent general purpose plastic film capacitor. Excellent stability, low moisture pick-up and a slightly negative temperature coefficient that can be used to match the positive temperature co-efficient of other components. Ideal for low power RF and precision analog applications

Polypropylene capacitors

This capacitor uses thin polyester film as the dielectric.They are not high tolerance, but they are cheap and handy.No polarity, mainly used when a higher tolerance is needed then polyester caps can offer. This polypropylene film is the dielectric.Very little change in capacitance when these capacitors are used in applications within frequency range 100KHz. Tolerance is about 1%.

Very small values are available.

Polyester Film capacitors

This capacitor uses a thin polyester film as a dielectric. Not as high a tolerance as polypropylene, but cheap and handy, temperature stable, readily available, widely used. Tolerance is approx 5% to 10%. Can be quite large depending on capacity or rated voltage and so may not be suitable for all applications. Temperature stability is poorer than paper capacitors. Usable at low (AC power) frequencies, but inappropriate for RF applications due to excessive dielectric heating.

Super capacitors

Supercapacitors are capacitors which have the ability to store large amounts of charge, and therefore energy, in a very small volume. Energy storage is by means of static charge rather than of an electro-chemical process that is inherent to the battery. Applying a voltage differential on the positive and negative plates charges the supercapacitor. This concept is similar to an electrical charge that builds up when walking on a carpet. The supercapacitor was first conceived in 1957 but now research is focused on using these as a light weight power sources as an alternative for batteries. the supercapacitor crosses into battery technology by using special electrodes and some electrolyte. Supercapacitors could find applications such as temporary back-up power supplies in the electrical power grid or providing the initial burst of energy to get electric cars moving.

Tantalum Capacitors

Tantalum capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small so they are used where a large capacitance is needed in a small size such as mobile phones or laptop computers. These capacitors have increasingly become an important as the demand for ever smaller electronic gadgets has grown. Columbite-tantalite - coltan, for short, the ore from which tantalum is refined is mined in Australia, Egpyt. The high demand for the ore has also financed civil wars in the Democratic Republic of the Congo. A UN security council report charged that a great deal of the ore is mined illegally and smuggled over the country's eastern borders by militas from neighbouring Uganda, Burundi and Rwanda providing the revenue to finance the military occupation of the Congo.

Variable capacitors

  Variable capacitors are devices that can be made to change capacitance values with the twist of a knob. These devices come in either air variable or trimmer forms. Air variable capacitors consist of 2 sets of aluminum plates (stator and rotor) that mesh together but do not touch. Rotating the rotor plates with respect to the stator varies the capacitor's effective plate surface area, thus changing the capacitance. Air variable capacitors typically are mounted on panels and are used in frequently adjusted tuning applications (eg. : fine tuning fixed frequency communications receivers, crystal frequency adjustments, adjusting filter characteristics). Trimmers may use a mica, air, ceramic, or glass dielectric and may use either a pair of rotating plates or a compression like mechanism that forces the plates closer together

Electrolytic Capacitors

Aluminum electrolytic capacitors are made by layering the electrolytic paper between the anode and cathode foils, and then coiling the result. The process of preparing an electrode facing the etched anode foil surface is extremely difficult. Therefore, the opposing electrode is created by filling the structure with an electrolyte. Due to this process, the electrolyte essentially functions as the cathode

Electrolytic capacitors are soaked in a liquid or paper impregnated with a liquid which is not a dielectric but when a voltage is applied creates a layer of aluminium oxide which acts a the dielectric. The reaction is dependent on the polarity of the applied voltage. If the polarity is reversed the capacitor will produce a gas and is likely to explode or burst because of the pressure inside and so are not suitable for altenating current applications

CERAMIC Capacitors

   This type is very popular non polarized capacitor that is small and inexpensive but has poor temperature stability and poor accuracy. It contains a ceramic dielectric and a phenolic coating. It is often used for bypass and coupling applications. Tolerances range from +/-5 to +/-100 percent, while capacitances range from 1 pf to 2.2 uF, with maximum voltage rating from 3 V to 6 kV

MICA Capacitors

This type is an extremely accurate device with very low leakage currents. It is constructed with alternate layers of metal foil and mica insulation, stacked and encapsulated. These capacitors have small capacitances and are often used in high frequency circuits (eg. : RF circuits). They are very stable under variable voltage and temperature conditions. Tolerances range from +/-0.25 to +/-5 percent. Capacitances range from 1 pf to 0.01 uF, with maximum voltage ratings from 100 V to 2.5 kV

Paper Capacitors

 A Paper Capacitor is made of flat thin strips of metal foil conductors that are separated by waxed paper (the dielectric material). Paper capacitors usually range in value from about 300 picofarads to about 4 microfarads. The working voltage of a paper capacitor rarely exceeds 600 volts. Paper capacitors are sealed with wax to prevent the harmful effects of moisture and to prevent corrosion and leakage

How to read the capacity of capacitors

Know the capacitance as the ability to store massive electrons, measured in unit Farad

Click to enlarge image

 

Inside a Capacitor

Capacitors

A capacitor is an electronic device for storing charge. Capacitors can be found in almost all but the most simple electronic circuits. There are many different types of capacitor but they all work in essentially the same way. A simplified view of a capacitor is a pair of metal plates separated by a gap in which there is an insulating material known as the dielectric. This simplified capacitor is also chosen as the electronic circuit symbol for a capacitor is a pair of parallel plates as shown in Figure 1. 

Figure 1. The symbol for an unpolarised capacito

Normally, electrons cannot enter a conductor unless there is a path for an equal amount of electrons to exit. However, extra electrons can be "squeezed" into a conductor without a path to exit if an electric field is allowed to develop in space relative to another conductor. The number of extra free electrons added to the conductor (or free electrons taken away) is directly proportional to the amount of field flux between the two conductors.
In this simplified capacitor the dielectric is air. When a voltage, V is applied to the terminals of the capacitor, electrons flow on to one of the plates and are taken off the other plate. The total number of electrons in the capacitor remains the same. There are just more on one the negative plate and fewer on the positive plate

Figure 2. Charging a capacitor with a battery

If the volage were increased the increased potential difference between the plates would push more electrons on to the negatively charged plate. We could measure the charge stored on the plate as a function of different applied voltages.
At zero voltage, the capacitor plates are neutral and so no charge is stored. (we assume that we started with a fully discharged capacitor), at a voltage V the charge on the plates is Q and at twice the voltage, the charge is doubled. We find that for increasing voltage the charge increases linearly. We can plot this as a straight line.
Suppose that we go away and do some research and come back with a better capacitor which stores more charge for a given voltage we can plot the result of the charge stored as a function of applied voltage
This would be represented as another line with a steeper slope. If we plotted lots of graphs for different capacitors we would get many straight lines. We can say that a measure of the capacitance is the how much the charge is stored for a given voltage. This is sometimes expressed as Q=CV.
Of course in charging the capacitor work must be done to move the charge. Therefore energy must be supplied and this energy is available when the capacitor is discharged

The work done is given by W=qV. Initially the charge is easily moved onto the plates of the capacitor, however as more charge is moved onto the capacitor plates the repulsive force between the charges makes it harder to add charge, when the repulsive force of the charges equals the power of the battery, no more charge can be moved onto the plates. Therefore the average work is 1/2qV. If we look at our graph of charge against voltage we can recognise this is the same as the area under the curve. In general, the work done is equal to the energy transferred. Mathematically