A capacitor is a bit like a battery, but it has a different job to do. A battery uses chemicals to store electrical energy and release it very slowly through a circuit; sometimes (in the case of a quartz watch) it can take several years. A capacitor generally releases its energy much more rapidly—often in seconds or less. If you're taking a flash photograph, for example, you need your camera to produce a huge burst of light in a fraction of a second. A capacitor attached to the flash gun charges up for a few seconds using energy from your camera's batteries. (It takes time to charge a capacitor and that's why you typically have to wait a little while.) Once the capacitor is fully charged, it can release all that energy in an instant through the xenon flash bulb. Zap!
There are many different kinds of capacitors available from very small capacitor beads used in resonance circuits to large power factor correction capacitors, but they all do the same thing, they store charge. In its basic form, a Capacitor consists of two or more parallel conductive (metal) plates which are not connected or touching each other, but are electrically separated either by air or by some form of a good insulating material such as waxed paper, mica, ceramic, plastic or some form of a liquid gel as used in electrolytic capacitors. The insulating layer between a capacitors plates is commonly called the Dielectric. A Typical Capacitor
Due to this insulating layer, DC current can not flow through the capacitor as it blocks it allowing instead a voltage to be present across the plates in the form of an electrical charge.
Capacitors and capacitance
The amount of electrical energy a capacitor can store is called its capacitance. The capacitance of a capacitor is a bit like the size of a bucket: the bigger the bucket, the more water it can store; the bigger the capacitance, the more electricity a capacitor can store. There are three ways to increase the capacitance of a capacitor. One is to increase the size of the plates. Another is to move the plates closer together. The third way is to make the dielectric as good an insulator as possible. Capacitors use dielectrics made from all sorts of materials. In transistor radios, the tuning is carried out by a largevariable capacitor that has nothing but air between its plates. In most electronic circuits, the capacitors are sealed components with dielectrics made of ceramics such as mica and glass, paper soaked in oil, or plastics such as mylar.
The size of a capacitor is measured in units called farads (F), named for English electrical pioneer Michael Faraday (1791–1867). One farad is a huge amount of capacitance so, in practice, most of the capacitors we come across are just fractions of a farad—typically microfarads (millionths of a farad, written μF), nanofarads (thousand-millionths of a farad written nF), and picofarads (million millionths of a farad, written pF).Supercapacitors store far bigger charges, sometimes rated in thousands of farads.
Types of Capacitors
There are many different types of capacitors and they each vary in their characteristics and each have their own advantages and disadvantages.
Some types of capacitors can charge up to higher voltages and, thus, can be used in high voltage applications. Some capacitors can charge up to very high charges, such as aluminum electrolytic capacitors. Some capacitors have very low leakage low leakage rates and others have very high leakage rates. All of these factors determine how and in what application each of the capacitors will be used in circuits.
Based on the design, capacitors are categorized in these different types:
For most of applications we use Electrolytic type Capacitors. They are very important for an electronic student as they are easy to get and to use, and they are inexpensive too.
Electrolytic Capacitors are generally used when very large capacitance values are required typically above 1μF. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode).
The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make capacitors with a large value of capacitance for a small physical size as the distance between the plates, d is very small.
The majority of electrolytic types of capacitors are Polarised, that is the DC voltage applied to the capacitor terminals must be of the correct polarity, i.e. positive to the positive terminal and negative to the negative terminal as an incorrect polarisation will break down the insulating oxide layer and permanent damage may result. All polarised electrolytic capacitors have their polarity clearly marked with a negative sign to indicate the negative terminal and this polarity must be followed. Electrolytic Capacitors are generally used in DC power supply circuits due to their large capacitance’s and small size to help reduce the ripple voltage or for coupling and decoupling applications. One main disadvantage of electrolytic capacitors is their relatively low voltage rating and due to the polarisation of electrolytic capacitors, it follows then that they must not be used on AC supplies. Electrolytic’s generally come in two basic forms; Aluminium Electrolytic Capacitors and Tantalum Electrolytic Capacitors.
An electrolytic capacitor is usually labeled with these things:
1. Capacitance value.
2. Maximum voltage.
3. Maximum temperature.
For an electrolytic capacitor, the capacitance is measured in micro Farad. Based on requirement the appropriate capacitor is chosen. With higher capacitance, the size of capacitor also increases.
Voltage Rating of a Capacitor
All capacitors have a maximum voltage rating and when selecting a capacitor consideration must be given to the amount of voltage to be applied across the capacitor. The maximum amount of voltage that can be applied to the capacitor without damage to its dielectric material is generally given in the data sheets as: WV, (working voltage) or as WV DC, (DC working voltage). If the voltage applied across the capacitor becomes too great, the dielectric will break down (known as electrical breakdown) and arcing will occur between the capacitor plates resulting in a short-circuit. The working voltage of the capacitor depends on the type of dielectric material being used and its thickness. The DC working voltage of a capacitor is just that, the maximum DC voltage and NOT the maximum AC voltage as a capacitor with a DC voltage rating of 100 volts DC cannot be safely subjected to an alternating voltage of 100 volts. Since an alternating voltage has an r.m.s. value of 100 volts but a peak value of over 141 volts!. Then a capacitor which is required to operate at 100 volts AC should have a working voltage of at least 200 volts. In practice, a capacitor should be selected so that its working voltage either DC or AC should be at least 50 percent greater than the highest effective voltage to be applied to it.
Polyester capacitors are capacitors composed of metal plates with polyester film between them, or a metallised film is deposited on the insulator.
Polyester capacitors are available in the range of 1nF to 15µF, and with working voltages from 50V to 1500V. They come with the tolerance ranges of 5%, 10%, and 20%. They have a high temperature coefficient. They have high isolation resistance, so they are good choice capacitors for coupling and/or storage applications. Compared with most other types, polyester capacitors have high capacitance per unit volume. This means more capacitance can fit into a physically smaller capacitor. This feature, together with their relatively low price makes polyester capacitors a widely used, popular, and cheap capacitor.
Tantalum Capacitors are capacitors that are made of tantalum pentoxide. Tantalum capacitors, just like aluminum, are electrolytic capacitors, which means they are polarized. Their main advantages (especially over aluminum capacitors) is that they are smaller, lighter, and more stable. They have lower leakage rates and less inductance between leads. However, their disadvantags are they have a lower maximum capacitance storage and lower maximum working voltage. They are also more prone to damage from high current spikes. For the last reason, tantalum capacitors are used mostly in analog signal systems that lack high current-spike noise.
Ceramic Capacitors or Disc Capacitors as they are generally called, are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For very low capacitance values a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitance’s can be obtained in a small physical size. Ceramic Capacitor
They exhibit large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads, ( μF ) but their voltage ratings are generally quite low. Ceramic types of capacitors generally have a 3-digit code printed onto their body to identify their capacitance value in pico-farads. Generally the first two digits indicate the capacitors value and the third digit indicates the number of zero’s to be added. For example, a ceramic disc capacitor with the markings 103 would indicate 10 and 3 zero’s in pico-farads which is equivalent to 10,000 pF or 10nF. Likewise, the digits 104 would indicate 10 and 4 zero’s in pico-farads which is equivalent to 100,000 pF or 100nF and so on. So on the image of the ceramic capacitor above the numbers 154 indicate 15 and 4 zero’s in pico-farads which is equivalent to 150,000 pF or 150nF or 0.15uF. Letter codes are sometimes used to indicate their tolerance value such as: J = 5%, K = 10% or M = 20% etc.
General uses of Capacitors
- Smoothing, especially in power supply applications which required converting the signal from AC to DC.
- Storing Energy.
- Signal decoupling and coupling as a capacitor coupling that blocks DC current and allow AC current to pass in circuits.
- Tuning, as in radio systems by connecting them to LC oscillator and for tuning to the desired frequency.
- Timing, due to the fixed charging and discharging time of capacitors.
- For electrical power factor correction and many more applications.
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