Electrolytic Capacitors

					Electrolytic Capacitors
        The main limitation of other capacitors are the limited capacitance value within
1μF. It is because of the practical limitation of the distance between the parallel plates.
Electrolyte capacitances solves this problem, because it uses a kind of electrolyte as the
medium to form the dielectric.

        Here two electrodes are immersed in the electrolyte form the plates of the
capacitor. Due to the chemical reaction, a thin oxide film is formed around one of the
plate and this thin film acts as the dielectric and it is in the order of few Å. Thus
capacitace values becomes high with small size capacitor. The capacitance value ranges
from 1μF to several thousands of μF.

       Different types of electrolytic capacitors are :
              1. Polarized
              2. Non-polorized

       According to the material used, electrolytic capacitors are classified into:
             1. Aluminium
             2. Tantalum

Aluminium Polarized type Electrolyte Capacitor
      The constructional details of these capacitors is shown below:




        The cathode is made up of nickel or copper and the metal case is also made up of
the same material. The metal case contain electrolyte, which is usually Al borate in
solution or dry form. The anode is made up of Al .
        When a dc potential is applied, with +ve potential to anode and –ve potential to
cathode a chemical reaction takes place between the electrolyte and Al foil. This causes a
very thin Al2O3 (Al oxide) is formed on the surface of the anode foil. This thin oxide
layer acts as dielectric and is highly polarized.
Aluminium Non-polarized Electrolytic Capacitor

        In ac applications, large capacitance values are required with the absence of dc
voltage. Such cases non-polarized electrolytic capacitors are used. The constructional
detail of this capacitor is as shown below:




      Here Al foil is used for the construction of both electrodes. Thus Al oxide film is
formed on both foils according to the potential.

Tantalum Electrolytic Capacitor

       The volume of capacitor with high capacitance can be provided by another type of
material to form dielectric film. For this Tantalum is used instead of Aluminium. The
constructional detail of the device is shown below:




        Here the cathode is made up of silver or copper and anode is formed by coating
Tantalum powder on a cylindrical metal and is called Tantalum pallet. The anode is
dipped in an acid solution of MnO2 and graphite. Thus a very thin layer of MnO2
(manganese oxide) film is formed on the Tantalum pallet during the operation and it acts
as the dielectric of the capacitor.
        Teflon bush is used to hold the anode cylinder.
Advantages of Al type Electrolytic Capacitor:

       1.   Very high capacitance value
       2.   Low cost/µF
       3.   Highly reliable (comparing to paper, mica…)
       4.   Highly stable (comparing to paper, mica,….)

Disadvantages of Al type Electrolytic Capacitor:

       1. Low operating voltage
       2. Not suitable for very high frequency applications.

Advantages of Tantalum type Electrolytic Capacitor:

       1.   Low leakage resistance
       2.   Long life
       3.   High stability
       4.   Highly reliable
       5.   Very high capacitance value (comparing to paper, mica…)
       6.   Low cost/µF

Disadvantages of these type capacitors are same as Al type.

Applications:

       Al type:
              1. Used as bypass capacitors
              2. Used as coupling capacitors
              3. Used as filter capacitors in power supplies

       Tantalum type:
              1. Used in timing circuits
              2. Used as starter component in induction motors.

Variable Capacitors
       It provides variation in capacitance and is useful for tuning purpose. Variation in
capacitance can be provided by varying either dielectric constant, area of plates or
distance between plates.

       Depending on the type of applications, variable capacitors can be classified into:
             1. Tuning type (Air ganged or air cored)
             2. Trimming type (Plastic cored)
Air Cored Variable Capacitors:

       It is mainly used in vacuum tube radios and are bulky, because of air core. The
main defect of these type capacitors is possibility for metal bending and short circuit.

       Constructional details of the device is shown below.




        It consists a set of movable Al vanes and a set of fixed Al vanes mounted on a
unit with air as dielectric. The vane acts as the plates of capacitor. The movable vanes are
made to rotate by means of a spindle. So that the areas of plates varies.

Plastic Cored Capacitor:

       It is mainly used to preset papoose and is small. It uses PVC film in between the
fixed and movable vanes to act as dielectric instead of air. The plastic film increases the
capacitance and reduces the size. The construction of this device is shown below.




        Here by varying the screw the distance between the plates can be varied, thus
capacitance also varied.
        Another type of variable capacitor is padder, which is air-cored trimmer. It
consists two tiny Al cups mounted concentrically, by varying the screw we can vary the
gap between the cups and thus the capacitance also varied.
Voltage Rating of Capacitors
       The voltage ratings of different capacitors mainly depend on the type of dielectric
used and are given below:

                     Mica      Paper    Ceramic      Plastic   Al electrolytic        Tantalum
                                                                                     electrolytic
 Voltage rating    Few kV      600V     150V to      750V       6V to 600V           35V to 75V
                                          6kV


Inductors
        They are passive devices, have energy storage property like capacitor. But the
energy stored in this device is in electromagnetic field. The symbol for inductance is L
and its unit is H (Henry),

       The electrical symbol for fixed and variable type inductors are shown below:




        Inductors are also called coils. It consists a long stretching wire made up of
copper with suitable guage, wound over an insulating former that may or may not carry a
core inside it.

       According to the nature of the core used, inductors can be classified into:
             1. Air cored
             2. Iron cored
             3. Ferrite cored

       According to the frequency of operation, inductors can be classified into:
             1. Low frequency (50Hz)
             2. Audio frequency (AF)
             3. Radio frequency (RF)
       According to the application, inductors can be classified into:
             1. High-voltage, high current coils for AC power line applications
             2. Low voltage coils in electronics circuits
             3. AF coils and chokes (small size)


        Inductors are used to construct transformers. Inductor works under self inductance
property, but transformers work under mutual inductance property. Transformer is a two
port device, which consist one set of input windings called primary coils and one or more
set of output winding called secondary coils.

       Constructional details of different fixed inductors are:

       Filter coils (Power coils)

              They are mainly used to produce smooth current to rectify ac to dc. It
consists many turns of wire wound on iron core. To reduce the power loss the core is
made up of laminated sheet of E and I shaped iron plates as shown below:




       The assembly of the choke is shown below:




       The inductance of these type inductors ranges from 5 to 20H.
AF Chokes

        They are used to produce high impedance to AF signals. Constructional
details of this inductor is similar to filter chokes, but the core is smaller size with
low inductance coil.

RF Chokes
         They are mainly used with RF circuits. It uses ferrite core in rod form or
core less construction. The coil is wounded on a wax former. The constructional
details is shown below:




        These type of winding is called universal winding, because it minimize
parasitic inter coil capacitance

				
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