Aluminium
electrolytic capacitors are widely used in power supply circuitry
of electronic equipment as there after several advantages over other
types of capacitances. The selection of a capacitor for an application
without knowing the basics may result in unreliable performance
of the equipment due to expanitor problems. It may lead to customer
dissatisfaction and damage market to potential or the image of a
reputed company. The aluminium eletrolytic capacitors are suitable
to be used when a great capacitance value is required in a very
small size. The volume of an electrolytic capacitor is more than
10 times less than a film one considering the same rated voltage
and capacitance. The cost per F is also less when compared with
all other capacitors.
CONSTRUCTION
An aluminium
electrolytic capacitor is composed of high-purity, thin aluminium
foil (0.05 to I mm thick) having a dieletric anidation on its surface
to prevent current flow in one direction. This outs as anode. Another
these two aluminium coils is an electrolytic impregnated paper,
which cuts as the dieletric. Since the capacitors is inversely propotional
to the dieletric thiclenen. And the dieletric thicknen is propotional
to the forming voltage, the relationship between capacitance and
cerming voltage is.
Capacitance
X Forming Voltage = Constant.
Aluminium tabs
attached to the anode and cathode coils act as the positive and
negative leads of the capacitor respectively. The entire element
is sealed into an aluminium can by using rubber, bakelite or phenolic
plastic. The construction of an aluminum electrolytic capacitor
is the following:
The anode (A):
The anode is
formed by an aluminium foil of extreme purity. The effective surface
area of the coil is greatly enlarged (by a factor upto 200) by electrochemical
etching in order to achive the maximum possible capacitance values.
The dieletric (O):
The aluminum
foil (A) is covered by a very thin oxidised layer of aluminium oride
(O=Al O3. This oxide is obtained by means of an eletro chemical
process. The typical value of forming voltage is 1.2 nm/v. the oxide
with stands a high electric field strength and it has a high dielectric
constant. Aluminium oxide is therefore well suited as a capacitor
dieletric in a polar capacitor. The A12O3 has a high insulation
resistance for voltages lower than the forming voltage. The oxide
layer consistitutes a nonlinear voltage dependent resistance: the
current increases more steeply as the voltage increases
The electrolytic
Paper, cathode (C,K)
The negative
electrode is a liquid electrolyte absorbed in a paper. The paper
also acts as a spacer between the positive foil carrying the dieletric
layer and the opposite Al-foil ( the negative Coil) acting as a
contact medium to the eletrolyte. The cathode foil serves as a large
contact area for passing current to the operating eletrolyte. Bipolar
Al electrolytic capacitors are also available. In this designs both
the anode foil and cathode foil are anodized. The cathode foil has
the same capacitance rating as the anode foil. This construction
allows for operation of direct voltage of either polarity as well
as operation of purely alternating voltages. Since it causes internal
heating the applied atternating voltage must be kept considerably
below the direct voltage rating. Since we have the series connection
of two capacitor elements, the total capacitance is equal to only
half the individual capacitance value. So compared to polar capacitor,
a bipolar capacitor requires upto twice the volume for the same
total capacitance.
