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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.
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.