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Monday, 4 November 2019

Types and Uses of Electrolytic Capacitors

Types and Uses of Electrolytic Capacitors

These capacitors have low volume for large capacitance especially at low working voltages and can be used for high energy pulse storage application. Electrolytic capacitors are for direct voltage working. Different types of electrolytic capacitors are dealt in succeeding paragraphs.

Aluminium Electrolytic Capacitor :

Aluminium oxide which is formed by a process of electrolysis on an aluminium electrode acts as dielectric in these capacitors. During the process of manufacture, the electrode is connected to positive of voltage source and is therefore called anode. Aluminium borate solution is used in the production of oxide film. The film is thin and therefore has high capacitance per unit space occupied by capacitor. The electrolyte in contact with aluminium anode, oxide coating behaves as the other electrode of the capacitor. The second coating behaves as the other electrode of the capacitor. The second terminal is connected to the non oxide aluminium foil. In the rolled electrolyte capacitor the anode consists of a long strip aluminium foil carrying the oxide film on both sides. Aluminium borate electrolyte solution is held in an absorbent paper tissue strip of similar length. A second strip of aluminium foil with no oxide film lies next to paper tissue and acts as the cathode. The three strips are rolled tightly and outer cover of aluminium is provided.

These capacitors are available in polarised or non-polarised form. In polarised type the oxide film is formed on the positive electrode only and the negative electrode is used only for connections. In the non-polarised form two polarised capacitors are to be connected back to back or by using both the electrodes with oxide films. Polarised capacitors are to be connected only in one direction. A change in polarity of connection allows the capacitor to draw high currents and gets heated. Such capacitors are used as filters for power supply. The non polarised form used in series with the starting winding of capacitor start inductions run motor and in the case of ceiling fan.

In some capacitors a solution of Boric Acid ammonium metaborate and glycol is used as electrolyte. The foil with the oxide film on both sides acts as the anode and an ordinary foil acts as cathode. These are rolled with kraft paper separations and encased in an aluminium casing and leads taken out. It has a polarity marking on it.

In the electrolyte capacitors since the oxide film is not a perfect insulator high leakage current will result and is of the order of 0.1 to 0.5 mA per Farad of capacitance and working temperature is limited to 80°C. The power factor of these capacitors is ten times the paper capacitor and is mainly due to the resistance film. The power may decrease with temperature and increases with frequency. The polarised capacitors are used as coupling capacitors in RC amplifier, as bias and decoupling capacitors, filters in rectifier circuits.

Tantalum Electrolytic Capacitors :

In the new type of electrolytic capacitors tantalum or Niobium is used. The dielectric oxide film Tantalum Pentoxide can be electrolytically formed on tantalum. This has higher dielectric constant and more stable than aluminium oxide. In the wet type electrolytic capacitor, the electrolytes used are sulphuric acid or lithium chloride or Boric Acid and in the solid type manganese dioxide is used.

Wet Type Tantalum Capacitors : 

In this type the anode is capsule shaped and is made of compressed powder of Tantalum and sintered in vacuum at 2000°C. An oxide layer is formed electrolytically. The cathode is cup shaped and made of silver. It contains a liquid electrolyte of very high conductivity. Lithium chloride is used as electrolyte. The anode is immersed in electrolyte and the cup sealed with a plate. The anode and cathode are separated by Teflon seal.
Tantalum Electrolytic Capacitors
Solid Type Electrolytic Capacitors :

In this type of tantalum powder is pressed to form a pellet in a die set and sintered at controlled temperature of 2000°C by which high bonding strength is acquired. The size of granules of tantalum powder determines the capacitance value. The pellet is then inserted in maganese sintrate. Decomposition from nitrate to oxide takes place and manganese oxide is formed. Graphite is then coated on the oxide layer and a copper coil or metal sprayed is covered. Lead wires are taken out and the entire assembly is encased in moulded tubular or rectangular plastic case with resin seal.

These are low voltage capacitors with a working temperature of 100°C. It has good ripple voltage rating and has low leakage due to high conductivity of manganese oxide layer.

Some of the features are :

(a) Its cost per microfarads is very low.
(b) Leakage current is less than wet type.
(c) Series resistance is less than wet type.
(d) Polarised and non polarised are available.
(e) The voltage ranges from 25V to 500 volts.
(f) The power factor is low and increases with frequency and applied voltages.

Uses of Electrolytic Capacitors :

Electrolytic capacitors are used in circuits that have a combination of de voltage and ac voltage. The dc voltage maintains the polarity. They are used as ripple filters where large capacitances are required at low cost in small space. They are also used as bias capacitors and decoupling capacitors. They are used as coupling capacitors in R-C amplifier stages.

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