Friday, 31 January 2014

Types of Capacitor-Constant value capacitors

Ceramic Capacitors
The non – polarized type ceramic capacitors which are also known as ‘Disc capacitors’ are widely used these days. These are available in millions of varieties of cost and performance. The features of ceramic capacitor depend upon:
Type of ceramic dielectric used in the capacitor which varies in the temperature coefficient and Dielectric losses

These are made by placing silver coated ceramic plates on two sides and assembled together to form the capacitor. The surface mount version consists of the ceramic dielectric in which a number of interleaved precious metal electrodes are contained. This structure gives rise to a high capacitance per unit volume. The inner electrodes are connected to the two terminations, either by silver palladium (AgPd) alloy in the ratio 65 : 35, or silver dipped with a barrier layer of plated nickel and finally covered with a layer of plated tin (NiSn).

The typical values range from 10pF to 1uF. The capacitance values are labeled by three digit codes where the first two digits represent a number and the third digit is the multiplier digit.
For example:  104 which is 10*104 pF which is 0.1uF
The tolerance is indicated by a letter like j=5%, K=10% and M=20%.

These capacitors are commonly used as a timing element in filter circuit and balancing oscillator circuits in radio frequency applications, coupling and decoupling networks.

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Types of Capacitor
Types of Capacitor-Constant value capacitors
Types of Capacitor-Variable value capacitors

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Electrolytic Capacitors

1. Aluminium electrolyte capacitor
These polarized capacitors are made of oxide film on aluminium foils. These are cheaper and easily available. The range of values typically varies from 1uF to 47000uF and large tolerance of 20%. The voltage ratings range up to 500V. They have high capacitance to volume ratio and used for smoothing in power supply circuits or coupling capacitors in audio amplifiers. These are available in both leaded and surface mount packages. 

2.Tantalum electrolyte capacitor
These capacitors utilize tantalum oxide which enables the fabrication of small size electrolytes. These are costlier than aluminium electrolytes and have lower maximum voltage up to 50V and are preferred where size matters. Their typical values range from 47uF to 470uF.

Film capacitors

These capacitors consist of a relatively large family of capacitors with the difference being in their dielectric properties. These include polyester (Mylar), polystyrene, polypropylene, polycarbonate, metalized paper, Teflon etc. Film type capacitors are available in capacitance ranges from 5pF to 100uF depending upon the actual type of capacitor and its voltage rating. 

Paper capacitor

Paper capacitors are made of paper or oil-impregnated paper and aluminium foil layers rolled into a cylinder and sealed with wax. These capacitors were commonly used but are now replaced by the plastic or polymer type of capacitors. The paper capacitors are bulky, highly hygroscopic and soaks moisture which causes loss to the dielectric degrading its overall performance is the major drawback with this type of capacitors.

Metallized paper capacitors

The metallized paper capacitors are smaller in size than the conventional paper capacitors. However, these capacitors are appropriate for only low current applications and are now replaced by metallized film capacitors.

Mica capacitor

The mica capacitor uses mica as the dielectric medium. Mica is inert in nature and hence the physical and chemical properties do not change as it ages. It provides good temperature stability and resistance to corona discharge i.e. electrical discharges due to ionization around conductor. However, the cost is very high and due to improper sealing the capacitor is highly prone to moisture which increases the power factor.

Silver Mica or metallized mica capacitor

These are a kind of mica capacitor which has an additional advantage of reduced moisture infiltration. These capacitors are expensive and are used often in HF and low VHF radio frequency circuits as low value accurate capacitors particularly in the oscillators and filters. The reasons that these capacitors are still in use regardless of high cost, large size and availability of other low cost capacitors are due to its remarkable features such as: 

  • Low tolerance of +/- 1% 
  • Positive temperature coefficient of 35 to 75 ppm/C 
  • Greater range from few pF to two or three pF 
  • Little voltage dependence, 
  • High stability 
  • Good Q factor.
Glass capacitor

These capacitors are fabricated of glass dielectrics and are very expensive which are used for highly accurate, stable and reliable operation in harsh environmental conditions. These are resistant to nuclear radiations and available in range of 10pF to 1000pF.

Polyester or PET capacitor

Polyester or PET capacitors are plastic capacitors available as leaded packages that replace the paper capacitors. These capacitors are made of polyester films which small in size and available at low cost. These have Operating voltages up to 60,000 V DC, operating temperatures up to 125 °C and low moisture absorption. These are mostly used as low frequency signal capacitors and integrators. They are preferred where cost plays an important role because they have high tolerances of 5 - 10 %.

Polystyrene capacitors

These are large size capacitors available in tubular shape leaded packages. They have high stability, negative temperature coefficient (NTC), high accuracy and low moisture absorption. The operating temperature is limited to +85 C. These are mostly preferred for low frequency applications as the tubular structure induces inductances which degraded the performance at high frequencies.

Kapton polyimide capacitor 

These capacitors are similar to polyester or PET capacitors that are made of Kapton polyimide film. They are expensive but offer high operating temperatures up to 250 C. These capacitors are not suitable for RF applications.

Polycarbonate capacitors

These are high performance capacitors which are least affected as it ages. These are characterized by good insulation resistance and dissipation factor. The operating temperature ranges from -55 to +125 C. The dielectric constant is 3.2 %, and dielectric strength is 38 KV/mm. The dissipative factor is 0.0007 at 50Hz and 0.001 at 1MHz. The water absorption is 0.16%. These are mostly used for filters, coupling and timing applications.

Polypropylene capacitors

These are used where higher tolerances than PET film capacitors are demanded. These are available in leaded packages and are used for low frequency operation. They have high operating voltages and are resistant to the breakdown. However, they get damaged by transient over-voltages or voltage reversals.

Polysulfone capacitor

These capacitors are similar to polycarbonate capacitors but can withstand full voltage at comparatively higher temperatures. These capacitors are very expensive and are not readily available. The stability is limited as the moisture absorption is typically 0.2%

TEFLON or PTFE fluorocarbon capacitor

These plastic capacitors are large in size and expensive. Due to low losses and higher stability these are used for some critical applications. The operating temperature ranges up to 250 C. The dielectric used is Polytetrafluoroethylene.

Metallized polyester or Metallized plastic capacitor

These capacitors have metallized plastic films which provide self heating advantage and also reduce the size of the capacitor over conventional plastic or polyester capacitor. However, they are limited by maximum current capacity. They are available in leaded package.

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