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Monday, 25 March 2019

Capacitors - Classification, Applications and Properties

Capacitors are classified in to different types based on various factors as given below :


1. According to the Type of Dielectric Used :

Ex : Mica, Paper, Ceramic, Air, Electrolytic Capacitors.

2. According to the Physical Construction :

(i) Fixed Capacitors : Whose capacitance value cannot be varied mechanically or by any other external means.
Ex : Mica, Paper, Ceramic, Electrolytic capacitors.

(ii) Variable capacitors : Whose capacitance value can be varied quite frequently or less frequently.
Ex : Tuning capacitors, and Trimming capacitors.

3. According to the Polarization :

(i) Polarized : Used in d.c applications
Ex : Aluminium, Tantalum Electrolytic Capacitors

(ii) Non polarized : Used in a.c applications.
Ex : Aluminium, Tantalum Electrolytic Capacitors

4. According to Voltage Rating :

(i) Low voltage capacitors (< 50V)
Ex : Ceramic, Electrolytic capacitors
(ii) High voltage capacitors (> 100V)
Ex : Mica, Glass, Ceramic capacitors.

FIXED CAPACITORS :


The Capacitance depends on the following factors :
(i) Directly proportional to the area of the plates (A) in square metres.
(ii) Inversely proportional to the distance (d) between the plates in metres.
(iii) Depends on to the permittivity of the medium between the plates (Ɛ).

Or C = ƐA/d  (since, Ɛ = Ɛ0 Ɛr)

Therefore, C = Ɛ0 Ɛr A/d Farad
where Ɛ0 = absolute permittivity of air = 8.8540 x 10-12 F/m
and Ɛr = relative permittivity of medium.

VARIABLE CAPACITORS:


Capacitors whose capacitance value cannot be varied mechanically or by any other external means. These capacitors usually have the fixed value with tolerance bearing from ± 1 to ± 20%. They are of various types depending on the use of dielectric between the plates.

1. Paper Capacitors :


In this type of capacitors paper is used as the dielectric medium. Based on the construction paper capacitors are divided into two types namely :

1. Impregnated paper capacitors
2. Metalised paper capacitors

Properties of Paper Capacitors :

1. They are usually high-voltage (> 100V) capacitors.
2. Their capacitance value is usually between 0.002 µF and 0.05 µF.
4. They are mechanically very strong.
4. They are very cheap.
5. They are quite bulky.
6. They have poor high frequency characteristics.

Applications of Paper Capacitors:

• Used as RF-suppression capacitors in circuits where noise interference from RF sources can occur.
• Used as bypass capacitors in amplifiers.
• Used in high voltage DC circuits.
• Used in commutating circuits of silicon-controlled rectifiers.

2. Mica Capacitors :


In this type of capacitors mica is used as the dielectric medium. Based on the construction mica capacitors are divided into two types namely :

1. Stacked mica capacitors
2. Silvered mica capacitors.

Properties of Mica Capacitors :

1. Mica capacitors have good mechincal strength.
2. They can be operated to temperatures as high as 900°C.
3. They can with stand very high voltages (Thousand of volts).
4. They are suitable for very high-frequency operation.
5. The capacitance value is generally between 5 to 3300 PE
6. The capacitance value is highly stable.
7. They are cheaper than polyester capacitors.

Applications of Mica Capacitors:

• Used as high-voltage capacitors in low-frequency power applications.
• Used as high-voltage RF capacitors.
• Used as high-voltage transmitter-capacitors.




3. Glass Capacitors:


In this type of capacitors Boro silicate glass or glass with potassium, barium or lead oxides is used as the dielectric medium.

Properties of Glass Capacitors:

1. Stability and frequency characteristics are better than mica capacitors.
2. Cost is higher.

Applications of Glass Capacitors:

These are used in medium power transmitters.

4. Polyester Capacitors :


These capacitors are made of polyethelene terapthalate (Mylar, Melinex, Terelyne etc).

Properties of Polyester Capacitors :

1. They have high capacitance (a few ,tF) in small volume.
2. They are very stable.
3. They have good high temperature (up to 250oC) properties.
4. They have good mechanical strength.
5. They have very low leakage.
6. Their cost is much higher than that of paper, ceramic and mica capacitors.

Applications of Polyester Capacitors:

• Used as coupling capacitors.
• Used as stable capacitors where capacitance stability over long years of operation is a must.
• They can replace leaky electrolytic capacitors in many R.F applications.

5. Polystyrene Capacitors :


These capacitors are made by rolling the polystyrene film with aluminium foil.
Properties of Polystyrene Capacitors
1. Excellent stability.
2. Low moisture pick-up.
3. Slightly negative temperature co-efficient.
4. Maximum operating temperature is only about 85°C.
5. Comparatively bigger in size.

Applications of Polysterene Capacitors :

• Used in precision timing circuits.
• Used in ICs.
• Used in high tuned circuits and sub standards.

6. Ceramic Capacitors :


These are the latest type capacitors. Ceramic is used as the dielectric material. Depending on the construction capacitors are of four types:

1. Tubular type
2. Disc type
3. Monolithic type
4. Barrier layer type.

1. Tubular Type:

These are in tubular form capacitance ranges from 5 to 1000 PF voltage ranges up to 5 KV and up to 10,000 PF in the lower voltage ranges.

2. Disc Type :

These are made up of thin ceramic film operated at voltage gradients up to 4000 V/mm. The disc types of capacitors have high capacitance per unit volume and are very economical. These capacitors are available from a few PF to 20,000 PF working voltage is 750V dc or 350V dc. These are low cost and small size capacitors used for coupling and by pass use in IF and RF circuits.

3. Monolithic Capacitors:

It consists of interleaved thin layers of ceramic - platinum electrodes fused. These capacitors are available from 10 PF to 0.47 μF at voltage ratings of 40 and 100 V dc. Insulation resistance is 1000 or 1 giga ohm. The size of the capacitor reduces by 40%.

4. Barrier Layer Type : 

These capacitors are made from barium titanate dielectric. These have much lower insulation resistance and are useful only for low voltage types. Operating temperature ranges from -40 to 80°C and insulation resistance values are about 1.5 MΩ for low capacitance values and 100 KΩ at 0.22 μF.

Applications:

Ceramic capacitors are used for by pass, decoupling and bias applications, used in scan correction circuits in TV receivers, these capacitors with prescribed temperature capacitance variations are used to compensate for impedance temperature changes in circuits. They are used in hybrid ICs.

Electrolytic Capacitors :

Electrolytic capacitors are of two types depending on the electrode material used
1. Aluminium Electrolytic capacitors
2. Tantalum Electrolytic capacitors

Both are available in wet and dry types.

1. Aluminium Electrolytic Capacitors :

Aluminium electrode is used in these capacitors. These capacitors have relatively high leakage resistance. The capacitance values are ranging from 1 to 2,70,000 µF and voltages ranging from 3 to 450 V with tolerance 50%. These capacitors are low cost and polarised.

2. Tantalum Electrolytic Capacitors: 

Tantalum Electrode is used in these capacitors. The features of these capacitors are rugged, low leakage resistance, small size and high capacity. Good for high reliability and critical industrial use. Generally polarised, but are also supplied as non polarized types and can be operated up to 175°C. These have the same uses as aluminium electrolytics but are superior in performance. Values are ranging from 1 to 1200 μF and voltage ratings ranging from 3 to 300V tolerance from 5 to 20%.

Applications of Electrolytic Capacitors:

These are used as by pass capacitors in amplifers, for inter-stage coupling between two amplifiers stages, as smoothing capacitors in D.C. Power supplies, as phase-shifting capacitors in single phase induction motors.



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