CAPACITORS Though any two conductors separated by an insulator have a capacitance ~ INFOTECHNO

11.1. CAPACITORS

Though any two conductors separated by an insulator have a capacitance ; but a system of two or more conductors duly separated by an insulator when given the shape of a component to present a definite amount of capacitance is called a capacitor.

A capacitor is also a very useful component of electronic circuits like inductors and resistors. It is used in amplifiers, oscillators, filter circuits and many other types of electronic circuits.

11.2. CLASSIFICATION OF CAPACITORS

The capacitors classified on the basis of their working are as follows :

1. Fixed Capacitors. A capacitor having a fixed capacitance value is called a fixed capacitor. Its capacity can't be changed easily, e.g., paper, mica, electrolyte etc.

2. Adjustable Capacitors. A capacitor whose value can easily be changed with the help of a screw driver is called an adjustable capacitor. Its capacitance can be set at the desired value, e.g., trimmer, padder etc.

3. Variable Capacitors. A capacitor whose value can easily be set with the help of its shaft between its minimum and maximum values, is called a variable capacitor, e.g., gang capacitor.

11.3. FIXED CAPACITORS

1. Paper Capacitors. This type of capacitor is made by two long aluminum foils duely separated by wax paper strips and rolled together to take a cylindrical shape. A connecting wire is joined to the each aluminum foil and the capacitor is covered with a suitable resin binder, see Fig. 11.1.

Fig.11.1 Paper capacitor

Characteristics :

• Value — 0.0001 µF to 2.0 µF

• Working voltage — Upto 2000 volts D.C.

• Type — Only fixed capacitors are made.

• Size — Small.

Demerit : Unsuitable for H.F. circuits.

2. Mica Capacitors. This type of capacitor is made by two brass or alloy metal plates separated by a thin sheet of mica. For making a capacitor of high capacitance value, a number of two plate systems are joined together. The Mica whole combination is covered with a suitable resin binder to take a cuboid shape, see Fig. 11.2.

Fig. 11.2. Mica capacitor

Characteristics :

• Value --2.5 pF to 0.05µF.

• Working voltage — 500 to 2500 volts.

• Types — Fixed and adjustable capacitors are made.

Merits :

(i) Suitable for H.F. circuits.

(ii) Unaffected from dampness.

(iii) Capacitance value remains almost stable.

3. Polyester or Styroflex Capacitors.

This type of capacitor is made in the same way as a paper made. But, instead of paper dielectric a very fine polythene sheet is used as dielectric.

Characteristics:

• Value — 2.5 pF to 0.05 µF

• Working voltage — Upto 400 volts D.C.

• Type — Only fixed capacitors are made.

• Size — Smaller than paper capacitors.

Merits : Suitable for H.F. circuits and miniature equipments.

4. Ceramic Capacitors. This type of capacitor is made in tubular, disc or pin-up shape. It employs a ceramic dielectric which is a compound of titanium, barium, magnesium and strontium. This compound has a dielectric constant of upto 6000. Aluminium, tin or silver is used for making conductor plates in these capacitors, See Fig. 11.3.

Fig. 11.3. Ceramic capacitors

Characteristics :

• Value 2.5 pF to 0.22 µF

• Working voltage — 50 to 1500 volts D.C.

• Type — Only fixed capacitors are made.

• Size — They are made in a very small size.

Merits : (i) Suitable for H.F. circuits. Rubber seal

(ii) It can be designed to have a positive, zero or negative temperature coefficient.

(iii) It has a long working life.

5. Electrolytic Capacitors. This type of capacitor consists of an electrolyte between two sets of plates called positive and negative, that container is why it is known as electrolytic capacitor. Like a secondary cell when an electrolytic capacitor is connected across D.C., a chemical action takes place and an insulating layer of aluminium oxide is formed on the plates positive plates. The layer so formed acts as a dielectric. There are two types of electrolytic capacitors which are as follows :

Fig. 11.4 Electrolytic capacitor

(i) Wet-type. This type of capacitor consists of a cylindrical aluminium pot which is filled with a solution of boric acid and sodium borate (Solutions of ammonium borate, sodium phosphate, aluminium borate etc. may also be used). A group of plates is placed in the solution Which may be of many diversified shapes. When the plate groups and the pot is connected to D.C. positive and negative respectively, the electrolysis action is started. As a result of the said action, a very thin layer of aluminium oxide is formed on the positive plates which acts as a dielectric, sec Fig, 11.4,

The above explained action is called 'forming' and it is performed in factories during manufacturing.

(ii) Dry-type. In a dry type electrolytic capacitor the solution is replaced by a paste, its construction process is similar to that used for paper capacitors, Long and thin aluminium foils separated by paper sheets embedded in the electrolytic paste are rolled to take a cylindrical shape and the roll is fitted in an aluminium can, see Fig. 11.5.

Fig. 11.5 Dry electrolytic capacitor

There is no possibility of a leakage in a dry electrolytic capacitor, hence these capacitors are used commonly in electronic circuits.

Characteristics :

• Value -1 to 2000 µF.

• Working voltage-- Upto 450 volts D.C.

• Type -- Only fixed capacitors are made.

• Size - Small size in consideration to their high capacity (It is due to very thin layer of dielectric)

Merit : Suitable for filter circuits.

6. Oil Dielectric Capacitors. This type of capacitor consists of Rubber brass or iron conductor plates separated by oil dielectric. A mineral oil seat or hydrogenated caster oil is suitable for the purpose. These capacitors are made in cuboid shape, see Fig. 11.6.

Fig. 11.6. Oil dielectric capacitor

Characteristics :

• Value — 0.001 µF to 0.1 µF

• Working voltage -- Upto 25,000 volts D.C.

• Type -- Only fixed capacitors are made.

Merits : (i) These capacitors have a long life.

(ii) Suitable for transmitters.

7. Air Dielectric Capacitors. In this type of capacitors, air is used as dielectric between conductor plates. Usually, these capacitors arc made in variable type.

Characteristics :

• Value — 5 pF to 600 pF.

• Working voltage — Upto 500 volts D.C.

• Type -- usually variable type capacitors are made.

• Size — These capacitors arc made in small and large both sizes.

Merit : Suitable for tuning circuits.

Note. The dielectric constant of air is not stable and it varies with a change in humidity, hence vacuum dielectric capacitors are preferred in high frequency circuits.

11.4. ADJUSTABLE CAPACITORS

1. Trimmer. Its capacitance value is kept between 3 to 30 pF or 4 to 70 pF. Its types are as follows :

(i) Parallel Plate Type. It consists of two alloy metal plates placed parallel to each other. One plate is fitted on a bakelite or a porcelain base and the other one is a tension plate which is fitted in such a way that the distance between the two plates can be adjusted by a screw. The variation in distance between the two plates varies the capacitance value of the trimmer. A mica sheet works as dielectric between the two plates, see Fig. 11.7(a).

(ii) Cup Type. It consists of two aluminium cups having different diameters. One cup is kept stable while the other one inverted over the first one can be adjusted to a desired position with the help of a screw. The variation of distance between the two cups varies the capacitance value of the trimmer. Air works as dielectric between the two cups. see Fig. 11.7(b).

Fig. 11.7 Various types of trimmers

(iii) Wire Type. It consists of a thick enamelled copper wire erected on the chassis by soldering its one end and a thin enamelled copper wire wound over the thick one. The stray capacitance which exists between any two turns of the thin wire is utilised in it. The value of its capacitance can be varied by changing the distance between the turns of the thin wire with the help of an insulated strip, see Fig. 11.7(c).

(iv) Disc Type. It consists of two semi-circular discs of brass. One disc is kept fixed and the other one can be rotated with the help of a screw over the first one. A mica sheet or a polythene sheet may be used as dielectric between the two discs. The value of its capacitance can be varied by changing the effective area between the two discs, see Fig. 11.7(d).

2. Padder. It is also an adjustable capacitor whose capacitance value can be adjusted between 400 to 600 pF with the help of a screw. It consists of two or more plate couples. The value of its capacitance can be varied by changing the distance between the plates. Mica sheets are used as dielectric between the plates, see Fig. 11.8.

Fig. 11.8. Padder

Trimmers and padders are used for frequency setting purpose in radio receivers.

11.5. VARIABLE CAPACITORS

A radio receiver consists of more than one tuning circuits which require more than one variable capacitors controlled by a single shaft. These capacitors are called 'gang capacitors'. A gang capacitor consists of two sets of plates — one is known as stator and the other as rotor. The rotor plates group can be rotated through an angle of 180°. The value of its capacitance can be varied by changing the effective area between the two Plate groups. Its capacitance value is kept between 30 pF to 600 pF. For a valve type radio receiver 135 pF to 500 pF and for a transistorized radio receiver 90 pF to 210 pF capacitors are used. Air works as dielectric between the two plate groups, see Fig. 11.9.

Fig. 11.9. Variable capacitor

11.6. CAPACITOR LOSSES

If a charged capacitor is kept aside then it is found that after some time its charge gets reduced or lost. The reason behind this said effect is the capacitor losses which are as follows:

1. Resistance Loss. There always exists some resistance in the capacitor plates, connecting lead, "nod wires and it causes loss of electrical energy. Therefore, the loss of electrical energy caused due to resistance, of the capacitor is known as resistance loss ( I_C².R).

2. Leakage Loss. A minute flow of electrons from negative to positive plate of the capacitor always exists in a charged capacitor through the dielectric. The capacitor gets heated up due to this leakage. In this way, the loss of electrical energy caused due to flow of current through the dielectric of the capacitor known as leakage loss.

Since, no ideal insulator is available hence the leakage loss cannot be eliminated completely. Of course, the use of a dielectric having high dielectric constant can reduce the leakage loss, e.g., ceramic.

3. Dielectric Loss. If a charged capacitor is short-circuited with a piece of wire, it gets discharged. But, on testing the capacitor after some time a little amount of charge is found in the capacitor. The reason behind the said effect is dielectric absorption. In this way, it becomes evident that the electron orbits disturbed while charging have not restored their original arrangement even after discharging the capacitor.

The aforesaid effect increases even further if A.C. is applied across the capacitor and it goes on increasing with a rise in the frequency. In this way, the absorption of electrical energy by the dielectric on high frequencies is termed as dielectric loss.

The electron orbits get continually disturbed if a.c. voltage is applied across the capacitor and the loss of electrical energy occurred is termed as dielectric hysteresis loss.

11.7. FACTORS TO BE OBSERVED WHILE CHOOSING A CAPACITOR

The following factors should be observed while choosing a capacitor:

(i) Working Voltage. The working voltage of a capacitor should be 10 to 20 percent higher than the circuit voltage.

(ii) Capacitance. The capacitance of the capacitor selected should be exactly or nearly equal to the required value.

(iii) Type of Dielectric. The selection of the dielectric used in the capacitor depends on the circuit frequency:

(a) For filter circuit (50 Hz) — Paper and electrolytic capacitors.

(b) A.F. circuits (upto 20 kHz) — Paper, mica and ceramic capacitors.

(iv) Circuit's Requirement. A fixed, variable or adjustable capacitor is selected depending on the circuit requirements.

(v) Price. Normally, the order of price of a capacitor lies in the following descending order —Electrolytic, ceramic, mica, paper etc. Hence, the selection of a capacitor depends on its price also.

11.8. CAPACITANCE AND TEMPERATURE RELATION

In general the capacitance of a capacitor increases with a rise in temperature due to increase in the area of conductor plates. Ceramic capacitors can be divided into following three types on the basis of temperature coefficient.

(i) Positive Temperature Coefficient Type. The capacitance of this type of capacitors increases with a rise in the temperature.

(ii) Negative Temperature Coefficient Type. The capacitance of this type of capacitors decreases with a rise in the temperature.

(iii) Zero Temperature Coefficient Type. The capacitance of this type of capacitors remains unaffected with increase or decrease in the temperature.

11.9. CAPACITOR'S COLOUR CODE

Like resistors the capacitance value of moulded paper, mica and ceramic capacitors is expressed by means of colour dots or bands while the capacitance of other First Second types of capacitors is printed on them. The method of digit digit expression of the value of a carbon resistor is recognised 1 2 3 internationally and there exists only one method. But, there are following four methods for the expression of the capacitance value of a capacitor :

1. Three Dots Method of E.I.A. The Electronics Industrial Association method is used for the expression of the capacitance value of moulded mica capacitors. Their working voltage is 500 volts and their tolerance is ± 20% and the capacitance value is expressed in picofarads, see Fig. 11.10.

Fig. 11.10. Three dots method of E.I.A.

Table 11.1. E.I.A. 3-dots Method

Colour	First Dot First Digit	Second Dot Second Digit	Third Dot Multiplier
Black Brown Red Orange Yellow Green Blue Violet Grey White	0 1 2 3 4 5 6 7 8 9	0 1 2 3 4 5 6 7 8 9	1 10 100 1,000 10,000 - - - - -

2. Six Dots Method of E.I.A. This method is also used for the expression of the capacitance value of moulded mica capacitors. The working voltage and the tolerance of the capacitor may also be expressed by this method and the capacitance value is expressed in pico farads, see Fig. 11.11.

Fig. 11.11. Six dots method of E.I.A.

Table. 11.2. E.I.A. 6-dots Method

Colour	First Dot = First Digit	Second dot = Second Digit	Third Dot = Third Digit	Fourth Dot = Multiplier	Fifth Dot = Tolerance	Sixth Dot = Working Voltage
Black Brown Red Orange Yellow Green Blue Violet Grey White Golden Silver No Colour	0 1 2 3 4 5 6 7 8 9 - - -	0 1 2 3 4 5 6 7 8 9 - - -	0 1 2 3 4 5 6 7 8 9 - - -	1 10 100 1,000 10,000 - - - - - 0.1 0.01 -	- 1% 2% 3% 4% - - - - - 5% 10% 20%	- 100 200 300 400 500 600 700 800 900 1000 2000 500

3. Six Dots Method of J.A.N. or A.W.S. The Joint Army Navy or American War Standard method is used for the expression of the capacitance value of moulded paper and mica capacitors, see Fig. 11.12. In this method also, the capacitance is expressed in pico farads.

Fig. 11.12 Six dots method of J.A.N.

Table 11.3. J.A.N. 6-dots Method

Colour	First Dot = Type of capacitor	Second Dot = First Digit	Third Dot = Second Digit	Fourth Dot = Multiplier	Fifth Dot = Tolerance	T.C. PPm/⁰C
Black Brown Red Orange Yellow Green Blue Violet Grey White Golden Silver No Colour	Mica - - - - - - - - - - Paper -	0 1 2 3 4 5 6 7 8 9 - - -	0 1 2 3 4 5 6 7 8 9 - - -	1 10 100 1,000 10,000 - - - - - 0.1 0.01 -	- 1% 2% 3% 4% - - - - - 5% 10% 20%	± 1000 ± 500 ± 200 ± 100 - 20 to + 100 0 to 70 - - - - - - -

Note. (i) Here T.C. PPm/⁰ C means temperature coefficient, part per million (i.e., = 10,00,000) per degree centigrade.

(ii) In both the above explained methods the upper dots are counted from left to right as 1, 2, 3 and the lower dots from right to left as 4, 5, 6.

4. Colour Code of Ceramic Capacitors : There are, following two methods for the purpose :

(i) Dot Method. In this method, 5 dots of different colours are marked on the capacitor. Out of 5 dots, the first one is First Multiplier larger than others and then there are second, third, fourth and digit fifth dots. In this method also, the capacitance is expressed in Fig. 11.13. Five dots method pico farads, see Fig. 11.13.

Fig. 11.13. Five dots method

Table 11.4. Colour Code of Ceramic Capacitors

Colour	First Dot Temp. Coeff. PPm/⁰C	Second Dot = First Digit	Third Dot = Second Digit	Fourth Dot = Multiplier	Fifth Dot = Tolerance
Black Brown Red Orange Yellow Green Blue Violet Grey White	0 - 30 -80 -150 -220 -330 -470 -750 30 500	0 1 2 3 4 5 6 7 8 9	0 1 2 3 4 5 6 7 8 9	1 10 100 1,000 10,000 - - - 0.1 0.01	± 20% ± 1% ± 2% - - ± 5% - - - ± 10%

(ii) Band Method, In this method, 5 bands of different Temperature Second To coefficient digit colours are marked on the capacitor in place of dots. The first band indicates temperature coefficient, second band indicates first digit of the capacitance value, third band indicates second digit of the capacitance value, fourth band indicates the no. of zeros to be put after the two digits and the fifth band indicates tolerance value of the capacitor.

The colour code for this method is shown in Table 11.4. In this method also the capacitance value is expressed in pico Fig. 11.14.

Fig. 11.14. Band method for ceremic capacitors

The aforesaid method of expressing a capacitor's value is most common method. Now a days, most of the capacitor's manufacturer use this method for expressing a capacitor's value.

Example

Colour of First Band	Colour of Second Band	Colour of Third Band	Colour of Fourth Band	Colour of Fifth Band	Capacitance Value
Brown Brown Grey	Brown Yellow Brown	Black Violet Black	Orange Orange Red	Brown Black White	10kPF ± 1% -30 PPm/⁰C 47 kPF ± 20% -30 PPm/⁰C 1kPF± 10% 30 PPm/⁰C