Capacitors and Inductors

Resistor: a passive element which dissipates energy only Two important passive linear circuit elements: 1) Capacitor 2) Inductor Introduction Capacitor and inductor can store energy only and they can neither generate nor dissipate energy.

Capacitors A capacitor consists of two conducting plates separated by an insulator (or dielectric). εa C d r 0 0 8.854 10 12 (F/m)

C εa d Three factors affecting the value of capacitance: 1. Area: the larger the area, the greater the capacitance. 2. Spacing between the plates: the smaller the spacing, the greater the capacitance. 3. Material permittivity: the higher the permittivity, the greater the capacitance.

Typical Specification of a Capacitor A capacitor is normally described using the following set of parameters. a. Purchase Tolerance : The maximum deviation of the C value from the specified value. b. Capacitive Reactance : c. Dielectric Absorption: This gives the time lag in charging and discharging of a capacitor due to an imperfect dielectric. d. Dielectric Strength: It is the ability of the dielectric to withstand the applied d.c. voltage without breaking down.

e. Insulation Resistance: The d.c. resistance measured across the capacitor terminal. f. Leakage DC: The direct current, which flows through the dielectric. g. Temperature Coefficient: The change in the capacitance per degree change in temperature per unit original capacitance. The Farad We now know that the ability of a capacitor to store a charge gives it its capacitance value C, which has the unit of the Farad, F. But the farad is an extremely large unit on its own making it impractical to use, so submultiple s or fractions of the standard Farad unit are used instead.

Capacitors which have a value of one Farad or more tend to have a solid dielectric and as One Farad is such a large unit to use, prefixes are used instead in electronic formulas with capacitor values given in micro- Farads (μf), nano-farads (nf) and the pico-farads (pf). For example:

Que: Convert the following capacitance values from a) 22nF to uf, b) 0.2uF to nf, c) 550pF to uf. Ans: a) 22nF = 0.022uF b) 0.2uF = 200nF c) 550pF = 0.00055uF

1. Fixed Capacitors i. Electrostatic Capacitors: The different types of electrostatic capacitors arise due to the different kinds of electric material used in between the two plates of capacitor. a) Paper Capacitor: This uses paper as a dielectric. Alternate strips of metal and paper are wound to make a tight roll. After winding, the foil are crushed over the paper and leads are then soldered to the crushed foil ends. The whole assembly is then impregnated with a wax or plastic resin.

Radial Lead Type Axial Lead Type Film Capacitor

Ceramic Capacitor Electrolytic Capacitor

b) Metal Paper Capacitor: This uses metal (e.g. Aluminum or Zinc) deposition on the surface of a paper. This construction gives larger capacitance gives larger capacitance value and higher voltage rating. c) Film Capacitor: Here, plastic films are used in between (aluminum) foils alternately. The whole construction is encapsulated in epoxy resin. d) Mica Capacitor: Mica sheets and metal foils are arranged with the foils interleaving the sheets. This construction is held together by clamps and the it is sealed in a mould of (wax/ bakelite/ araldite/ glass/ ceramic).

e. Glass Capacitor: These are very reliable and precise capacitors. They are very stable, unaffected by temperature, aging, moisture, vibrations and shocks. Glass in the form of a ribbon is used as a dielectric in between metal plates. f. Ceramic Capacitor: These give high dielectric constant, using sintered inorganic component (e.g. barium titanate, calcium titanate, strontium titanate, lead nibonate) as a dielectric. These capacitors are available in various forms such as disc, tabular, monolithic, ceramic and chip.

ii. Electrolytic Capacitors: These capacitors have a definite polarity. Two plates are called anode and cathode respectively. These capacitors have different constructions as follows: a) Foil Style b) Sintered anode style a) Foil Style: Here the dielectric material is an oxide layer deposited on the anode plate of the capacitor. In this construction, the two plates of the capacitor are the anode and the cathode. The cathode is normally copper or nickel.

The anode is normally aluminium or tantalum. On this anode, an oxide layer is deposited using an electrolytic cell. This oxide layer works as a dielectric. The oxide film is highly polarized and hence forward direction of this film is characterized by high resistence. This, when connected in reverse direction, increases the reverse current which may cause excessive heating of capacitor and it may get damaged. b) Sintered Anode Style: In this type, anode is made from powdered tantalum. This is pressed in the form of a pellet to fuse tantalum particles together. The pellet is placed is placed in anodic bath to form oxide coating. The cathode is an electrolyte. The whole assembly is sealed to avoid leaking.

2. Variable Capacitors Variable capacitors are very commonly used in our radios to tune a particular station. Tuning is normally done using a resonant RC or LC circuit. The trimming type variable capacitors are used to make film adjustments.

i. Tuning Capacitors: These are normally used when we need frequent movement of the capacity. Here, we have two sets of parallel intermeshed metal plates- one fixed and the other connected to a shaft. As we move the variable capacitor knob, the shaft rotates and the area of crosssection changes, introducing the change in capacitance. ii. Trimmer Capacitors: Here, air is the dielectric. The plates are mounted on a rotor and a stator. The plates are in a cylindrical fashion to reduce the size. The capacitance is varied by changing the effective plate area, by intermediate stator and rotor cylinders. Mica, ceramic, plastic film or glass can be used as a dielectric, instead of air, thereby giving rise to various types of trimmer capacitors.

Capacitor Colour Codes Generally, the actual values of Capacitance, Voltage or Tolerance are marked onto the body of the capacitors in the form of alphanumeric characters. However, when the value of the capacitance is of a decimal value problems arise with the marking of the Decimal Point as it could easily not be noticed resulting in a misreading of the actual capacitance value. Instead letters such as p (pico) or n (nano) are used in place of the decimal point to identify its position and the weight of the number.

For example, a capacitor can be labelled as, n47 = 0.47nF, 4n7 = 4.7nF or 47n = 47nF and so on. Also, sometimes capacitors are marked with the capital letter K to signify a value of one thousand Pico-Farads, so for example, a capacitor with the markings of 100K would be 100 x 1000pF or 100nF.

To reduce the confusion regarding letters, numbers and decimal points, an International colour coding scheme was developed many years ago as a simple way of identifying capacitor values and tolerances. It consists of coloured bands (in spectral order) known commonly as the Capacitor Colour Code system and whose meanings are illustrated below:

We look at Capacitor Colour Codes and see the different ways that the capacitance and voltage values of the capacitor are marked onto its body.

Capacitor Colour Code Table Band Colour Digit A Digit B Multiplier D Tolerance (T) > 10pF Tolerance (T) < 10pF Black 0 0 x1 ± 20% ± 2.0pF Temperat ure Coefficien t (TC) Brown 1 1 x10 ± 1% ± 0.1pF -33 10-6 Red 2 2 x100 ± 2% ± 0.25pF -75 10-6 Orange 3 3 x1,000 ± 3% -150 10-6 Yellow 4 4 x10,000 ± 4% -220 10-6 Green 5 5 x100,000 ± 5% ± 0.5pF -330 10-6 Blue 6 6 x1,000,000-470 10-6 Violet 7 7-750 10-6 Gray 8 8 x0.01 +80%,-20% White 9 9 x0.1 ± 10% ± 1.0pF Gold x0.1 ± 5% Silver x0.01 ± 10%

Capacitor Voltage Colour Code Table Band Voltage Rating Colour Type J Type K Type L Type M Type N Black 4 100 10 10 Brown 6 200 100 1.6 Red 10 300 250 4 35 Orange 15 400 40 Yellow 20 500 400 6.3 6 Green 25 600 16 15 Blue 35 700 630 20 Violet 50 800 Gray 900 25 25 White 3 1000 2.5 3 Gold 2000 Silver

Capacitor Voltage Reference Type J Dipped Tantalum Capacitors. Type K Mica Capacitors. Type L Polyester/Polystyrene Capacitors. Type M Electrolytic 4 Band Capacitors. Type N Electrolytic 3 Band Capacitors.