Direct Current And Its Applications

Direct current is the unidirectional flow of electrons from an area of negative charge to an area of positive charge through a conductor. A direct current always flows in one direction as it flows from one point to another. A DC voltage is always positive or it is always negative. DC voltage has a fixed polarity and the magnitude of the current is constant. It may increase or decrease in intensity but constant in amplitude. The flow of electric current originates from the positive terminal towards the negative terminal. The direction of the electron flow is from the negative terminal of the battery to the positive terminal. The graphical representation of a DC voltage is a straight line across the time-line with a constant voltage level.

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Definition Of DC

A DC refers to a voltage with a single polarity of voltage or current. It also refers to constant, zero frequency, or slowly moving mean value of a voltage or current. The voltage across a DC voltage source is constant as is the current through a DC current source. Direct current in an electric circuit has constant voltage and constant current. A stationary voltage or current has a DC component and a zero time varying component. 

Sources of DC

The sources of direct current are batteries, thermocouples, solar cells, capacitive storage etc. Fuel cells also produce DC by mixing hydrogen and oxygen with a catalyst, producing DC electricity and water as byproduct. Direct current may also be obtained by rectification and filtration of an alternating current or AC from a generator, which is a pulsating DC. The direct current can flow through a conductor such as electrical wires and cables. DC can also flow through semiconductors, insulators, or even through vacuum.

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Most electronic circuits require a direct current that is steady and constant in amplitude for their smooth functioning. Some circuits such as lamps, heaters, and some type of motors may work with a pulsating DC.

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DC Applications
Direct current is used for powering most electronic devices, charge batteries, run motors etc.  It is also commercially used in the production of aluminum and other metals, separation of metals, electroplating, and other electrochemical processes. Telephones are connected to wires that carry DC for their functioning and telephone exchanges have communication equipment that works on direct current. Direct current is used in driving electrical vehicles such as electric cars, bikes, carts, trams, rail etc.  High voltage direct current is used to transmit electricity from the generator site to the place where it is used and to interconnect intermittent power grids.

Working Of a Semiconductor Diode

A semiconductor diode is an electronic component made up joining a block of P-type of semiconductor and an N-type of semiconductor in a single crystalline form. Most of these diodes are made of silicon, although germanium diodes are also available. The semiconductor diodes are also called as a PN junction diode. These diodes are used for a variety of applications such as rectification, voltage regulation, detection, light emission, solar cells, photo detection, etc.

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When a potential is given to the terminals of a diode, the current can flow only in one direction. The current flows through the diode only when the positive of the battery is connected to the P side of the diode and the negative of the battery is connected to the N side of the diode. This is called as the forward bias condition. The diode also has a junction potential of 0.6 volts where  the conduction or flow of current takes place through the  diode. Only when the voltage across the terminals crosses 0.6 volts in  the forward bias mode the conduction of the diode takes place. In the forward bias condition, the positive holes are repelled by the positive anode and the free electrons are repelled by the cathode the negative terminal of the battery. The electrons and holes move across the junction and they combine together. More electrons are siphoned off by the  positive anode and move through the terminals and through the battery. The holes thus created moves across to the N side through the PN junction. Thus a steady current flows through the diode.

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In the reverse bias condition, the positive of the battery is connected to the N side of the diode and the negative  of the battery is connected to the P side of the diode.  Here the diode is at an off or non conducting position. The electrons are attracted to the positive pole of the battery. The holes in the P side of the diode are attracted to the negative of the battery.  The  electrons and holes leave the junction creating an area completely devoid of electrons or holes. This area which is devoid of either electrons or holes is called as a depletion zone. There is no conduction of the diode in the reverse biased diode.

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When an AC voltage is applied to the terminals of the diode, the diode conducts during the alternate halve cycles of the AC waveform and cuts off the voltage during the reverse cycles. The resultant waveform is the broken DC pulses during alternate half cycles of the  AC waveform. A full waveform can be obtained using a transformer and diode combination. The DC pulses can be smoothed out by various waveform filters to obtain a smooth DC voltage.

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When the voltage is increased in a reverse biased diode, the depletion zone becomes wider.  At a critical point, the depletion field becomes large enough so that the carrier, either the electron or hole gain sufficient momentum to break electrons from the atoms with which it collides. These stripped electrons gains sufficient speed until it collides with other atoms striking still more electrons. The operation is accelerated by heat and heated generated carriers in the depletion zone. This process is called as the avalanche. At the avalanche area, the diode begins to conduct in the reverse mode. Usually the breakdown voltage is much higher than the the voltage used in the circuit. There are diodes which operate in the avalanche region and these are called as the zener diodes, which are voltage regulator diodes.

Conduction In a Semiconductor

Semiconductors are substances that are used to make electronic components such as diodes, transistors, solar cells, integrated circuits, microprocessors etc. These semiconductor electronic components are used in the construction of all electronic gadgets and devices ranging from radios, music systems, television, solar panels, telephones, mobile phones, computers, laptops, e notebooks etc. Therefore semiconductors are fundamental in the field of electronics and construction of electronic devices.

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The semiconductors have replaced the valves in most electronic circuitry. Semiconductor devices are light weight and have small physical dimensions, high efficiency and low power losses, ability to overcome vibrations, long life and durability of the components, and requires low voltage and power supplies.

 

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A semiconductor is a substance with an electrical conductivity that lies between a conductor and an insulator.  The semiconducting materials are mostly crystalline in nature and include carbon, silicon, germanium, mixtures of arsenic, selenium, tellurium, etc. The atoms of germanium and silicon  have 4 electrons in their outer orbits.  As there are many atoms in their crystalline structure, electron sharing occurs. This sharing behavior causes their outer electrons become tightly bonded to their respective places in the crystal.  This type of bonding is called  covalent bond.  Although these are covalently bonded in their crystalline structure, these covalent bonds are weak in semiconductors.  The bonds are broken at higher temperatures above absolute zero and more  bonds are broken when the temperature goes higher. When the covalent bonds are broken,  an electron becomes free that can be moved by an electric field.  The place that is left by the free electron becomes a hole. A free place is available in the atom where the electron has left which is the hole. An electron is electrically negative and a hole is electrically positive in charge.

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In the absence of any electrical charge, free electrons are distributed evenly throughout the semiconductor. The electrical charge in semiconductor exists as electron-hole pairs. The electrons are being captured by the holes resulting in the destruction of the  electron-hole pairs. When a hole captures an electron, the hole no longer exists and the electron is no longer a free electron.  Electron and hole pairs are being constantly created and destroyed in about equal numbers.

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When an electrical charge is applied across a block of semiconductor, electrons are attracted to the positive end of the semiconductor block. Electron hole pairs are formed rapidly throughout the block due to the electric charge. There is heavy concentration of electrons at the positive end and only fewer electrons are left at the negative end of the block. More electrons move towards the positive end by the steady application of electrical charge. The holes at the negative end of the block cannot capture electrons because there are almost none for them to capture. At the positive end of the semiconductor block there is an excess of electrons and at the negative end there is an excess of holes. More electron hole pairs are formed at the positive end as long as the temperature breaks the electrons loose from their bonds. Thus the conduction on a pure semiconductor is dependent on the temperature and the charge given.

Capacitance And Its Measurement

A capacitor is a device, which is used to store an electrical charge or electrical energy. Capacitor consists of 2 electrical conductors separated by a dielectric medium. Capacitance is the measure of the capacitor to store electrical charge. It is the property of an electric circuit which tends to oppose a change in voltage when a potential difference is applied. Capacitance exists when two electrical conductors are separated by a nonconducting medium or a dielectric material.

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Charging Of a Capacitor

When a battery is connected across 2 metal plates, the positive terminal attracts the negative  electrons in plate 1 of the plates. The excess of electrons on the negative of the battery rush to the plate 2 of the capacitor and gets accumulated. More and more electrons leave the plate 1 to the positive terminal of the battery and and more electrons crowd to the plate 2 from the negative terminal. A positive charge accumulates in the plate 1 and a negative charge gets accumulated in the plate 2.  The charges on the two plates increase in opposite directions until the difference between them is exactly equal to the difference in potential between the two terminals of the battery. At this point, the electrons stop flowing as there is a balance in forces of the charged plates. When a dielectric material such as glass or plastic is placed in between the plates, the capacitance increases as seen by the deflection of an ammeter connected in the circuit.

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The unit of capacitance is farad. A capacitor has a capacitance of 1 farad when 1 volt difference in voltage results in the storage of 1 coulomb of charge. Usually capacitors are denoted in micro farads, nanofarads, picofarads, etc. An ideal capacitor is characterized by a constant capacitance C, defined as a ratio of charge + or - Q on each conductor to the voltage V between them.

C = Q/V

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The charge build up in the capacitor affects the capacitor mechanically that varies its capacitance, where the capacitance is measured in incremental changes.

C = dq/dv 

Calculating current flow through the capacitor

Formula of current, I = dQ/dt

The formula for current with respect to time is dQ/dt = d(CV)/dt

it is expressed as I = C (dV/dt)

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Capacitors are used in electronic circuits for blocking DC voltage, allowing passage of AC, resonant circuits in radio, filter circuits, power supply smoothing, timing circuits, switching circuits, stabilizing power and voltage flow etc.

A capacitor has undesirable characteristics and limitations such as leakage current when the dielectric between the conductor plates of the capacitor passes a small leakage current through it. Capacitor has an electric field strength limit, resulting in a breakdown voltage for each. Also the leads and plates of the capacitor introduces some unnecessary inductance and resistance to the circuit.

Factors That Determine the Capacitance

The capacitance of a capacitor differs based on

1. The area of the plate surface that are directly opposite to each other. Increasing the plate area will increase the capacitance.

2. Distances between the plates. The smaller the distance between the plates, the bigger is the capacitance.

3. Type of dielectric. The type of dielectric that the capacitor is using does have an influence on the capacitance. Some dielectrics offers more capacitance than that using other types of dielectric.

The greater the surface area of the plates, the higher is the capacitance. The closer the distance between the plates, the higher is the capacitance. The greater is the dielectric constant of the dielectric, the higher is the capacitance.

Capacitance in Series and Parallel

Capacitors can be connected in series and parallel combination for various reasons. The need for connecting capacitance in combination are to increase or decrease the capacitance, reduce fluctuations due to heat, improved filtering, achieve higher voltage rating, obtaining high Q etc. Capacitors are combined in series to achieve higher voltage as in smoothing a higher voltage AC, the voltage rating of each capacitor adds up. Series combination is also used to connect polarized capacitors  connected back to back to form a bipolar capacitor.

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Capacitors Connected in Series

When capacitors are connected in series, the total capacitance can be reduced. The separation distance of the capacitor increases and not the plate area. Each capacitor stores the charge that is equal to that of every other capacitor in series. The voltage difference at the terminals of each capacitor is distributed among the capacitors according to the inverse of their capacitance. The effective capacitance is smaller than the smallest of any one of the capacitor connected to it.

The capacitors of capacitance C1, C2, C3,  - - - Cn are connected in series and the total capacitance is measured by the formula.

1/C = 1/C1 + 1/C2 + 1/C3 + - - - 1/Cn

For example, the three capacitors with capacitance 10 microfards, 5 microfarads, 5 microfarads are connected in series, it is written as

1/C = 1/10 + 1/5 + 1/5

1/C =  1+2+2/10

C = 2 microfarads

Capacitors Connected in Parallel

When capacitors are connected in parallel, the total capacitance is the sum of all the capacitors. All the capacitance adds up and the effective capacitance increases. Each capacitor have the same applied voltage across its terminals. There is equal distribution of charge through all capacitors depending on the size. Each capacitor contributes to the total surface area of the capacitor.

The capacitors of capacitance C1, C2, - - - C3 are connected in parallel and the total capacitance is measured by the formula.

C = C1 + C2 + C3 - - - + Cn

For example, the three capacitors with capacitance 10 microfards, 5 microfarads, 15 microfarads are connected in parallel, it is written as

C = 10 + 5 + 15

C =  30 microfarads

Capacitor Color Codes

The capacitors have specific values which are marked on to the body of the capacitor, that helps to understand the capacitance and other properties of it. The important values for the capacitor are the capacitance, tolerance, voltage rating, temperature coefficient etc. These are usually marked on the body of the capacitor in the alphanumeric form. There are many decimal values that are used as capacitor values where the reading becomes difficult without the help of a capacitor value chart. To reduce the confusion involved with the letters, numbers, and decimals used in representing capacitor values and also to prevent misreading the values of the capacitor, an international color coding scheme was introduced.  The capacitor color code represents a simple and efficient way of reading capacitor values and tolerances.

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There are different build of non polarized capacitors such as the axial lead, radial lead, and ceramic disc capacitors. The disc type of capacitor color code is read from the top of the capacitor to the bottom of the capacitor. The extreme end of the body of the capacitor is considered as top of the capacitor and towards the axial leads is considered as the bottom of the capacitor. The color marking at the top of the capacitor is considered as the first color band. In radial and axial lead type of capacitors the ring near any of the lead is considered as the first color band.

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The first band A is usually the value for the temperature coefficient of the capacitor, which is denoted as A. The second band B is the first digit of the capacitor in picofarads, denoted as B. The third band C is the second digit of the capacitor in picofarads, denoted as C. The fourth band D is the multiplier of the capacitance in multiples of 10, denoted as D.  The fifth band E is the tolerance of the capacitor in percentage, denoted as E.

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More recently a different type of capacitor coding has come to be in use. In small types of capacitors such as film or disc form, instead of the color coding, the capacitance is given as a letter or a number code. The code consists of 2 or 3 numbers and an optional tolerance letter code to identify the tolerance.

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When a 2 letter code is used, the value of the capacitor is denoted in picofarads such as 10 = 10pf, 22 = 22 pf, and 100 = 100 pf etc. The 3 letter code is used to denote the value of capacitor, the first two digits for the first and the second value of the capacitor and the third one is multiplier in picofarads which multiplies in multiples of 10. For example a capacitor with a value 251 = 25 * 10 = 250 pf and 102 = 10 * 100 = 1000 pF etc. There is an additional letter included in a 3 digit code to include the tolerance measurement of the capacitor. For example a capacitor with a value 103J printed on the body, which denotes the 1st and 2nd digits as the 1st and 2nd value of the capacitor and the 3rd one is denoted as the multiplier in picofarads and the letter J is the tolerance.

10 * 1000 = 10,000 pf and the letter J denotes a tolerance of +/- 5%

The capacitance is 10,000 pF which is equivalent to 10 nF or 0.010 mF with a tolerance of +/- 5%

Resistor Color Codes

An electronic color code is used in most electronic components including the resistors. Most of the axial lead resistors are marked with the color rings, which gives the value of the resistor. The rings are printed on the body of the resistor and there will be 3 to 6 rings in total. The resistor color code gives the exact value of the resistor and its tolerance. The nearest ring to the end is taken as the first ring. The resistor value rings for the 1st digit, 2nd digit, and 3rd digit will be adjacent to each other. The resistor is held with the first ring to the left and then the color code is read from the left to the right. Reading the resistor is easier with the 5% and 10% tolerance resistor as it is at the opposite end of the first resistor ring, which is marked as silver or gold.

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For a 4-band resistor, the first ring indicates the first figure of the value in ohms. The second ring indicates the second figure of the value in ohms. The third ring indicates the multiplier which is the number of zeroes that will come on the right side of the value. The fourth ring indicates the tolerance of the resistance, which is expressed in percentage. If there is only 3 rings and no fourth ring, it indicates that the tolerance is 20% for that resistor.

For a 6-band resistor, the first ring indicates the first figure of  the value in ohms. The second ring indicates the second figure of the value in ohms. The third ring indicates third significant digit of the value in ohms. The fourth ring will be taken as the multiplier. The fifth ring indicates the tolerance of the resistance. The sixth ring indicates the temperature coefficient of the resistor.

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Resistance Value of The Resistor

The first 2 bands indicate the value of the resistor in a 4 band resistor. The first 3 bands indicate the value of the resistor in a 6 band resistor.

Black – 0

Brown – 1

Red – 2

Orange – 3

Yellow – 4

Green – 5

Blue – 6

Violet – 7

Grey – 8

White – 9

Multiplier Value of The Resistor

The third band indicates the multiplier value in a 4 band resistor. The fourth band indicate the multiplier value in a 6 band resistor.

Black – x1

Brown – x10

Red – x100

Orange – x1K

Yellow – x10K

Green – x100K

Blue – x1M

Violet – x10M

Grey – x100M

White – x1G

Gold – .1

Silver – .01

Tolerance of The Resistor

The fourth band indicates the tolerance of the resistor in a 4 band resistor. The fifth band indicate the tolerance of the resistor in a 6 band resistor.

Brown – 1%

Red – 2%

Orange – 3%

Yellow – 4%

Green – .5%

Blue – .25%

Violet – .1%

Gray – .05%

Gold – 5%

Silver – 10%

No color - 20%

Temperature Coefficient of The Resistor

Resistor values change with the change in temperature. The sixth band in a 6-band resistor is a temperature coefficient or tempco of the resistor, which represents the amount of resistance value that will change with temperature. It is measured in ppm/degree C. The sixth band represents the following.

Brown – 100 ppm/degreeC

Red – 50 ppm/degreeC

Orange – 15 ppm/degreeC

Yellow – 25 ppm/degreeC

Blue – 10 ppm/degreeC

Violet – 5 ppm/degreeC