1.• means that increase in the voltage drops the

1.• Zener Diode – This type of semiconductor diode is used excessively in electronic circuits. They are also known as reference diodes as they are capable of providing a stable voltage considered as the reference in circuits. They work the same way as an ordinary diode in the forward direction.• However, in reverse direction they work differently.  Voltage across the diode remains constant once the breakdown voltage is attained irrespective of the amount of voltage supplied to it. This particular behavior in the breakdown region can be obtained by either zener effect or by avalanche effect.• Signal Diode – These diodes are devised for processing information in the form of electrical signals in circuits. It needs to pass small current only which is upto 100mA only.These switching diodes have many applications as they are found in many of the electronic devices. 1N4148 is such a high speed switching diode.• Tunnel Diode – It is also known as Esaki diode. It is based on the electron tunneling effect discovered by Leo Esaki. These diodes exhibit negative resistance, which means that increase in the voltage drops the current through it.These diodes work properly in the microwave radio frequency region. They are used in oscilloscope trigger circuits. High speed and longevity are two important advantages of this type of diode. • These diodes are mainly found in simple light meters. One important quality of these diodes is that they give instant response which can be measured in nanoseconds. The characteristics of all photodiodes get affected by change in temperature.• Crystal Diode – It is composed of two different types of semiconductors which are placed just next to each other. These are also known as Cat’s whisker diodes. It is made of a thin shaped metal wire which is kept in pressing position against the semiconductor crystal.The metal wire works as the anode, whereas the semiconductor crystals act as the cathode. These diodes are not used much today and hence are considered obsolete.• Schottky Diode – It makes use of a metal semiconductor junction which provides a lot of advantages in certain conditions.This is due to the feature of very low forward voltage drop  possessed by this diode. It is also known for its switching speed; together, these qualities make it suitable to be used in RF applications. It also possesses much higher current density than in comparison to an ordinary PN junction.• LED (Light Emitting Diode) – It is one of the most widely used semiconductor diodes. It is made of a specialized PN junction made by using a heavily doped semiconductor material. The PN junction of this diode is covered by a transparent and hard plastic resin shell. This shell is for the protection of the LED from vibrations.They are preferred over normal bulbs for their energy efficiency and durability. They are made from semiconductor compounds like Gallium Arsenide and Silicon Carbide, due to which it is able to generate a distinct wavelength of color.• Shockley Diode – It comprises of two PN junctions, and therefore is also known as four layer diode. It is primarily used in switching applications. It acts as an open switch when is in the condition of forward biased.It has a very high forward resistance. It possesses an inherent trigger voltage. In case the applied voltage is lower than trigger voltage, the diode exhibits high resistance, however if the voltage applied is greater than the trigger voltage, diode is switched on and also has very low resistance.• Laser Diode – This diode is quiet the same as LED, but it is known for producing a narrow beam of a very high intensity. These diodes are usually used in optical fiber systems, laser printers, remote control devices, etc.• They generate a wide angle beam and have low power requirements. It is made by doping a vey thin layer on the surface of the crystal wafer. As the charge injection is used for providing power to these diodes, they are also known as injection lasers or injection laser diodes.• Varactor Diode – It is also known as a parametric diode or tuning diode. It provides a voltage-dependent variable capacitance. These diodes are used in radio frequency circuits.• They work under reverse bias conditions and are made just like a capacitor. The maximum capacitance level decides the maximum voltage that can be handled by the varactor diode.It is usually connected to a tuned circuit in parallel with already exiting capacitance or inductance..2. Half-wave Rectifier During the positive half cycle of the source, the ideal diode is forward biased and operates as a closed switch. The source voltage is directly connected across the load. During the negative half cycle, the diode is reverse biased and acts as an open switch. The source voltage is disconnected from the load. As no current flows through the load, the load voltage vo is zero. Both the load voltage and current are of one polarity and hence said to be rectified.  • Full wave center tap rectifier:  only two diodes are used, and are connected to the opposite ends of a centre-tapped secondary transformer as shown in the figure below. The centre-tap is usually considered as the ground point or the zero voltage reference point.  Peak CurrentThe instantaneous value of the voltage applied to the rectifier can be written asVs = Vsm SinwtAssuming that the diode has a forward resistance of RFWD ohms and a reverse resistance equal to infinity, the current flowing through the load resistance RLOAD is given asIm = Vsm/(RF + RLoad). Output CurrentSince the current is the same through the load resistance RL in the two halves of the ac cycle, magnitude od dc current Idc, which is equal to the average value of ac current, can be obtained by integrating the current i1 between 0 and pi or current i2 between pi and 2pi.  DC Output VoltageAverage or dc value of voltage across the load is given as Root Mean Square (RMS) Value of CurrentRMS or effective value of current flowing through the load resistance RL  is given as . Root Mean Square (RMS) Value of Output VoltageRMS value of voltage across the load is given as  Rectification EfficiencyPower delivered to load,  Ripple FactorForm factor of the rectified output voltage of a full wave rectifier is given as  RegulationThe dc output voltage is given as • The Diode Bridge RectifierA bridge rectifier is an arrangement of four or more diodes in a bridge circuit configuration which provides the same output polarity for either input polarity. It is used for converting an alternating current (AC) input into a direct current (DC) output. A bridge rectifier provides full-wave rectification from a two-wire AC input, therefore resulting in lower weight and cost when compared to a rectifier with a 3-wire input from a transformer with a center-tapped secondary winding. Peak CurrentInstantaneous value of the voltage applied to the rectifier is given asvs =  Vsmax Sin wt If the diode is assumed to have a forward resistance of RF ohms and a reverse resistance equal to infinity, then current flowing through the load resistance is given asi1 = Imax Sin wt and i2 = 0 for the first half cycleand i1 = 0 and i2 = Imax Sin wt for second half cycleThe total current flowing through the load resistance RL, being the sum of currents i1 and i2 is given asi = i1 + i2 = Imax Sin wt for the whole cycle.Where peak value of the current flowing through the load resistance RL is given asImax = Vsmax/(2RF + RL) Output CurrentSince the current is the same through the load resistance RL in the two halves of the ac cycle, magnitude od dc current Idc, which is equal to the average value of ac current, can be obtained by integrating the current i1 between 0 and pi or current i2 between pi and 2pi.  DC Output VoltageAverage or dc value of voltage across the load is given as  Root Mean Square (RMS) Value of CurrentRMS or effective value of current flowing through the load resistance RL  is given as  Root Mean Square (RMS) Value of Output VoltageRMS value of voltage across the load is given as  Rectification EfficiencyPower delivered to load,  Ripple FactorForm factor of the rectified output voltage of a full wave rectifier is given as  RegulationThe dc output voltage is given as ?3.Block Diagram of a Power Supply From the block diagram, the basic power supply is consti¬tuted by four elements viz a transformer, a rectifier, a filter, and a regulator put together. The output of the dc power supply is used to provide a constant dc voltage across the load. Let us briefly outline the function of each of the elements of the dc power supply.Transformer is used to step-up or step-down (usually to step-down) the-supply voltage as per need of the solid-state electronic devices and circuits to be supplied by the dc power supply. It can provide isolation from the supply line-an important safety consideration. It may also include internal shielding to prevent unwanted electrical noise signal on the power line from getting into the power supply and possibly disturbing the load.4. Diode Application• Power Conversion: One significant application of diodes is to convert AC power to DC power. A single diode or four diodes can be used to transform 110V household power to DC by forming a half-way (single diodes) or a full-wave (four diodes) rectifier. A diode does this by allowing only half of the AC waveform to travel through it. When this voltage pulse is used to charge a capacitor, the output voltage appears to be a steady DC voltage with a small voltage ripple. Using a full wave rectifier makes this process even more efficient by routing the AC pulses so both the positive and negative halves of the input sine wave are seen as only positive pulses, effectively doubling the frequency of the input pulses to the capacitor which helps keep it charged and deliver a more stable voltage.Diodes and capacitors can also be used to create a number of types of voltage multipliers to take a small AC voltage and multiply it to create very high voltage outputs. Both AC and DC outputs are possible using the right configuration of capacitors and diodes.• Demodulation of Signals: Diodes also function well as protection devices for sensitive electronic components. When used as voltage protection devices, the diodes are non-conducting under normal operating conditions but immediately short any high voltage spike to ground where it cannot harm an integrated circuit. Specialized diodes called transient voltage suppressors are designed specifically for over-voltage protection and can handle very large power spikes for short time periods, typical characteristics of a voltage spike or electric shock, which would normally damage components and shorten the life of an electronic product.• Current Steering: The basic application of diodes is to steer current and make sure it only flows in the proper direction. One area where the current steering capability of diodes is used to good effect is in switching from power from a power supply to running from a battery. When a device is plugged in and charging, for example, a cell phone or uninterruptible power supply, the device should be drawing power only from the external power supply and not the battery and while the device is plugged in the battery should be drawing power and recharging. As soon as the power source is removed, the battery should power the device so no interruption in noticed by the user