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NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 UNIT-I Syllabus ⮚ PN Junction Diode: Characteristics ⮚ Half wave rectifier, Full wave rectifier, filters, ri...

NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 UNIT-I Syllabus ⮚ PN Junction Diode: Characteristics ⮚ Half wave rectifier, Full wave rectifier, filters, ripple, regulation, TUF and efficiency ⮚ Zener diode and Zener diode regulators. ⮚ CRT construction and CRO applications PN Junction Diode: Characteristics P-type semiconductor: When the trivalent impurity is added to an intrinsic or pure semiconductor (silicon or germanium), then it is said to be an p-type semiconductor. Trivalent impurities such as Boron (B), Gallium (G), Indium (In), Aluminum (Al) etc. are called acceptor impurity. In p-type semiconductors holes are the majority carriers while electrons are the minority carriers. N-type semiconductor: When pentavalent impurity is added to an intrinsic or pure semiconductor (silicon or germanium), then it is said to be an n-type semiconductor. Pentavalent impurities such as phosphorus, arsenic, antimony etc are called donor impurity. In n-type semiconductors electrons are the majority carriers while holes are the minority carriers  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 What is Diode? A diode is a two-terminal electronic component that conducts current primarily in one direction it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other Formation of p-n junction: when an n-type semiconductor is joined with the p-type semiconductor, a p-n junction is formed. The region where the p-type and n-type semiconductors are joined is called p-n junction. It is also defined as the boundary between p-type and n-type semiconductor. This p-n junction forms a most popular semiconductor device known as diode. A P-type material has holes as the majority carriers and an N-type material has electrons as the majority carriers. As opposite charges attract, few holes in P-type tend to go to n-side, whereas few electrons in N-type tend to go to P-side. As both of them travel towards the junction, holes and electrons recombine with each other to neutralize and forms ions. Now, in this junction, there exists a region where the positive and negative ions are formed, called as PN junction or junction barrier as shown in the figure.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The formation of negative ions on P-side and positive ions on N-side results in the formation of a narrow-charged region on either side of the PN junction. This region is now free from movable charge carriers. The ions present here have been stationary and maintain a region of space between them without any charge carriers. As this region acts as a barrier between P and N type materials, this is also called as Barrier junction. This has another name called as Depletion region meaning it depletes both the regions. There occurs a potential difference VD due to the formation of ions, across the junction called as Potential Barrier as it prevents further movement of holes and electrons through the junction. V-I Characteristics of P-N junction Diode : The relationship between the current flowing through the diode and the voltage due to the applied voltage in forward bias and reverse bias is shown by a graph. Thus showing the voltage and diode current through the graph is called VI characteristics of the diode. The voltage applied determines one of three biassing conditions for p-n junction diodes: Zero Biased Condition In this case, no external voltage is applied to the P-N junction diode; and therefore, the electrons diffuse to the P-side and simultaneously holes diffuse towards the N-side through the junction, and then combine with each other. Due to this an electric field is generated by these charge carriers. The electric field opposes further diffusion of charged carriers so that there is no movement in the middle region. This region is known as depletion width or space charge. Forward Bias In the forward bias condition, the negative terminal of the battery is connected to the N-type material and the positive terminal of the battery is connected to the P-Type material. This connection is also called as giving positive voltage. Electrons from the N-region cross the junction and enters the P-region. Due to the attractive force that is generated in the P-region the electrons are attracted and move towards the positive terminal. Simultaneously the holes are attracted to the negative terminal of the battery. By the movement of electrons and holes current flows. In this condition, the width of the depletion region decreases due to the reduction in the number of positive and negative ions.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Circuit diagram for Forward bias V-I Characteristics of PN junction diode in forward bias By supplying positive voltage, the electrons get enough energy to overcome the potential barrier (depletion layer) and cross the junction and the same thing happens with the holes as well. The amount of energy required by the electrons and holes for crossing the junction is equal to the barrier potential 0.3 V for Ge and 0.7 V for Si. This is also known as Voltage drop/knee voltage/cut-in voltage. The voltage drop across the diode occurs due to internal resistance. This can be observed in the below graph. Forward bias V-I Characteristics Reverse Bias When the p-type is connected to the battery’s negative terminal and the n-type is connected to the positive side, the P-N junction is reverse biased. In this case, the electrons from the N-type semiconductor are attracted towards the positive terminal and the holes from the P-type semiconductor are attracted to the negative terminal. This leads to the reduction of the number of electrons in N-type and holes in P-type. In addition, positive ions are created in the N-type region and negative ions are created in the P-type region. Therefore, the depletion layer width is increased due to the increasing number of positive and negative ions. Due to this strong electric field across the junction,  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 electrons and holes want more energy to cross the junction so they cannot diffuse to the opposite region. Hence, there is no current flow due to the lack of movement of electrons and holes. Circuit diagram for Reverse bias V-I Characteristics of PN junction diode in reverse bias: Due to thermal energy in crystal minority carriers are produced. Minority carriers mean a hole in N-type material and electrons in P-type material. These minority carriers are the electrons and holes pushed towards P-N junction by the negative terminal and positive terminal, respectively. Due to the movement of minority carriers, a very little current flows, which is in nano Ampere range (for silicon). This current is called as reverse saturation current. Saturation means, after reaching its maximum value, a steady state is reached wherein the current value remains same with increasing voltage.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The magnitude of reverse current is of the order of nano-amperes for silicon devices. When the reverse voltage is increased beyond the limit, then the reverse current increases drastically. This particular voltage that causes the drastic change in reverse current is called reverse breakdown voltage. V-I Characteristics Graph for Reverse Bias V-I Characteristics of P-N junction Diode Diode Current Equation:  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 V-I Characteristics of P-N junction Diode Diode Resistance: The two types of resistance takes place in the p-n junction diode are: Forward resistance Reverse resistance Forward resistance Forward resistance is a resistance offered by the p-n junction diode when it is forward biased. In a forward biased p-n junction diode, two type of resistance takes place based on the voltage applied. The two types of resistance takes place in forward biased diode are Static resistance or DC resistance Dynamic resistance or AC resistance  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Static resistance or DC resistance When forward biased voltage is applied to a diode that is connected to a DC circuit, a DC or direct current flows through the diode. Direct current or electric current is nothing but the flow of charge carriers (free electrons or holes) through a conductor. In DC circuit, the charge carriers flow steadily in single direction or forward direction. The resistance offered by a p-n junction diode when it is connected to a DC circuit is called static resistance. Static resistance is also defined as the ratio of DC voltage applied across diode to the DC current or direct current flowing through the diode. The resistance offered by the p-n junction diode under forward biased condition is denoted as Rf. Dynamic resistance or AC resistance The dynamic resistance is the resistance offered by the p-n junction diode when AC voltage is applied. When forward biased voltage is applied to a diode that is connected to AC circuit, an AC or alternating current flows though the diode. In AC circuit, charge carriers or electric current does not flow in single direction. It flows in both forward and reverse direction. Dynamic resistance is also defined as the ratio of change in voltage to the change in current. It is denoted as rf. Reverse resistance Reverse resistance is the resistance offered by the p-n junction diode when it is reverse biased. When reverse biased voltage is applied to the p-n junction diode, the width of depletion region increases. This depletion region acts as barrier to the electric current. Hence, a large amount of electric current is blocked by the depletion region. Thus, reverse biased diode offer large resistance to the electric current. The resistance offered by the reverse biased p-n junction diode is very large compared to the forward biased diode. The reverse resistance is in the range of mega ohms (MΩ). Purpose of a Diode/Applications of diode:  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 ⮚ A diode is also used as a Switch. It helps a faster ON and OFF for the output that should occur in a quick rate. ⮚ A diode is used to block the electric current flow in one direction, i.e. in forward direction and to block in reverse direction. This principle of diode makes it work as a Rectifier. Rectifier definition A rectifier is an electrical device that converts an Alternating Current (AC) into a Direct Current (DC) by using one or more P-N junction diodes. CLASSIFICATION OF RECTIFIERS: Using one or more diodes in the circuit, following rectifier circuits can be designed. 1) Half - WaveRectifier 2) Full – WaveRectifier 3) BridgeRectifier HALF-WAVERECTIFIER: A Half – wave rectifier as shown in fig 2 is one, which converts a.c. voltage into a pulsating voltage using only one half cycle of the applied a.c. voltage.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The a.c. voltage is applied to the rectifier circuit using step-down transformer-rectifying element i.e., p-n junction diode and the source of a.c. voltage, all connected is series. The a.c. voltage is applied to the rectifier circuit using step-down transformer V=Vm sin (wt) The input to the rectifier circuit, Where Vm is the peak value of secondary a.c. voltage. Operation: For the positive half-cycle of input a.c. voltage, the diode D is forward biased and hence it conducts. Now a current flow in the circuit and there is a voltage drop across RL. The waveform of the diode current (or) load current is shown in fig 3. For the negative half-cycle of input, the diode D is reverse biased and hence it does not  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Conduct. Now no current flows in the circuit i.e., i=0 and Vo=0. Thus, for the negative half- cycle no power is delivered to the load. Analysis: In the analysis of a HWR, the following parameters are to be analyzed. Let a sinusoidal voltage Vi be applied to the input of the rectifier. Then V=Vm sin (wt) Where Vm is the maximum value of the secondary voltage. Let the diode be idealized to piece-wise linear approximation with resistance Rf in the for ward direction i.e.in the ON state and Rr (=∞) in the reverse direction i.e., in the OFF state. Now the current ‘i’ in the diode (or) in the load resistance RL is given by V=Vmsin(wt) i) AVERAGE VOLTAGE V = Vm dc π ii).AVERAGE CURRENT: I = Im dc π iii) RMS VOLTAGE: V rms = V rms=  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Analysis: In the analysis of a HWR, the following parameters are to be analyzed. Let a sinusoidal voltage Vi be applied to the input of the rectifier. Then V=Vm sin (wt) Where Vm is the maximum value of the secondary voltage. Let the diode be idealized to piece-wise linear approximation with resistance Rf in the for ward direction i.e.in the ON state and Rr (=∞) in the reverse direction i.e., in the OFF state. Now the current ‘i’ in the diode (or) in the load resistance RL is given by V=Vmsin(wt) i) AVERAGE VOLTAGE V = Vm dc π ii).AVERAGE CURRENT: I = Im dc π iii) RMS VOLTAGE: V rms =  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 V rms= V 𝑉𝑚 rms= 2 vi) Ripple Factor: is the ratio of the AC component's RMS value and the DC component's RMS value V RF = ac V dc 2 2 Vac =√(𝑉𝑟𝑚𝑠 -𝑉𝑑𝑐 ) 2 2 V -V -= rms dc V dc = -1 =1.21  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 vii) Efficiency (Ƞ): Efficiency is defined as the ratio of DC power (output power) to the applied input AC power viii) Transformer Utilization Factor(TUF): The d.c. power to be delivered to the load in a rectifier circuit decides the rating of the transformer used in the circuit. Therefore, transformer utilization factor is defined as ix) Peak Inverse Voltage(PIV): It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. The peak inverse voltage across a diode is the peak of the negative half- cycle. For half- wave rectifier, PIV is Vm. x) %Regulation: The variation of dc output voltage as the function of dc load current is called regulation. The percentage regulation is defined as  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 DISADVANTAGES OF HALF-WAVE RECTIFIER: 1. The ripple factor is high. 2. The efficiency is low. 3. The Transformer Utilization factor is low. Because of all these disadvantages, the half-wave rectifier circuit is normally not used as a power rectifier circuit. FULL WAVE RECTIFIER: A full-wave rectifier converts an ac voltage into a pulsating dc voltage using both half cycles of the applied ac voltage. In order to rectify both the half cycles of ac input, two diodes are used in this circuit. The diodes feed a common load RL with the help of a center-tap transformer. A center-tap transformer is the one, which produces two sinusoidal waveforms of same magnitude and frequency but out of phase with respect to the ground in the secondary winding of the transformer. The full wave rectifier is shown in the fig 4 below  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Fig. 5 shows the input and output wave forms of the ckt. During positive half of the input signal, anode of diode D1 becomes positive and at the same time the anode of diode D2 becomes negative. Hence D1 conducts and D2 does not conduct. The load current flows through D1 and the voltage drop across RL will be equal to the input voltage. During the negative half cycle of the input, the anode of D1 becomes negative and the anode of D2 becomes positive. Hence, D1 does not conduct and D2 conducts. The load current flows through D2 and the voltage drop across RL will be equal to the input voltage. It is noted that the load current flows in the both the half cycles of ac voltage and in the same direction through the load resistance. i) AVERAGEVOLTAGE ii) AVERAGECURRENT  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 iii) RMSVOLTAGE: V  rms V  rms IV) RMS CURRENT 𝐼𝑚 Irms = √2  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 vi) Ripple Factor: is the ratio of the AC component's RMS value and the DC component's RMS value vii) Efficiency (Ƞ): Efficiency is defined as the ratio of DC power (output power ) to the applied input AC power  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 viii) Transformer Utilization Factor(TUF): The d.c. power to be delivered to the load in a rectifier circuit decides the rating of the transformer used in the circuit. So, transformer utilization factor is defined as %Regulation: The variation of dc output voltage as the function of dc load current is called regulation. The percentage regulation is defined as V noload=2Vm/π Vfull load=Idc.RL 2𝑉𝑚/ π Idc = 𝑅𝑓+𝑅𝐿 %Regulation=Rf/RL Peak Inverse Voltage(PIV): It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. The peak inverse voltage across a diode is the peak of the negative half- cycle. For Full- wave rectifier, PIV is 2Vm. Advantages 1) Ripple factor = 0.482 (against 1.21 forHWR) 2) Rectification efficiency is 0.812 (against 0.406 forHWR) 3) Better TUF is 0.693 (0.287 forHWR)  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Disadvantages: 1) Requires center tapped transformer. 2) PIV is more BRIDGE RECTIFIER: Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below. The Diode Bridge Rectifier The Positive Half-cycle The four diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The Negative Half-cycle During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch "OFF" as they are now reverse biased. The current flowing through the load is the same direction as before. As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax. However in reality, during each half cycle the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply)  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Peak Inverse Voltage(PIV): It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. The peak inverse voltage across a diode is the peak of the negative half- cycle. For bridge rectifier, PIV is Vm. Comparison Table for half wave ,full wave and Bridge Rectifiers :  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 RECTIFIERS WITH FILTERS Filter: The ripple in the signal denotes the presence of some AC component. This ac component has to be completely removed in order to get pure dc output. So, we need a circuit that smoothens the rectified output into a pure dc signal. A filter circuit is one which removes the ac component present in the rectified output and allows the dc component to reach the load. The following figure shows the functionality of a filter circuit. A filter circuit is constructed using two main components, inductor and capacitor. An inductor allows dc and blocks ac. A capacitor allows ac and blocks dc. Let us try to construct a few filters, using these two components. Some important filters are: 1. Inductor filter 2. Capacitor filter 3. LC or L section filter 4. CLC or Π-type filter  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 1. Series Inductor Filter As an inductor allows dc and blocks ac, a filter called Series Inductor Filter can be constructed by connecting the inductor in series, between the rectifier and the load. The figure below shows the circuit of a series inductor filter. The rectified output when passed through this filter, the inductor blocks the ac components that are present in the signal, in order to provide a pure dc. This is a simple primary filter.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 2. Shunt Capacitor Filter As a capacitor allows ac through it and blocks dc, a filter called Shunt Capacitor Filter can be constructed using a capacitor, connected in shunt, as shown in the following figure. The rectified output when passed through this filter, the ac components present in the signal are grounded through the capacitor which allows ac components. The remaining dc components present in the signal are collected at the output. 3. L-C Filter A filter circuit can be constructed using both inductor and capacitor in order to obtain a better output where the efficiencies of both inductor and capacitor can be used. The figure below shows the circuit diagram of a LC filter.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The rectified output when given to this circuit, the inductor allows dc components to pass through it, blocking the ac components in the signal. Now, from that signal, few more ac components if any present are grounded so that we get a pure dc output. This filter is also called as a Choke Input Filter as the input signal first enters the inductor. The output of this filter is a better one than the previous ones. Π- Filter (Pi filter) This is another type of filter circuit which is very commonly used. It has capacitor at its input and hence it is also called as a Capacitor Input Filter. Here, two capacitors and one inductor are connected in the form of π shaped network. A capacitor in parallel, then an inductor in series, followed by another capacitor in parallel makes this circuit. If needed, several identical sections can also be added to this, according to the requirement. The figure below shows a circuit Working of a Pi filter In this circuit, we have a capacitor in parallel, then an inductor in series, followed by another capacitor in parallel. Capacitor C1 − This filter capacitor offers high reactance to dc and low reactance to ac signal. After grounding the ac components present in the signal, the signal passes to the inductor for further filtration. Inductor L − This inductor offers low reactance to dc components, while blocking the ac components if any got managed to pass, through the capacitor C1. Capacitor C2 − Now the signal is further smoothened using this capacitor so that it  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 allows any ac component present in the signal, which the inductor has failed to block. Thus we, get the desired pure dc output at the load. RECTIFIERS WITH FILTERS  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 CAPACITOR FILTER: Capacitor filter An inexpensive filter for light loads is found in the capacitor filter which is connected directly across the load, as shown in Fig. 18.8(a). The property of a capacitor is that it allows a.c. component and blocks the d.c. component. The operation of a capacitor filter is to short the ripple to ground but leave the d.c. to appear at the output when it is connected across a pulsating d.c. voltage. During the positive half-cycle, the capacitor charges up to the peak value of the transformer secondary voltage, V, and will try to maintain this value as the full-wave input drops to zero. The capacitor will discharge through R, slowly until the transformer secondary voltage again increases to a value greater than the capacitor voltage (equal to the load voltage). The diode conducts for a period which depends on the capacitor voltage. The diode will conduct when the transformer secondary voltage becomes more than the 'cut-in' voltage of the diode. The diode stops conducting when the transformer voltage becomes less than the diode voltage. This is called cut-out voltage.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Referring to Fig. 18.8(b) with slight approximation, the ripple voltage waveform can be assumed as triangular. From the cut-in point to the cut-out point, whatever charge the capacitor acquires is equal to the charge the capacitor has lost during the period of non-conduction, i.e. from cut-out point to the next cut-in point. The charge it has acquired = Vr, pp x C The charge it has lost = Id.c. X T2 Therefore, Vr, pp X C = Id.c. X T2 If the value of the capacitor is fairly large, or the value of the load resistance is very large, then it can be assumed that the time T2 is equal to half the periodic time of the waveform.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Zener diode A Zener diode can be defined as a heavily doped semiconductor device that is designed to operate in the reverse bias conditions. Working Principle of Zener Diode A Zener diode functions similarly to a regular diode when forward-biased. However, in reverse-biased mode, a small leakage current flows through the diode. As the reverse voltage increases and reaches the predetermined breakdown voltage (Vz), current begins to flow  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 through the diode. This current reaches a maximum level determined by the series resistor, after which it stabilizes and remains constant across a wide range of applied voltages. There are two types of breakdowns in a Zener Diode: Avalanche Breakdown and Zener Breakdown. Avalanche Breakdown in Zener Diode Avalanche breakdown occurs in both normal diodes and Zener diodes when subjected to high reverse voltage. When a significant reverse voltage is applied to the PN junction, the free electrons gain enough energy to accelerate at high velocities. These high-velocity electrons collide with other atoms, causing the ejection of additional electrons. This continuous collision process generates a large number of free electrons, resulting in a rapid increase in electric current through the diode. In the case of a normal diode, this sudden surge in current could permanently damage it. However, a Zener diode is specifically designed to withstand avalanche breakdown and can handle the sudden current spike. Avalanche breakdown typically occurs in Zener diodes with a Zener voltage (Vz) greater than 6V. Zener Breakdown in Zener Diode When the reverse bias voltage applied to a Zener diode approaches its Zener voltage, the electric field within the depletion region becomes strong enough to attract and remove electrons from their valence band. These valence electrons, energized by the intense electric field, break free from their parent atoms. This phenomenon takes place in the Zener breakdown region, where even a slight increase in voltage leads to a rapid surge in electric current.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 V-I Characteristics of Zener Diode The V-I characteristics of a Zener diode are divided into two parts which are mentioned as follows: Forward Characteristics of Zener Diode Forward characteristics of the Zener Diode are similar to the forward characteristics of any normal diode. It is clearly evident from the above diagram in the first quadrant that the VI forward characteristics are similar to other P-N junction diodes. Reverse Characteristics of Zener Diode In reverse voltage conditions a small amount of current flows through the Zener diode. This current is because of the electrons which are thermally generated in the Zener diode. As we keep increasing the reverse voltage at any particular value of reverse voltage the reverse current increases suddenly at the breakdown point this voltage is called Zener Voltage and is represented as Vz Advantages of zener diode Power dissipation capacity is very high High accuracy Small size Low cost Applications of zener diode It is normally used as voltage reference  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 Zener diodes are used in voltage stabilizers or shunt regulators. Zener diodes are used in switching operations Zener diodes are used in clipping and clamping circuits. Zener diodes are used in various protection circuits Voltage Regulator Zener Diode as Voltage Regulators The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current and the zener diode will continue to regulate the voltage until the diodes current falls below the minimum IZ(min) value in the reverse breakdown region. It permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage. The Zener diode specially made to have a reverse voltage breakdown at a specific voltage. Its characteristics are otherwise very similar to common diodes. In breakdown the voltage across the Zener diode is close to constant over a wide range of currents thus making it useful as a shunt voltage regulator. The purpose of a voltage regulator is to maintain a constant voltage across a load regardless of variations in the applied input voltage and variations in the load current. A typical Zener diode shunt regulator is shown in Figure. The resistor is selected so that when the input voltage is at VIN(min) and the load current is at IL(max) that the current through the Zener diode is at least Iz(min). Then for all other combinations of input voltage and load current the Zener diode conducts the excess current thus maintaining a constant voltage across the load. The Zener conducts the least current when the load current is the highest and it conducts the most current when the load current is the lowest. Fig: Zener diode shunt regulator IS=IZ+IL  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 RS=(VIN-VZ)/IS VZ=V0 If there is no load resistance, shunt regulators can be used to dissipate total power through the series resistance and the Zener diode. Shunt regulators have an inherent current limiting advantage under load fault conditions because the series resistor limits excess current. A zener diode of break down voltage Vz is reverse connected to an input voltage source Vi across a load resistance RL and a series resistor RS. The voltage across the zener will remain steady at its break down voltage VZ for all the values of zener current IZ as long as the current remains in the break down region. Hence a regulated DC output voltage V0 = VZ is obtained across RL, whenever the input voltage remains within a minimum and maximum voltage. Basically, there are two type of regulations such as: a) Line Regulation In this type of regulation, series resistance and load resistance are fixed, only input voltage is changing. Output voltage remains the same as long as the input voltage is maintained above a minimum value.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 b) Load Regulation In this type of regulation, input voltage is fixed and the load resistance is varying. Output volt remains same, as long as the load resistance is maintained above a minimum value. Cathose Ray Oscilloscope: Block Diagram of CRO: CRO is made up of different blocks such as 1. Cathode Ray Tube (CRT) 2. Vertical amplifier 3. Delay Line 4. Trigger circuit 5. Timebase generator 6. Horizontal amplifier CRO Block Diagram  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 CRO Working: Cathode Ray Tube (CRT): CRT Produces a sharply focused beam of electrons, accelerated to a very high velocity. This electron beam travels from the electron gun to the screen. The electron gun consists of filament, cathode, control grid, accelerating anodes and focusing anode. While travelling to the screen, electron beams passes between a set of vertical deflecting plates and a set of horizontal deflection plates. Voltages applied to these plates can move the beam in vertical and horizontal plane respectively. The electron beam then strikes the fluorescent material (phosphor) deposited on the screen with sufficient energy to cause the screen to light up in a small spot. Vertical Amplifier - The input signals are amplified by the vertical amplifier. Usually, the vertical amplifier is a wide band amplifier which passes the entire band of frequencies. Delay Line - As the name suggests, this circuit is used to delay the signal for a period of time in the vertical section of CRT. The input signal is not applied directly to the vertical plates because the part of the signal gets lost, when the delay time is not used. Therefore, the input signal is delayed by a period of time. Trigger Circuit - The signals which are used to activate the trigger circuit are converted to trigger pulses for the precision sweep operation whose amplitude is uniform. Hence input signal and the sweep frequency can be synchronized. Time Base (Sweep) Generator - Time base circuit uses a uni-junction transistor, which is used to produce the sweep. The saw tooth voltage produced by the time base circuit is required to deflect the beam in the horizontal section. The spot is deflected by the saw tooth voltage at a constant time dependent rate. Horizontal Amplifier - The saw tooth voltage produced by the time base circuit is amplified by the horizontal amplifier before it is applied to horizontal deflection plates. Power supply - The voltages required by CRT, horizontal amplifier, and vertical amplifier are provided by the power supply block. It is classified into two types - (1) Negative high voltage supply (2) Positive low voltage supply  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The voltage of negative high voltage supply is from -1000V to -1500V. The range of positive voltage supply is from 300V to 400V. Cathode Ray Tube Construction and working: 1. Electron gun The electron gun is used for generating, controlling and focusing the beam of electrons enclosed in a vacuum tube. The electron gun again internally consists of five components. They are: A. Heater B. Cathode C. Control grid D. Accelerating anode.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 E. Focusing anode 2. Horizontal deflection plates The horizontal deflection plates deflects the electrons horizontally. 3. Vertical deflection plates The vertical deflection plates deflects the electrons vertically. Both the horizontal and vertical deflection plates controls the path of electrons beam so that it can be directed towards a specified positions on the phosphor-coated screen. 4. Fluorescent screen Fluorescent screen is a transparent screen coated on one side with a phosphor that glows brightly when exposed to cathode rays. How cathode ray tube works The electron gun is used for generating, controlling and focusing the beam of electrons enclosed in a vacuum tube. The electron gun again internally consists of five components. They are: heater, cathode, control grid, accelerating anode, and focusing anode. The heater at the left side in the figure heats the cathode to a high temperature. Cathode is a conductor that emits electrons from its surface when heated to a high temperature. A high positive voltage is applied to the accelerating anode of the order of 1 to 20,000 volts, relative to the cathode. This potential difference generates an electric field between the accelerating anode and the cathode which accelerates electrons from cathode to accelerating anode. Electrons passing through the hole in the anode form a narrow beam and travel with constant horizontal velocity from the anode to the florescent screen. The area where the electrons beam strikes the screen glows brightly. The control grid controls the flow of electrons between the cathode and the accelerating anode. Hence, it controls the brightness of the spot on the screen. The focusing anode ensures that the electrons emitted from the cathode in slightly different directions are focused down to a narrow beam and all arrive at the same spot on the screen.  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 The assembly of heater, cathode, control grid, accelerating anode and focusing anode is called the electron gun. The beam of electrons passes between two pairs of deflection plates: horizontal deflection plates and vertical deflection plates. The electric field between the horizontal deflection plates change the direction of electrons horizontally, while the electric field between the vertical deflection plates change the direction of electrons vertically. The screen consists of a glass which is coated by some florescent material like zinc silicate which is semitransparent phosphor substance. The phosphor substance converts the electrical energy into light energy. When the high velocity electrons strike the phosphorescent screen, the light is emitted from it. The property of phosphor to emit light when its atoms are excited is called fluorescence. The intensity of the glow produced at the screen is determined by the number of electrons striking the screen. Oscilloscope Application: 1. Voltage Measurement 2. Time Period measurement 3. Frequency measurement 4. Phase difference measurement 5. Component test 6. To find Modulation Index  NEIL GOGTE INSTITUTE OF TECHNOLOGY Subject: Basic Electronics Unit:1 1. A half wave rectifier is connected to a transformer with turns ratio 4:1 find its average DC output voltage, PIV of diode and ripple frequency if input voltage is 200 V rms, 50 Hz mains supply. (Ans : VDC​=22.52 V, PIV of diode =70.72 V Ripple freq. =50 Hz ) 2. A bridge rectifier is connected with a transformer whose secondary voltage is 30 volt rms/60 Hz find its average DC voltage, PIV of diode and ripple frequency. Draw necessary diagram. (Ans : VDC​=27.027 V PIV =42.43 V Ripple freq. =12OH2​) 3. The turn's ratio of the transformer used in a Bridge rectifier is 10:1. The primary is connected to 220 V,50 Hz power mains. Find the output DC voltage, PIV of diode and ripple frequency under no load condition assuming the voltage drop across the diode to be zero. (Ans : VDC​=19.81 V PIV =31.12 V Ripple freq. =100H2​) 4. A230 V,60Hz voltage is applied to the primary of 5:1 step-down , center-tap transformer use in a full wave rectifier having aload of 900 ohm.If the diode resistance and secondary coil resistance together has a resistance of 100 ohm ,determine a) d.c voltage across the load b) d.c current flowing through the load c) d.c. power delivered to the load d) PIV across each diode ,e) ripple voltage and its frequency and (Vdc=20.7V, Idc=20.7mA,Pdc=0.386W,PIV=65V, Ripple Voltage=10.05V,frequency of ripple voltage=120Hz,) 5. A HWR has a load of 3.5Kohm.If the diode resistance and secondary coil resistance together have a resistance of 800 ohm and the input voltage has a signal voltage of peak value 240V.calculate a) Peak average and rms value of current flowing b) dc power c) ac power d) efficiency of the rectifier.