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Prof.R.S.Kakade (Sub- Electronics) Chapter no. 4 RECTIFIERS, FILTERS & REGULATORS ( 20 Marks) RSK Rectifier: Rectifier is an electronic device which is used for converting an alternating (ac) voltage or current into a unidirectional (dc) voltage or current. Rectification is the process of converting the alternating voltage or current into the corresponding direct (dc) quantity (direct voltage or current) Need of Rectification: Most of the electronic devices require DC supply for their operation, where as the electric supply available is almost always AC. A rectifier circuit is, therefore, commonly included in most of the electronic equipments to convert the alternating supply into DC. Types of Rectifier circuits: Rectifier is a circuit which converts an alternating (ac) voltage into a direct (dc) voltage. Half Wave Rectifier: The circuit configuration of a half wave rectifier is as shown in fig. It is the input step down transformer. RL is the load resistance. In the positive half wave cycle of the ac supply, the secondary voltage VAB is positive i.e. A is positive with respect to B. Hence the diode is forward biased and starts conducting. 2nd Sem/CM/IF Rectifiers, Filters & Regulators 1 Prof.R.S.Kakade (Sub- Electronics) In the negative half cycle the ac supply secondary voltage V AB is negative i.e. A is negative with respect to B. Hence diode is reverse biased and offers a very high resistance due to this load voltage and load current both are zero. the o/p waveforms are as shown in fig. or Operation: The ac voltage across the secondary winding AB changes polarities after every half cycle. During the positive half cycle of input ac voltage, end A becomes positive w.r.t. end B. This makes the diode forward biased and hence it conducts current. during the negative half cycle, end A is negative w.r.t. B. under this condition the diode is reverse biased and it conducts no current. Therefore, current flows through the diode during positive half cycles of input ac voltage only. it is blocked during the negative half cycles. In this way, current flows through load RL is always in the same direction. Hence d.c.load output is obtained across RL. Parameters of Half wave Rectifier: 1) Iav- DC or Average Load Current (ILdc):Definition – The average value of a periodic function is given by, the area under one cycle of the function divided by the base (period) 2nd Sem/CM/IF Rectifiers, Filters & Regulators 2 Prof.R.S.Kakade (Sub- Electronics) Average Load Current Idc = Im where Im = peak amplitude of the load current. 2) Vac - DC or Average Load Voltage (VLdc)VLdc = ILdc X RL VLdc = Vm 3) Irms - AC or RMS Load current (Irms)Consider one complete cycle of the load current waveform (0-2) Irms = Im 2 4) Vrms - AC or RMS value of load voltage VLrms Vm ..... (approximate) 2 5) Ripple factor (RF)The rectifier output consists of AC as well as DC components. The ripple factor indicates how close the rectifier is to the pure ideal dc voltage waveform. Small values of ripple factor indicate that the rectifier output waveform is close to being pure dc. The ideal value is should be as small as possible. Ripple Factor = Rms values of the AC component of o/p DC or Average value of the o/p Ripple factor is defined as the rms value of the ripple voltage (a.c. component of the output) to the dc output voltage. 6) DC output Power (PLdc)PLdc = V2Ldc RL 2nd Sem/CM/IF Rectifiers, Filters & Regulators 3 Prof.R.S.Kakade (Sub- Electronics) 7) AC Input Power - (Pac) Pac = I2s rms X (Rs + RF + RI) 8) Rectification efficiency () It is the ratio of output power to the a.c. input power of a rectifier. = DC o/p power AC I/p power = PLdc Pac 9) Peak Inverse Voltage (PIV)Peak Inverse voltage is the maximum negative voltage which appears across a no conducting reverse biased diode. Full wave rectifier with Center Tapped Transformer: The full wave rectifier configuration is as shown in fig. A center tapped secondary winding AB is used with two diodes connected so that each used one half cycle of input ac voltage. Diode D1 utilizes the ac voltage appearing across the upper half (OA) of secondary winding rectification while diode D2 uses the lower half winding (OB). 2nd Sem/CM/IF Rectifiers, Filters & Regulators 4 Prof.R.S.Kakade (Sub- Electronics) Operation – During the positive half cycle of secondary voltage, the end A of the secondary winding becomes positive and end B negative. This makes the diode D1 forward biased and diode D2 reverse biased. Therefore diode D1 conducts while diode D2 does not conduct. The conventional current flow is through diodes D1, load resister RL and the upper half of secondary winding as shown in fig. During the negative half -cycle, end A of the secondary winding becomes negative and end B positive. Therefore diode D2 conducts while diode D1 does not . The conventional current flows through diode D2, Load RL and lower half winding referring the fig. it may be seen that current in the load RL is in the same direction for both half cycles of the input ac voltage. Therefore, d.c. is obtained across the load RL. 1) Average Load Current (ILdc)ILdc = 2Im π 2) Average Load Voltage (VLdc) – VLdc = 2Vm .. . Approximate π 3) RMS Load Current (ILrms) VLrms = Im √2 2nd Sem/CM/IF Rectifiers, Filters & Regulators 5 Prof.R.S.Kakade (Sub- Electronics) 4) RMS Load voltage (VLrms) VLrms = Vm .. . Approximate √2 5) Ripple factor (Rf) – Ripple Factor = [ V2Lrms – V2Ldc] 1/2 VLdc 6) Dc output Power (PLdc) – PLdc = I2Ldc X RL 7) Ac input Power (Pac) – Pac = I2s rms X (Rs + RF + RL) 8) Ripple Efficiency (η) – η= PLdc Pac 9) Peak Inverse Voltage (PIV) – PIV = 2 Vm Volts 10) Ripple frequency The Ripple frequency of full wave rectifier is twice that of the Half wave rectifier. i.e. 100 HZ. Full Wave Bridge Rectifier: 2nd Sem/CM/IF Rectifiers, Filters & Regulators 6 Prof.R.S.Kakade (Sub- Electronics) Full wave bridge rectifier contains four diodes D1,D2,D3 and D4 connected to form a bridge as shown in fig. The a.c. supply to be rectified is applied to the diagonally opposite ends of the bridge through the transformer. And in between other two ends of bridge, the load resistance RL is connected. Operation During the positive half – cycle of secondary voltage, the end P of the secondary windings becomes positive and end Q becomes negative. This makes diodes D1 & D3 forward biased while diodes D2 & D4 are reverse biased. Therefore, only diodes D1 & D3 conduct. The conventional current flows from A to B through load Resistance RL as shown in fig (a) by dotted lines. During the negative half cycle of secondary voltage, end P becomes negative and end Q becomes positive. This makes diodes D2 & D4 forward biased where as diodes D1 & D3 are reverse biased. Therefore only diodes D2 & D4 conducts. These two diodes will be in series through the load RL. The conventional current flows through the ckt. As shown in fig. /it may be seen that load RL. i.e. in the same direction, as for the positive half cycle. Therefore d.c. output is obtained across the load RL. 2nd Sem/CM/IF Rectifiers, Filters & Regulators 7 Prof.R.S.Kakade (Sub- Electronics) Why Bridge Rectifier is better than centre tapped full wave rectifier? Ans:- 1) It requires a small size transformer. Centre tap transformer is not required . This makes the bridge rectifier cost effective. 2) The input transformer is not a must. It is possible to operate the bridge rectifier directly on the 230V ac supply. 3) This circuit is most suitable for the high voltage applications. 4) High avarae output voltage. 5) Rectifier efficiency is high. 6) Transformer utilization factor TUF is high. Filters and regulators Filters: Filter is nothing but electronic circuit which gives ripple free voltage (i.e. no A.C. Voltage) or pure D.C. Voltage. Need of Filters: The purpose of a steady d.c. voltage. Through the output of full wave rectifier is better unidirectional still it provides pulsating d.c. voltage. (i.e. it contain some what a.c. voltage). This pulsating d.c. voltage produces losses in load resistor (RL). Therefore in order to obtain steady d.c. output voltage and remove a.c voltage use of filter is necessary. The filter is connected between rectifier and load as shown in fig. Below. The filter circuits use: Resistors, Capacitors and inductors. Therefore they are called passive filter. 2nd Sem/CM/IF Rectifiers, Filters & Regulators 8 Prof.R.S.Kakade (Sub- Electronics) Types of Filter: Filters are classified depending on the components used and depending on the configuration in which they are connected. 1) Capacitor filter (Shunt capacitor filter) 2) Choke input filter (Series inductor filter) L Filter 3) LC Filter. 4) π Type filter (CLC filter) 1) Capacitor filter (Shunt capacitor filter) C Filter. As shown in above fig. Capacitor is connected across the load resistor. O/P of rectifier is given the load resistor. As the capacitor is connected in Parallel with load and acts as filter. Therefore this filter is called as shunt capacitor filter. Operation: 2nd Sem/CM/IF Rectifiers, Filters & Regulators 9 Prof.R.S.Kakade (Sub- Electronics) The initial voltage on capacitor is assumed to be zero. In the first positive half cycles diode D1 is forward biased and D2 is reverse biased and acts as open switch. As Diode D1 conducts its supplies the charging current to the capacitor. Capacitors starts charging through diode D1 and capacitor charges to the peak value of voltage VM as shown in wave form at point A after point A the secondary voltage starts decreasing as shown in dotted lines. As secondary voltage decreases. This will reverse bias the diode D1. Thus diode D1 is turned off. During this period the voltage on capacitor is higher than rectifier o/p (as shown by dotted lines). The diode D1 and D2 both remains off. Then the capacitor starts discharging exponentially through load resistor RL as shown in portion AB in o/p wave forms. Again at point B Secondary voltage starts increasing. Therefore the diode D2 becomes forward biased its starts conducting and capacitor charges to peak value (VM) through D2 as shown in waveform by portion of voltage BC. Again at point C both the diodes are reversed biased and turned off and capacitor starts discharging through load resistor RL as shown in o/p wave form by the portion CD. Thus we get the pure D.C. o/p shown by portion ABCD. Ripple factor: The expression for the ripple factor of HWR and FWR with center tap or bridge rectifier are as follows 1) Ripple factor for C filter with FWR or bridge rectifier r= 1 . 4 √3 fcRL 2) Ripple factor for HWR is r= 1 . 2√3 fcRL Where f= supply frequency in Hz C = Value of filter capacitor in Farad RL= Load resistance. The value of ripple factor should be as low as possible. How to reduce the ripple factor from above expression we can reduce ripple factor by, 1) Increasing value of filter capacitor C or 2) Increasing the load resistance RL 2nd Sem/CM/IF Rectifiers, Filters & Regulators 10 Prof.R.S.Kakade (Sub- Electronics) L Filter (Series inductor filter) (Choke input filter) The circuit diagram for series inductor filter is as shown in figure below As shown in figure inductor filter is connected in series with load resistor (RL). Operation: An inductor has fundamental property of opposing any charge in current flowing through it whenever the current tends to change a back emf is induced in the inductor. As inductor filter is connected in series with load resistor RL. This offers high reactance to a.c. components of frequency 2w, 4w, 6w etc. and small reactance to D.C. components. Amplitude of a.c. component in the output voltage are considerably reduced. If the amplitude of a.c. component increases above the average value inductor stores energy in its magnetic field and if the current decreases below the average value the inductor releases the energy. The higher the current flowing through it better is the filtering. The expression for ripple factor is given by Rf = R . 3√2 wL 2nd Sem/CM/IF Rectifiers, Filters & Regulators 11 Prof.R.S.Kakade (Sub- Electronics) π filter or CLC filter: The π Type filter is a combination of capacitor input filter and LC filter as shown in above figure. It consists of two capacitors C1 and C2 along with inductors L. Generally both the capacitors are of same value. The o/p of full wave rectifier to given to the π filter. The capacitor C1 comes first therefore C1 acts as shunt capacitors filter. The capacitors C1 and C2 provide a low reactance path for the ripple where as the series inductor L provides a high reactance to the a.c. ripple. The combine effect of this two filter (L1 and Lc) reduces a.c. o/p and improves D.C. output. Waveform: Expression for ripple factor: The expression for ripple factor with full wave rectifier is given by r = 1 . 2 2 √3 (w L c1 c2 RL) and with half wave rectifier, r = 1 . 2 √3 (w L c1 c2 RL) From the expression ripple factor of π filter is inversely proportional to the load. Resistor RL for RL= ∞ i.e. at no load ripple is best as RL decreases the value of ripple factor increases. LC Filter: 2nd Sem/CM/IF Rectifiers, Filters & Regulators 12 Prof.R.S.Kakade (Sub- Electronics) The above figure shows the LC filter with full wave rectifier. This is combination of inductor filter and capacitor input filter. We know that inductor filter is preferred for low values of R and the capacitor input filter is preferred for high values of load resistance. As it is combination of the two filters LC filter gives low ripple (i.e. Pure DC o/p) irrespective of load. The series connected inductor filter offers high reactance to AC component and small reactance to DC components and parallel capacitor provides allow reactance by pass path for AC signal. Thus combination of this two filter gives better DC o/p. Expression for the ripple factor: The expression for ripple factor of LC filter is given by Rf 1 r = . 6 √2 ( w2LC ) Comparison of C,L,Lc and π filter. Sr. Parameter 1) Component used 2) Connection 3) Ripple contend in o/p 4) Size of Filter 5) Suitable for 6) 7 8) Cost Expression for RF Voltage regulation Series Low LC Filter Capacitor & Inductor Parallel Very Low π Filter Two Capacitor and Inductor Parallel Negligible Bulky Heavy load application Moderate Bulky Light as well as heavy Higher Bulky All type of loads Highest C Filter Capacitor L Filter Inductor Parallel High Small Light load application Lowest r = 1 . 4 √3 FCR Poor 2nd Sem/CM/IF Rectifiers, Filters & Regulators r = 1 3 √2 WL Moderate . r = 1 . 6 √2w2 LC Good r = 1 . 2√3w2(C1C2RL) Very Good 13 Prof.R.S.Kakade (Sub- Electronics) Regulator: Definition : The voltage regulator is a circuit which gives constant o/p voltage irrespective of change in i/p voltage or change in load current or change in temp. Need of Regulator: The combination of rectifier and filter can be used as a d.c. power supply for certain application. Such as eliminators. But the problem with this type of D.C. power supply is that its o/p voltage will not remain constant in the event of change in i/p voltage or change in load current i.e. we get unregulated D.C. voltage. We can not use this type of unregulated power supply for expensive electronic instruments. Therefore we need regulator. Zener Diode as Regulator: There are two types of regulator that is shunt voltage regulator and series voltage regulator we can use zener diode as shunt voltage regulator as shown in fig below. The series resistance Rs is connected to limit the total current drawn from i/p supply. The zener diode is connected across the load resistance RL. Operation: The voltage across zener diode remains constant when it is operated in the zener region in reverse bias condition. This property of zener is utilized for the regulation of voltage at o/p. 1) If RL is constant and Vin (ip) voltage is changes. If Vin i/p voltage increases then current I increases. But load current does not increased because Vz zener diode voltage and RL load resistance is constant there fore the increase in current I will increase the zener current Iz. If the Zener current Iz is less than IZmax then zener diode operated in the zener region and o/p voltage remains constant. If Vin (i/p voltage) decreased the current I decreases but IL remains constant because RL. Vz is constant and hence naturally Iz is greater than Izmin. The zener diode is operated in the zener region and o/p voltage remains constant. 2) Vin (i/p voltage is constant) and load Resistor RL is varying. If RL increases then IL decreases but the total current I is constant. I = Vin - Vz Rs Therefore decrease in load current (IL) the zener current Iz will increase as long as Iz is less than Izmax. Zener diode is operated in zener region and o/p voltage will remain constant. If RL I reduced IL increases but I is constant and zener current Iz will decrease as long as Iz is higher than Iz min. the zener diode is operated in zener region and o/p voltage remains constant. 2nd Sem/CM/IF Rectifiers, Filters & Regulators 14 Prof.R.S.Kakade (Sub- Electronics) Load regulation: The load regulation is defined as change in output voltage. When the load current is changed from zero (no load) to maximum (Full load) value. LR = VNL - VFL Where VNL – o/p voltage on no load VFL - - o/p voltage on full load The % load regulation is defined as % LR = VNL – VFL X 100 VFL The L R of a power supply should be ideally equal to zero. Regulation characteristics Line Regulation: It is also called as source regulation. It is defined as the change in regulated load voltage due to change in line voltage. SR or LR = VLH – VLL Where VLH: Load voltage with high line voltage. VLL : Load Voltage with low line voltage. Regulated Power Supply: 2nd Sem/CM/IF Rectifiers, Filters & Regulators 15 Prof.R.S.Kakade (Sub- Electronics) Waveforms at various points in a regulated dc power supply A typical Regulated Power supply A typical regulated dc power supply is as shown in fig. It uses a bridge rectifier, a capacitor and a zener voltage regulator. Advantages of the Regulated Power supplies: 1) Very low ripple contents in the output voltage. 2) We get a very high quality (pure) dc output voltage. 3) Output voltage remains constant in spite of variation in input voltage, load current or temperature. --o-- 2nd Sem/CM/IF Rectifiers, Filters & Regulators 16