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Electronics Principles & Applications Fifth Edition Charles A. Schuler Chapter 4 Power Supplies ©1999 Glencoe/McGraw-Hill INTRODUCTION • The System • Half-Wave Rectification • Full-Wave Rectification • RMS to Average • Filters • Multipliers • Ripple and Regulation • Zener Regulator Circuit A Power Supply Circuit B Circuit C The power supply energizes the other circuits in a system. Thus, a power supply defect will affect the other circuits. dc ac Power Supply Circuit A dc Circuit B dc Circuit C Most line operated supplies change ac to dc. The cathode makes this the positive end of the load. + 0 - + ac 0 Half-wave pulsating dc A series rectifier diode changes ac to dc. The cathodes make this the positive end of the load. + 0 Full-wave pulsating dc + 0 ac - Two diodes and a transformer provide full-wave performance. One-half of the transformer secondary conducts at a time. C.T. VTOTAL VTOTAL VLOAD is equal to one-half the total secondary voltage. + 0 - ac + 0 Full-wave pulsating dc The bridge circuit eliminates the need for a transformer. + 0 - ac + 0 Full-wave pulsating dc Reversing the diodes produces a negative power supply. Power supply basics quiz Most line-operated power supplies change ac to ________. dc A single diode achieves ________ -wave rectification. half Two diodes and a center-tapped transformer provide ________ -wave rectification. full A bridge rectifier uses ________ diodes. four The positive end of the load is the end in contact with the diode ________. cathodes Vac Vac Vdc Vdc Ignoring diode loss, the average dc is 45% of the ac input for half-wave. ac Converting rms to average Vac Vdc Vac Vdc ac Ignoring diode loss, the average dc is 90% of the ac input for full-wave. Vac Vdc Average dc quiz The average dc voltage with half-wave is equal to ______ of the ac voltage. 45% The effective ac voltage in a two-diode, full-wave rectifier is _______ of the secondary voltage. half The average dc voltage with a full-wave rectifier is _________ of the effective ac voltage. 90% The average dc voltage with a bridge rectifier is equal to ________ of the ac voltage. 90% Filter capacitor Discharge VP + ac 0 Charge - A relatively large filter capacitor will maintain the load voltage near the peak value of the waveform. Discharge time + ac 0 - Full-wave is easier to filter since the discharge time is shorter than it is for half-wave rectifiers. VP Vac Vdc ac Adding a filter capacitor increases the dc output voltage. Ignoring diode loss and assuming a large filter, the dc output is equal to the peak value for both half-wave and full-wave. Vac Vdc Vac Vdc Ignoring diode loss and assuming a large filter, the dc output is equal to the peak value for both half-wave and full-wave. ac Vac Vdc Vac Vdc ac Full-wave doubler Ignoring diode loss and assuming large filters, the dc output is twice the peak ac input. Vac Vdc The charge on C1 adds to the ac line voltage charged.to twice the peak value. and CC21isischarged ac C1 C2 Half-wave voltage doubler Capacitive filter dc output quiz (assume a very light load for this quiz) The dc output in a well-filtered half-wave supply is _____ of the ac input. 141% The dc output in a well-filtered full-wave supply is _____ of the ac input. 141% The dc output in a well-filtered half-wave doubler is _____ of the ac input. 282% The dc output in a well-filtered full-wave doubler is _____ of the ac input. 282% Vac Vdc An ideal dc power supply has no ac ripple. ac ripple = 0% Vac Vdc Vac Vdc ac Real power supplies have some ac ripple. 1.32 Vac x 100% = 11 % ac ripple = 12 Vdc Vac Vdc An ideal power supply has perfect voltage regulation. The voltage does not change. Vac Vdc Voltage Regulation = = ac DV VFL x 100 % 1V x 100 % 12 V = 8.33 % The output of a real supply drops under load. Vac Vdc A zener diode can regulate voltage. Vac Vdc ac But, if the zener stops conducting, the regulation is lost. Power supply quality quiz The voltage regulation of an ideal power supply is ___________. 0% The ac ripple output of an ideal power supply is ___________. 0% DV in a real power supply should be as ___________ as is feasible. small The ac component of an ideal dc power supply should be as ________ as is feasible. small A device that is commonly used to regulate voltage is the ________ diode. zener REVIEW • The System • Half-Wave Rectification • Full-Wave Rectification • RMS to Average • Filters • Multipliers • Ripple and Regulation • Zener Regulator