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+ 0 Fu Full-Wave Center ll-Wave Ce nter TTap ap Figure Three Fig ure Th ree - A - C B ++ 0 Full-Wave Bridge Full-W ave Bridge Figure Four Figur e Four + + RIPPLE 0 O FFigure igure FFive ive + 5 volt volt logic logic supply supply in in video video and and +5 processor games. games. processor needed to to smooth smooth the the DC DC pulses. pulses. isis needed The basic basic function function of of aa capacitor capacitoris is to to The store aa charge charge or or voltage voltage across across the the store leads. IfIf aa capacitor capacitor isis charged charged and and leads. left alone, alone, itit will will hold hold that that charge charge left or until until aa resistance resistance or or indefinitely or indefinitely load isis put put across across it. it. Then Then itit will will load discharge at at aa rate rate determined determined by by the the discharge capacitance and and the the resistance. resistance . This This capacitance called an an "RC" "RC" time time constant. constant. isis called in seconds seconds itit takes takes aa The time time in The capacitcr to to discharge discharge to to half half its its capacitor original voltage is equal to the original voltage is equal toequals the capacitance in in farads farads (1 (1 farad farad equals capacitance 1,000,000 micro micro farads) farads) times times the the 1,000,000 resistance inin ohms. ohms. (T (T=R = R xx C). C). With With resistance the capacitor capacitor hooked hooked across across the the the rectifier circuit circuit and and no no load load on on the the rectifier output , the the capacitor capacitor will will charge charge output, immediately to to the the peak peak voltage voltage on on immediately the DC DC pulse pulse and and stay stay there there because because the no load load resistance resistance to to there isis no there discharge it. it. discharge With aa load load added added to to the the power power With supply, the the cap cap will will charge charge instantly instantly supply, the DC DC pulse pulse because because there there isis no no with the with resistance through through the the rectifier, rectifier, and and itit resistance will discharge discharge slowly slowly until until the the next next will will be. be. there will there Now itit can can be be seen seen that that itit takes takes Now much capacitance capacitance to to filter filter aa twice as as much twice as itit does does for for aa half-wave supply supply as half-wave full-wave supply supply with with the the same same load. load. full-wave The DC DC pulses pulses happen happen half half as as often often The in the the half half wave wave rectifier; rectifier; so so T T in in the the in formula isis doubled. doubled. formula In other other words, words , for for the the same same In capacitance and and load, load , aa half-wave half-wave capacitance will have have about about twice twice as as rectifier will rectifier much ripple ripple as as aa full-wave full-wave supply. supply . much power supply supply Knowing why why aa power Knowing works can can make make troubleshooting troubleshooting works fu ll-wave supply supply has has aa bad bad easy. IfIf aa full-wave easy. rectifier , the the amount amount of of ripple ripple will will rectifier, double and and the the frequency frequency will will be be cut cut double in half. half. in Take aahum hum bar bar on on aa TV TV monitor, monitor, Take for example. example. IfIf there there are are two two hum hum for bars on on the the screen screen at at once, once, the the bars diodes are are good good because because the the vertical vertical diodes second, the the scans at at sixty sixty times times aa second, scans same as asthe the AC ACline line frequency. frequency. A A full full same wave rectifier puts out two pulses per wave rectifier puts out two pulses per cycle, or or aa ripple ripple frequency frequenc y or or 120 120 cycle, HZ (HZ (HZ= Hertz= Cycles per per secsecHZ = Hertz = Cycles ond). The The problem problem in in this this case case isis aa ond). bad capacitor or too much load on on bad capacitor or hum too bar much load the supply. supply. One Onehum means aa 60 60 the bar means HZ ripple ripple frequency; frequency ; the the output output of of aa HZ half-wave rectifier. rectifier. Since Since most most monimonihalf-wave full wave wave supplies, supplies, the the tors use use full tors problem isis usually usually an an open open diode. diode. problem When taking taking voltage voltage measuremeasureWhen in aa supply supply with with aa VOM, \!OM, the the ments in ments AC input to the rectifier wJI be about AC input to the rectifier will be about the same same as as the the DC DC output output under under the heavyload. load . With With no no load load the the output output heavy will be be about about 1.4 1.4 times times the the measured measured will AC input. input. Keep Keep in in mind mind that that there there isis AC .7 volt volt drop drop at at the the diode diode and and there there aa .7 resistance in in the the transformer transformer isis aa resistance so the the pure pure sine sine wave wave input input windings so windings will flat top under load . A half wave will flat top under load. under A half heavy wave power supply supply output output under power heavy will have have aa much much lower lower output output load will load voltage than than input input because because of of its its voltage efficiency. efficiency. equipment sensitive sensitive to to ripple, ripple , For equipment For regulator must must be be used. used. ItIt takes takes the the aa regulator filtered output output of of the the rectifier rectifier and and filtered uses aa semiconductor semiconductor to to give give aa set set uses output that that stays stays the the same same even even DC output DC the load load changes. changes. Under Under heavy heavy ifif the load the the input input to to the the regulator regulator will will load drop in in voltage voltage and and increase increase in in drop ripple , but but the the ripple ripple must must not not come come ripple, within one one volt volt of of the the output, output, or or the the within output will will start start to to show show ripple. ripple. The The output simplest regulator regulator isis the the LM309 LM309 or or simplest any LM323 five -volt regulator. For LM323 five-volt regulator. For any reasonable input input voltage voltage over over six six reasonable the regulator regulator will will put put out out aa solid solid volts the volts Other five volts volts for for I.C. I.C. logic logic supply. supply. Other five types of regulators use pass transistypes of regulators use pass transisZener diodes diodes and and regulator regulator tors , Zener tors, chips but they need their own chips but they need their own introduction; we'll we'll leave leave that that for for the the introduction; next article. article. next 1 1 , - C The full full wave wave bridge bridge rectifier rectifier (in (in The Figure Four) Four) isis sometimes sometimes used used when when Figure center tap tap winding winding isis not not available. available . aa center It is easy to recognize because uses It is easy to recognize because itit uses four diodes. diodes. Remember Remember the the orientaorientafour thediodes diodescan can make make itit easy easyto to tionofofthe tion identify wires wires when when working working on on aa identify powersupply. supply. For Foreach each half-cycle half-cycle or or power AC only only two two diodes diodes are are conducting. conducting. AC the second second half half of of the the cycle, cycle , During the During the polarity polarity isis reversed, reversed, thinking thinking of of the first half half cycle cycle with with the the top top the first the and the the bottom bottom negative. negative. positive and positive Only diodes diodes BB and and C are are Only conducting because because their their cathodes cathodes conducting and the the anodes anodes are are are negative negative and are the voltage voltage reverses reverses positive . When When the positive. during the the other other half-cycle half-cycle diodes diodes A A during and DDconduct conduct while while BB and and CC are are cut cut and At any any time time there there are are two two diodes diodes off. At off. series with with the the winding. winding. For For aa inin series the negapositive full-wave supply, positive full-wave supply, the negative output output isis grounded; grounded; grounding grounding tive thepositive positive output outputmakes makes aa negative negative the ·oltage power power supply. supply. 'oltage After the the rectifier, rectifier, aa capacitor capacitor filter filter After AND comes along, along, as as shown shown in in DC pulse pulse comes DC Figure Five. The further the cap Figure Five. The further the cap discharges, the the more more ripple ripple voltage voltage discharges,