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Adjustable 3-Terminal Regulator Low-Cost Battery Charging Systems
Adjustable 3-Terminal Regulator Low-Cost Battery Charging Systems
With introduction LM317 3-terminal adjustable regulator becomes relatively easy design high-performance low-cost battery charging systems Even single battery cells charged this regulator which adjustable down internal protection circuitry used limit charging current well protect against overloads output voltage easily adjusted multiple voltage chargers made ability accurately adjust output voltage LM317 makes especially attractive constant voltage battery charging applications Batteries most quickly charged ``constant-voltage'' charging circuits however close control charging voltage necessary prevent overcharging especially with nickel cadmium cells internal protection circuitry LM317 helpful protecting against accidental overload conditions commonly occurring charging systems INTERNAL CURRENT LIMIT peak charging current output current controlled internal current limit LM317 This current limit will work even battery connected backwards output charger Should fault condition exist extended period time thermal limiting circuitry will decrease output current protecting regulator well transformer constant voltage charger circuit shown Figure output voltage with resistors given VOUT
National Semiconductor Linear Brief August 1976
generated LM317 also used limit peak charging current partially charged battery value other than regulator current limit With circuit output impedance Including feedback loop decreases value resistor needed particular output impedance reducing cost power dissipation example with gelled electrolyte battery regulator give output Nominally battery discharged about making output impedance limiting charging current start charging rather than internal current limit regulator With fully discharged battery under short circuit conditions peak output current still LM317K with resistor dissipating opposed resistor were used directly series with battery Resistor included provide ``topping-up'' current charged battery This regulator configuration provides some other important features charger input power removed fully charged battery connected charger output there damage Under these conditions about current will drawn divider Since there ground connection LM317 regulator very little current flows through LM317 this respect LM317 differs from other 3-terminal regulators which damaged applying power output terminal with input open-circuited battery connected backwards LM317 will current limit thermal limit normally protecting charger ZOUT
DECREASING CURRENT LIMIT Adding single transistor used decrease current limit charge shown Figure
8484
FIGURE Constant Voltage Charging Circuit Since cost applications filter capacitors used output rectifier battery only charged peaks sine wave This requires peak output voltage from transformer least greater than battery voltage plus However little cost premium should result since average current from transformer lower than capacitive input filter circuits Optional resistors used further control charging characteristics Resistor controls output impedance charger allowing ``taper-charge'' characteristic
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FIGURE Constant Voltage Charger with Peak Current Limiting
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C1995 National Semiconductor Corporation
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RRD-B30M115 Printed
Adjustable 3-Terminal Regulator Low-Cost Battery Charging Systems
Resistor senses output current turns when IOUT equals about Transistor pulls adjustment terminal negatively decreasing output voltage controlling output current limitation this circuit that does work direct short circuits output voltage must above about external current limiting active internal current limit LM317 course still operative This usually problem since batteries charge above very quickly Resistors protect regulator transistor both direct short circuits reverse battery connections
simple constant current charger type battery shown Figure resistor between adjustment terminal output regulator sets output current IOUT
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FIGURE Constant Current Charger Current anywhere between appropriate resistor choice Current regulation very tight current level since only flows adjustment terminal This circuit also immune damage from shorts reverse battery connections input voltage regulation should also about times battery voltage plus UNIQUELY SUITED ability adjust output LM317 3-terminal regulator makes uniquely suited battery charging systems Little been included about charging specific types batteries since characteristics charger should matched battery These charger circuits although very simple perform well They easily modified voltage current even temperature coefficient making divider string temperature sensitive More complex chargers made since output LM317 easily controlled driving adjustment terminal Finally chargers inherently protected against overloads fault conditions
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FIGURE Charger with Battery Loading when Power ``OFF'' illustrated Figure float standby applications desirable remove loading from battery when input power ``OFF When power ``ON saturated grounding voltage setting divider circuit works similar manner charger circuit Figure When power ``OFF open eliminating loading battery separate pair current diodes necessary bias rather than power bridge rectifier tied output bridge reverse current flow through LM317 would keep ``ON'' loading battery
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