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9 TECHNICAL TOPICS By Ed Ed Anderson Anderson of of Electronic Electronic Service Service By Voltage regulators Voltage regulators Last month's month's Update Update covered covered the the Last basics of power supplies. Unfortubasics of power supplies. Unfortunately, I.C. I. C. logic logic needs needs aa very very stable stable nately, DC supply, free from ripple and DC supply, free from ripple and transient voltage voltage spikes, spikes, even even under under transient changing load. load. Simple Simple regulators regulators aa changing can supply supply aa set set DC DC voltage voltage but but are are can compensate for rapid or unable to unable to compensate for rapid or widely changing changingloads loads on on the the output. output. widely The more more sophisticated sophisticated regulators regulators The the supply supply line line and and can sample sample the can quickly correct correct for for the the slightest slightest error. error. quickly Some have have thermal thermal shutdown shutdown and and Some high current current overload overload protection. protection. high The simplest simplest and and most most widely widely The is the the LM LM 309 309 integrated integrated used is used regulator (See (See Figure Figure One). One). ItIt comes comes regulator in aa TO-3 T0-3 case case and and has has an an input input lead, lead, in an output output lead, lead , and and case case ground. ground . It It an has full full protection protection and and can can handle handle has over one one amp amp of of load load with with aa heat heat over sink. The The newer newer LM LM 323R 323R isis nearly nearly sink. and can can supply supply over over three three identical and identical amps. Other types are available for amps. Other types are available for positive or ornegative negative 5V, 5V, 12V, 12V, 15V 15V or or positive 20V, low low and and medium medium power power 20V, applications. applications. There are are also also ways ways of of increasing increasing There the current current capabilities capabilities of of these these the regulators . If a low resistance is regulators. If a low resistance is placed in in parallel parallel with with the the regulator regulator placed some of of the the current current flow flow may may be be some routed around around it. it. This This only only works works for for routed some applications applications because because ifif the the load load some TRANSFORMER becomes too too light, light, the the resistor resistor will will becomes raise the the output output voltage voltage above above its its set set raise level and and introduce introduce ripple ripple from from the the level unregulated supply. supply. unregulated The simplest simplest regulator regulator isis aa Zener Zener The diode . As you know, a diode only diode. As you know, a diode only in one one direction, direction, but but conducts current currentin conducts every diode has a breakdown '{Oltage every diode has a breakdown voltage or P.I.V. P.I.V. (Peak (Peak Inverse Inverse Voltage) Voltage) or common power rectifiers rating . For rating. For common power rectifiers this may may be be in in the the area area of of 200 200 volts, volts, this and not not of of any any concern concern in in low low and voltage supplies. supplies. voltage Zener diode, diode, on on the the other other hand hand A Zener A manufactured to to have have aa precise precise isis manufactured breakdown voltage. voltage . Valves Valves range range breakdown three volts volts on on up. up . from three from we put put aa twelve-volt twelve-volt Zener Zener diode diode IfIf we in series series with with aa resistor resistor and and hook hook in them across across aa twenty-volt twenty-volt DC DC them unregulated supply, supply, with with the the anode anode unregulated hooked to to the the negative negative side, side, the the hooked diode conducts conducts current current at at its its Zener Zener diode breakdown voltage voltage of of twelve twelve volts. volts. breakdown The resistor resistor limits limits the the amount amount of of The current drawn drawn through through the the diode. diode . current Zener diodes diodes are are rated rated in in watts watts but but Zener the current current with aa little little calculation calculation the with rating can can be be found. found. It's It's as as easy easy as as rating P =IE (Power (Power equals equals current current times times P=IE common rating rating might might be be voltage). A A common voltage). one watt. watt. For For aa one one watt watt twelve-volt twelve-volt one Zener, 11 watt watt= amps xx 12 12 volts, volts, Zener, = II amps is maximum maximum current current rating rating or or there is there .. J~~Dr :01 -r--1-1 LM309 ,. .I.. I D1 117 VAC LM309 02 D2 C1 I 8000uF 8000µF T Figure One One Figure ~ +5VDC +5voc C2 C2 F 250p 250µF + 20 V DC 1 50 mg 4 km ( 1 WATT) -0 + 12 v 11. VOLT vo1--r 12 ZENER.. ZENER 1)\00£ DIODE W"'TT')) (( 11 WATT Figure Two Two Figure 12 amp amp or or 83 83 milliamps. milliamps. To To be be 11=1/ = 1/12 safe, the the resistor resistor should should only only allow allow safe, 50 MA MA to to flow flow through through the the Zener. Zener. 50 The resistor resistor should should be be the the same same The rating as as the the Zener: Zener: 11 watt. watt. rating With ohms ohms law law the the valve valve of of the the With current limiting resistor is found . The current limiting resistor is found. The voltage across across the the resistor resistor is is 20v20vvoltage 12v== 88 volts. volts. Ohm's Ohm's law law says says voltage voltage 12v equals current current times times resistance. resistance. 88 equals volts = .05 amps x R ohms, so RR = = volts = . 05 amps x R ohms, so 4000 ohms. ohms. 4000 Now we we have have aa regulated regulated twelvetwelveNow volt source source at at the the anode anode of of the the Zener Zener volt than can supply up to 50 MA {:,f than can supply up to 50 MA of current. IfIfthe the current current drawn drawn isis higher higher current. than that, that, the the voltage voltage will will fall fall below below than Zener voltage voltage of of the the diode diode and and will will Zener show ripple from the unregulated show ripple from the unregulated full current current supply. This This circuit circuit draws draws full supply. or from the unregulated supply, load or from the unregulated supply, load no load load and and in in medium medium power power no