Play Meter

applications bulky components and applications components and . too much bulky heat become problems too much heatcanbecome Some of this be curedproblems. by adding Some of this can be cured by adding a pass transistor. a pass A transistor. transistor basically controls A transistor basically controls current flow. There are two main current flow. There are two have types, NPN and PNP. Bothmain have Bothand PNP.base types, three NPN leads:and emitter colthree emitter base current and colflow lectorleads: . The base controls lector. The base controls current flow . between the collector and emitter between the collector and emitter. When current flows through the base When current flows through the base is emitter junction, the transistor emitter junction, the transistor "turned on" and allows current isto "turned on"collector and allows current flow from to emitter . Intoan flow from collector to emitter. In NPN transistor, current begins to an flow NPN transistor, begins+to.7flow is at about volts when the basecurrent when base istoat the aboutemitter. +.7 volts with the respect The with respect emitter. The in relation collector musttobethe positive collector be positive relation to both .must A PNP transistorin works the tosame both. way A PNP transistor works the except that the voltages same way except that the voltages are negative . are negative. Looking at Figure Two a + 5. 7 a is+5.7 Figure Two Looking Zeneratregulator circuit ronvolt volt Zener regulator circuit is con- V + UNREGULATED SUPPLY nected to the base of an NPN nected to The the collector base of is an NPN transistor. connected transistor. The collector is connected to the unregulated positive supply toand the the unregulated positive emitter is the outputsupply of the and the mess emitter is the of thebe whole . The baseoutput will always whole will always beat at 5 . 7mess. volts The and base the emitter will sit at55.7 volts and the emitter will sit volts. Say a heavy load is addedatto 5 the volts.output Say a, heavy load increase is added to the this will the output, this will increase the current flow from base to emitter current from baseturn to on emitter makingflow the transistor harder making the transistor turn on harder to keep the output at five volts . This tocircuit keep can the output five more volts. power This handle at much circuit handlealone muchand more power than can the Zener it dissipates than Zener alone and it dissipates heatthe only under load since the Zener heat onlyneeded under load thereference Zener is only as a since voltaQe is and onlydraws needed as aa voltage onlv few MAreference . and This drawsregulating only a few MA. isn 't quite scheme This regulating isn't quite under heavy loads perfect because scheme perfect because under heavy loads the higher base current in the the higher base current the transistor reduces the in current transistor reduces currentits through the Zener,thelowering through the Zener, its voltage slightly. This oflowering course shows voltage slightly. This of course shows up at the output. Care must also be up at the output. Care must also be NPN TRANSISTOR OUTPUT E C + 12 V +12.7 V Z!N~ft. R ZENE 11.·1'1 12.7 V -- "': Figure Three Figure Three UMRU.. '/-t' UNREG. 51.JpPLY V+ SUPPLY NP"' c c. NPN OIJIPUI TPUT 0 OU E - OP AMP --=---.. Figure Four Figure Four it£•A.E~£ NCE JI"REFE'\fOREL.or~'-~ E VOLTAG taken not to short the output to taken notortotheshort the output to ground transistor will destroy ground itself. or the transistor will destroy itself. For a very stable output under For a very outputreference under different loads,stable the Zener different loads, the Zener must be isolated from,reference the pass must be isolated passbe amp, the IL can transistor. An OPfrom transistor. An error OP amp IL can be a used as an amplifier. It has used as an error amplifier. It has a very high input impedance so that very inputthe impedance so that A will high not affect Zener reference. will not affect the Zener reference. voltage dropping network is usedAto voltage is used to get a dropping sample ofnetwork the output voltage getand a sample the output voltagerefurther ofisolate the Zener and further reTheisolate samplethe fromZener the output ference. ference. The sample from the output and reference voltage are fed to and reference are amp. fed The to separate inputsvoltage of the OP separate inputs of the OP amp. The difference between the two is difference two is amplified between and used the to correct the amplified and used to correct the base current in the transistor to obtain base the transistor the current desiredinoutput voltage.to obtain the desired output voltage. If a pot is added to the resistor If a pot is added, to resistorto dropping network it isthepossible dropping network, it is possible to trim the output to the exact voltage trim the output to the exact voltage desired . desired. The result is a very stable variable The result stable variable supply usedis ainvery many video game supply used .inFormany videoreliability, game applications increased applications. For increased reliability, current limiting and thermal shutcurrent and thermal , but theshutparts down limiting can be added down but the parts out of hand for normal countcan can be getadded, can get .out of hand for normal count applications applications. Some manufacturers use one of Some manufacturers use one !C's of the many precision regulator theavailable many to precision IC'sof reduce regulator the number available to reduce the a number of components and give high quality components give alogic. high quality sensitive Most of supply for and supply sensitive Most of protection and these for chips have fulllogic. these full external protectionresistors and only chips needhave a few only few external resistors and need a passatransistor for high current and a pass transistor for high current capability . capability. Servicing can be done in the field Servicing be ,done in themust fieldbe with only acan VOM but care with onlynot a VOM, but care themust logicbeby taken to damage taken notonto adamage the logic by turning supply with a shorted turning on . aAsupply shorted Zener with diodea will check regulator regulator. diode will using checkan the sameAasZener any other diode the same as any other silicon diode using ohmmeter . A good diodean will ohmmeter. good silicon diode will show con A resistance with the positive show resistance with the positive on the cathode , negative to the leadcon lead on the cathode, negative anode , and infinite resistancetointhe the anode, and infinite resistanceare in easily the other direction. Transistors other direction. Transistors are easily tested out of circuit by checking the tested out of. ofcircuit checking eacn by junction in the both resistance resistance .of junction both directions . Aeach transistor actsinlike two directions. transistor acts tolike two series , cathode cathode diodes in A diodes in series, to anode cathodefor for NPN type , cathode anode to forPNP NPN type, anode to anode for type . There is infinite resistance PNP type. There is infinite resistance between emitter and collector in both between emitter collector indidirections . Anyand leakage at inat!both directions. Any leakage at indiwill cates a bad trapsistor. Theatlbase cates a bad The base will show the transistor. same reading to either show theor same reading to either emitter collector , and reversing the emitter collector, andopen reversing theat will show an circuit leads or leads show. an open circuit at eachwill junction eachSome junction. indications of a bad regulaSome a bad in a indications video gameofare humregulabars on tor torthe in ascreen, video game on , humareinhum thebarsaudio the screen, hum in the audio, characters flashing on the screen, characters thelines screen, large, oronwavy at the picture tooflashing picture too large, or wavy lines the edge of the screen . Always beatcareful edge of the screen. regulators Always be careful when replacing because when regulators because the replacing wrong supply voltage can thedamage wrong supply voltage can some components . damage some components.