Tektronix 494A 106 +5 V Voltage ReferenceSupply, Theory of Operation 4g4Ll4g4ApService, Vot

Theory of Operation - 4g4Ll4g4ApService, Vot. 1

Secondary& FanDriveCircuits

(Diagram47)

The secondarycircuits includethe rectifier_filtercir- cuit, which rectifiesand filters the secondaryvoltages; the voltagereferencecircuit, which furnistrei a stiOte and precisereferencefor the regulators;and the regula- tor circuits,which control the voltageand currEntlor the suppliesthat requirepreciseregulation.

The Fan Driver board (A30Al) contains the Fan Driver circuit, which furnishes the appropriatedrive current for the fan motor. lt also containsthe Over- Voltage Protection circuit, which shuts down the +5 V supplyin caseof over-voltage.

Reclifler-Filter Circuits

TransformerT4071 has three secondarywindings. The first furnishescurrent to the +3OOV and +100 V supplies;the second furnishes current to the -7 V, +7 V, and +9 V supplies;and th€ third furnishescurrent to the +17V and -17V suppliesThe. linearregulated supplies (+5 V reference, +5 V, -5 V, +iS t, and -15 V) derive their cunent from the rectifier-filtercir-cuits.

The ac voltage from pins 7 and g of T4071 is qp]i"_q to_ a bridge rectifier composed of CR3053, CR3056,CR3055, and CR3054. The output of this rectifteris filtered,then appliedto the r€mainderof th€ lnstrumentas the +100 V supply.

. The+300V supplyis derivedby stackinga 2X multi- plieron the +100 V suppty. CR3O52,CRiO42,CR1O34, CR1022and associatedcapacitors,composethis cir- cuit.

- The ac voltag€ from pins 9 and 10 supplycurrentto full-waverectifier CR4061/CR4062. The output is fltered and sent to the rest of the instrumentas the *9 V supply. Two othertaps off the samewinding(pins 11 and 12) supply current to the bridge rectitierihat consists of CR4063, CR40S7,CR40S3,and CR4065. The output dividEsacross filter capacitorsC9051and C4051 to become the +7 V and -7V supplies. The +7 V supply is only used on the Main power Suppty board;the -7V supptyis used by other circuitsin ihi instrument.

. .Thethird windingof T407.1(pins13, 14, and 15)fur_ nishes current to full-wave bridge rectifier CR5052, CR5062,CR5065,and CR5055. The output is divided to bEcometh€ +lZV and -17Vsupplies.The -lZV s.upflyl: used onty on the Main power Supplyboard; the +17 V supplyis used bothon the MaineowerSup_ ply boardand elsewherein the instrument.

7-106

+5 V Voltage ReferenceSupply

The +17V is divideddown by a voltagedividerto Zener diode VR6026. The 6.2 V from VR6026is divided across R6029,R6028,and R6023. CR5031providesa regulatedsourceof bias to VR6020after +15 V comes up. The +5 V REF adjustment,R6028,is set by moni- toring the +15v supply and setting it tor a precise +15.00v.

Regulator Circuits

The +15V, -15V,+5V. and -5Vare regulated. Sinceall four regulatorsare basicaltythe same,onlythe +5 V regulatoris described.Significantdifferencesare discussedfollowingthis description.

U2O37A,the voltage regulatorpart of the circuit, compares the +5 v^g. and +5 v sENsE voltages, amplifies the difi€r€nce, and applies the change to drivertransistorQ2023.The changeis amplifiedby this stage and appliedto the base of seriss-passtransistor Q2024to changeits conductionand correct for the ori- ginal changeto the +5 V. The +5 V sensesamplesthe +5 V at a distributionpoint on the Mother board. This signalcompensatesfor voltage(lR)lossesto that point.

U20i!78 is the curent limiterportionof the reguta- tor. The amplifierdet€cts the voltagedifferentialacross the current sensingresistorR2017,which is in seri€s with the output load. When the overloadthresholdis reached,as set by R2017,R2039,R3032,and R3031. U20378 removes bias current from driver transistor 02023 and Q2024. The negativebias on R3031ailows the limiter to rernainactive under short circuit condi- tions.

The +15 V regulatoris identicalto the +5 V regula, tor, except that the ounent limiter, U2O37Dsupplies additionalpositivebias for Q2031when it is not active. The -15Vregulatoris virtuallyidenticalto the +5V regulator. The -5V regulatordiffersfrom the othersin that a driver stage is not required,so the preamplifiers clrives€ries-passtransistorQ5013 directly.

*5 V Over-Voltage Protection Circuit

Zener diode VR1015and SCR 01010 form the over-voltage protection circuit. lf the +5 V supply exceeds +6 V, th€ potential on the gate of 01010 biases it into conduction.This forces the +5 V oupply to ground potential;it remainsat ground potentialuntil the mainspoweris turnedoff andturnedon again.

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