Olympus BH-2 BHT - Page 3

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OlympusBH‐2(BHT/BHTU)Electronics   Revision2    Page3of24
ElectricalpowerfromtheAClineisappliedtotheequipmentviatheAClinecord,whichplugsintoJ101onthebackof
the microscope base.Power transformer T101 converts the AC line voltage (120V or 240V, depending on  which
transformer is installed) to approximately 9V RMS on the secondary
 winding .The output of the secondary winding
feeds into bridge rectifier DB101, whose full‐wave‐rectified output in turn feeds the lamp dimmer circuitry.Bridge
rectifierDB101isnecessarytoprovideDCtothelamp dimmercircuitry,sincetransistorsQ101andQ201inthedimmer
circuitarebipolarjunctiondevices
whichcanconductcurrentinonlyasingledirection.
PotentiometerRV101controlsthelampintensity. WhenRV101isadjustedforminimumintensity(i.e.,thewiperis at
the top end of its travel), the full output voltage of bridge rectifier DB101 is fed to the base of transistor Q201 via
resistor
 R202.Under this condition, transistor Q201 will not conduct since the base‐emitter junction is not forward
biasedandtherewillbenocollectorcurrentinQ201,andthere forenobasecurrentintransistorQ101.Withoutbase
current,transistorQ101willbecutoffandnocurrentwillflowthroughthe
lamp.
AspotentiometerRV101ismovedfromitsminimumintensitypositioninthedirectionofincreasingintensity(i.e.,the
wiperismovedtowards thezero referenceground), thecontrol signalfeedingthebaseoftransistor Q201decreases,
progressivelyforwardbiasingits base‐emitterjuncti on andallowing  ittoincreasinglyconduct collector
current.Since
thecollectorcurrentoftransistorQ201isthebasecurrentoftransistorQ101,Q101inturnbeginstoconductincreasing
amountsofcollectorcurrent, whichflowsthroughthehalogenlamp.Thelowertheamplitudeofthecontrolsignalfrom
thepotentiometer,thehighertheresultinglampcurrentand
correspondinglampintensity.
The overall DC current gain of the lamp dimmer is considerable, and is approximately equal  to the product of the
individualβvaluesoftransistorsQ201andQ101(i.e.,β
OVERALL
~β
Q201
xβ
Q101
).Becauseofthishighvalueforβ
OVERALL
,the
halogen lamp will  be fully illuminated with little base current in transistor Q201, thereby introducing  only a minimal
voltagedropacrossresistorR202.Neglectingthisminimaldrop,thevoltagepresentonthebottomterminalofthelamp
is approximately 0.7V higher than the control signal from the potentiometer, at
 any setting of the potentiometer
(excluding the minimum‐intensity setting, where there is no forward bias on the base‐emitter junction of transistor 
Q201).Thismeansthatthelampvoltagemaybeadjustedfromzeroatoneextremeoftheintensitycontroltonearly
thefulloutputvoltageofDB101
attheotherextremeoftheintensitycon trol.
Figure2isthelampdimmercircuitryshowninFigure1withtheadditionoftransistorQ202,resistorR201,andcapacitor
C202.Thesethreecomponentsprovideacurrent‐limitingfunctionforthedimmer,topreventdamagetothelampin
theeventof
powersurgesontheAClineandtominimizethermalstressesonthelampatpower‐up.
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Figure2–BHT/BHTULampDimmerCircuitry(withCurrentLimiting)