Samsung SCH-1900 Service Manual

Also see for SCH-1900: User guide Owner's manual

Samsung SCH-1900 Service Manual Manual pdf 21 page image

Circuit DescriptionSamsung Electronics 4-114-7-1 Input Filter and RegulatorThe AC input converts to a high-voltage DCthrough overcurrent protector F1 and F2, surgeprotector VA1, noise eliminator CO1 and LF1, andregulator D1 and C3.4-7-2 Switching Controller andTransformerDC voltage is biased to the gate of Q1 throughR3-R7, resulting Q1 to be on. When the current issupplied to the primary winding of T1, the gatevoltage of Q1 is increased.The voltage detected by R2 is biased to the base ofQ2 through R10 and C7. It turns Q2 on. The gatevoltage of Q1 is reduced so that Q1 turns off. Theflyback current transfers to the secondarywinding.4-7-3 FeedbackThe transferred current is rectified and flattenedby D20, C21 and C22. If there is no batteryinstalled or Q2 turns off, the current is biased toU21 through R26 and R27, then applied to OP1according to the value set through R27-R29. Itcontrols the bias voltage of Q2 base. It causes theoutput voltage to be 8.0V.4-7-4 Battery ChargingWithout battery installed, Q24 turns on. It turnsLED off. The output status of U23 and U24 are asfollows:OUT-A OUT-B Q21 Q20U23 L LU24 L L OFF OFFWith battery installed, and when the outputvoltage of U23A pin 3 is smaller than pin 2, theoutput status of U23 and U24 are as follows. Q24turns off, the LED is lit in red.OUT-A OUT-B Q21 Q20U23 L LU24 H H OFF OFFWhen the battery voltage of pin 3 of U23A isgreater than pin 2, Q21 turns off, the output ofU23A becomes low, the output status of U23 andU24 are as follows:OUT-A OUT-A Q21 Q20U23 H HU24 H H ON ONWhen Q20 and Q21 turn on, the battery is charged.The output voltage detected by R21 is increaseduntil the output voltages of U22A pin 6 and pin 5are the same level. When the output voltage of pin5 becomes greater than that of pin 6, U22 becomeshigh, resulting U21 reference voltage to be high.When the OP1 current increases, Q2 turns on, Q1turns off, and the output voltage is reduced.The battery voltage is detected by U23B pin 5.U23B compares the voltage with reference voltagein pin 6, and remains low status until the voltagereaches the reference voltage. It causes U24A to beremained high.4-7 Travel Charger