OHAUS SCOUT Service Manual

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2-5CHAPTER 2 THEORY OF OPERATION2.3.5 Full Bridge Weight A/D Converter (Cont.)Instead of comparing the sawtooth against a reference signal, this circuit compares itagainst a ramp signal generated by transistor Q2, resistors R2, R3 and capacitor C12.At time zero, capacitor C12 starts to charge through resistors R2 and R3. The outputof the ramp circuit starts high and goes low. The output of integrator U5B on pin 7 startslow and goes high. When the ramp signal from the junction of resistors R2 and R3 whichis input to pin 3 of U3A crosses the sawtooth signal from integrator U5B which is on pin2 of comparator U3A, the comparator output on pin 1 of U3A changes from high to low.The microprocessor pin 15 then sends a signal to the electronic switches U4B, U4C,U4D and U4E and turns on Q2. With Q2 turned on, capacitor C12 is discharged andthe ramp is reset. The ramp signal goes from the high point to the crossing point of thesawtooth of the integrator.2.3.6 Half Bridge EmulatorThe primary function of the Half Bridge Emulator is to simulate a full bridge circuit byusing two 1K resistors R45 and R46 to form the other half of the bridge when two StrainGauges are used on the Transducer. The output of the half bridge Transducer isconnected to the input of integrator U5B, pin 6. All other circuit functions of the FullBridge Weight A/D Converter apply.2.3.7 Microprocessor U1Microprocessor U1 processes the A/D converter signals, controls electronic switchesU4, accepts calibration data from EEPROM U6 and sends control signals to the displaydriver U7. Microprocessor U1 also resets an internal counter for the next result,monitors low supply voltage and provides an output signal to the display driver for lowbattery indication on the display. Reset circuitry for the microprocessor consists ofresistor R17, capacitor C6 and diode D5.2.3.8 EEPROM U6Integrated circuit U6 is an EEPROM which stores the necessary calibration data for themicroprocessor U1. The data is stored during manufacturing of the balance.2.3.9 Display Driver U7The Display Driver U7 converts the serial display information from microprocessor U1to the individual signals which drive the individual segments of the LCD display DS1.Resistors R25 and R26 determine the contrast of the display and the viewing angle.