7-3-1-6. Power
Supply
Relay RLA selects which secondary tap of transformer T1 is applied to the bridge rectifier composed of
diodes D103-D106. Capacitor C101 smoothes the rectified AC voltage. Relay RLA is controlled by the
comparator circuit to provide a higher voltage to the output stage to reduce settling time when an inductive
load is being charged.
7-3-1-7.
Charging Inductor Mode
The gate voltage of TR103 is compared to a reference voltage defined by D107-D109. When the gate voltage
exceeds the reference voltage, TR105 conducts which energizes relay RLA and opto-coupler IC102-B.
Transistors TR104 and TR105 form a Schmidt trigger circuit to reduce relay chattering. Buffer IC101-B
isolates the gate drive circuit from the comparator to reduce rail injected noise.
7-3-2. Voltmeter/Control Logic
The operation of the voltmeter and control logic is described in the following paragraphs. Component
references in this section can be found on Schematic 4300-072 Sheet 2.
7-3-2-1. A-D
Converter/Clock
Most of the A-D converter circuitry is contained in IC's 202 and 203 along with their associated components.
IC202 contains the digital section of a 4½ digit dual slope A-D converter; the analog section is contained in
IC203.
The operating cycle of the dual slope integrating A-D converter is divided into three phases, whose duration is
set by an external clock. The clock is provided by IC201, a self contained 100KHz CMOS crystal controlled
oscillator.
During the first, or Auto Zero phase, the A-D input is internally shorted to analog ground and C203 is charged
to the integrator's output offset voltage.
During the next two phases, the capacitor voltage subtracts from the analog input to cancel the offset.
During the second, or input integrate phase, the input voltage is connected to the integrator via the buffer
composed of IC204 and associated components. IC203 integrates the input for a precise time (10,000 counts
of the clock) by charging
C204 linearly towards the input voltage. The buffer gain is controlled by analog switch IC207 and relay RLK.
The gain of the buffer along with the reference voltage determines the A-D converter's full scale range. The
A-D converter's full scale voltages are tabulated in Table 7-1. The gain is determined by the position of the
voltage range selector on the 4300B front panel.
