This section describes in detail the operation of the main circuits of the Model 4300B. The reference
designators used in this section refer to the drawings included at the back of this manual.
7-3-1 Current
Source
The operation of the Current Source is described in the following paragraphs. All components in this section
can be found on Schematic 4300-072 Sheet 1.
7-3-1-1. Reference
IC103 along with resistors R116-R118 and RV101 form a temperature-stabilized voltage reference. IC103 is
a precision 6.95 volt ovened zener diode. The zener current is limited by resistor R116. A voltage divider
composed of R117, RV101 and R118 divide the zener voltage down to 1.0000 VDC. Capacitor C103
removes any noise riding on the reference output.
7-3-1-2. Error
Amplifier
The error amplifier is composed of C102, IC101A, R104, R103, RLB-2 and TR103. IC101A provides high
DC gain to drive transistor TR103 via resistors R104 and R103. C102 provides local AC feedback around
IC101A for stability. TR103 provides base current to the output stage. Relay contact RLB-2 is closed when
the test current is turned off which removes the gate drive to TR103.
7-3-1-3. Output
Stage
The output stage, composed of TR101, TR102, R101, R102, R126 and RLB-1 provides current gain capable
of sourcing 10 amps. Base drive for transistors TR101 and TR102 is provided by the error amplifier.
Resistors R101 and R102 equalize the emitter currents in the two output devices. Relay contact RLB-1 is
closed when the test current is turned off, which connects resistor R126 across the output terminals to reduce
the output voltage to less than 20mV.
7-3-1-4. Range
Switching
Range selection is accomplished by selecting one of the precision current shunts R105, R107, R109, R111,
R113 or R115. Relays RLC thru RLH select the appropriate combination of shunts. Only one range relay is
ever energized at any one time. Resistors R106, R108, R110, R112, and R114 along with potentiometers
RV102 through RV105 are used to trim the shunts to produce a precise current output.
7-3-1-5. Protection
Circuitry
The protection circuitry is composed of diodes D101, D102, opto-isolator IC102-A, R123 and R124. The two
diodes provide a discharge path for inductive loads. D102 conducts when the back EMF from the load is
greater than 5 volts which turns on the opto-coupler through R123 and R124. This circuitry is used to
determine the Disconnect Status LED condition. A Metal Oxide Varistor (MOV) installed across the voltage
input terminals is used to protect the A-D converter by clamping voltages that exceed 35 volts.
