A1 Main Board Circuits; Rail And Bias Circuits - Agilent Technologies 6631B Service Manual

Principles of Operation - 4
output stages and keep the power supply output off. The OV_SCR* control signal is used to fire the SCR
and keep the power supply output off when an overvoltage condition has occurred.
The EEPROM (electrically erasable programmable read-only memory) chip on the A2 interface board
stores a variety of data and configuration information. This information includes calibration constants,
GPIB address, present programming language, and model-dependent data, such as the minimum and
maximum values of voltage and current. One of the EEPROM storage locations holds a checksum value
which is used to verify the integrity of the EEPROM data. Access to the calibration data in the EEPROM is
controlled by the combination of a password and switch settings on A2S201, located on A2 interface board
(See Chapter 3 "Inhibit Calibration Switch").
The Dual 12-bit DAC converts the programmed value of voltage and current on the bus into the CV_Prog
and CC_Prog signals, which are sent to the CV control circuits in order to control the magnitude of the
output voltage in the CV mode and output current in CC mode. The CV_Prog and CC_Prog signals are in
the 0 to -5 V range, which corresponds to the zero to full-scale output ratings of the dc power supply.
The Quad 8-bit DAC converts programmed information for the following circuits into analog format:
negative offset trim (OS_Trim_Neg), overvoltage setting (OV_Prog), current measurement range select
(Range_Select), and fan speed programming (Fan_Prog). The OS_Trim_Neg signal allows the negative
current control circuit to be calibrated at zero. The OV_Prog signal is applied to the OV detect circuit,
which compares the programmed overvoltage setting with the actual output voltage. The Range_Select
signal selects either the high or the low (20mA) measurement range. The Fan_Prog signal is applied to the
fan speed control circuit in order to speed up the fan as temperature increases, and to slow the fan speed
down as temperature decreases.
The 16-bit ADC in conjunction with a 4x1 multiplexer returns data from the following measurement signals
to the logic array: monitored output voltage (VMon), monitored high-range current (Imon_H), monitored
low-range current (Imon_L), and monitored peak current (Imon_P). All measurement signals are in the
range of 0 to +5V, which corresponds to the zero to full-scale readback capability of the dc power supply.
The 8-channel, 8-bit ADC returns the following signals to the logic array: high-range output current
(Imon_H), high range negative current (Imon_H-), overvoltage (V_Mon), ambient temperature
(Temp_Amb), heatsink temperature (HS_Therm), and output fuse state (Fuse). Five of these signals are for
fan control. The logic array varies the Fan_Prog signal depending upon the ambient temperature, the
heatsink temperature, and the present output voltage and current. The Fuse signal informs the logic array if
the output fuse (F300) is open.

A1 Main Board Circuits

Rail and Bias Circuits

Figure 6-4 shows the transformer, positive and negative output rails and primary and secondary bias
circuits. All bias circuits are located on the A1 pc board. Bias voltage test points are shown in table 6-1 and
transformer wiring diagrams are shown in Figure 3-3.
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