HP 70427A User Manual page 511

Block Diagram
Controller, I/O Interface and Power Supply Assembly A6
blowing out fuses. At module power-up, a pulse of current is generated in
each of the six supplies to force the output enable op-amps into the "normal"
power supply "on" condition. These op-amps are configured as bi-stable
flip-flops, using positive feedback. They will remain in the "normal" state
until an over-current fault occurs. If the maximum current is exceeded, the
flip-flop will go to the "overload" state, and shut down the supply. The
supply will remain off until a new start-up pulse resets the flip-flop. Power
must be cycled on the module for power to be restored.
The over-current protection circuit generates an analog voltage which is
proportional to the output current, and used to compare with the over-current
trip point. This voltage is equal to 5 A/volt, and is available via the
diagnostic multiplexer system.
The power supply output voltage is monitored for each of the eight power
supplies. The comparator thresholds are set to 0.66 of the nominal output
voltage. If any one of the eight supplies falls below the comparator
threshold, the "voltage good" flag is set low, and the microprocessor turns on
all of the front-panel status lights.
13-76 HP 70427A/HP 70428A User's Guide