HP 8133A Service Manual page 160

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7-28 Theory of
Operation
to damp ripple ,voltages. At high line voltage tolerance only Q1 is on
and thus only the low tapping path is activated.
Limitation.
Feedback signal R + 15 and
output current sensing resistor. it is compared by U1A. It achieves
control in a similar manner to voltage regulation, but limits the output
current when active. Diodes CR1 and CR2 decouple operation during
the most positive control voltage, therefore the limiting function is
active.
When below the current limit, the voltage of U l a pin 1 is close
voltage control. The foldback characteristic is achieved by lowering
the threshold. A s the output voltage decreases, so does the R + 15
voltage in current limiting operation. At the output, the short circuit
minimum foldback current limit is reached and R + 15 is almost zero.
The output voltage of the operational amplifier U2
and R19. The gate signals G+5H and G + 5 L are controlled by voltage,
rather than current, via R17 and R18. Therefore at differential stage
the active tapping path. At the same time the conducting transistor
switches the other tapping path off. CR7 and CR8 lower the maximum
output voltage, to prevent saturation of U2 at the highest gate voltage
and conduction points. R73 prevents gate oscillation.
There is a separate voltage signal, -4.5VSENS, from the motherboard,
that compensates for voltage drops of up to 0.1V. R34 in conjunction
with the -4.5V signal prevents the output voltage rising more than
approximately 0.1V if the -4.5VSENS line is open circuit.
15V represent the voltage drop of the
buffered by Q6
is