HP 3465B - Battery Low-Voltage Detection

Model 3465B
Section IV
the primary and secondary windings of T1 inverts and
forward biases CR18 and CR32. The energy stored in the
primary winding inductance of T1 is transferred to the
output capacitors, C24 and C34, and their loads.
4-76. The cycle ends as Q33 turns on. This results when
either the voltage at the base of Q33 decreases to the point
that Q33 begins to turn on or the energy stored in
transformer T1 goes to zero. Both events cause the
collector of Q33 to begin a positive transition. Pin 3 of
transformer Ti begins a negative transition at the same time
and is applied to the base of Q33 through the feedback
circuit of R81 and C25. This action causes Q33 to saturate
and the cycle bevins again.
4-77. Converter Regulation. Regulation of the dc—to—dc
converter is accomplished by controlling the energy transfer
to the load. The energy transfer to the load is controlled by
the switch, Q33 and the current source I. The magnitude of
I is determined by Q34, R98 and the voltage at the base of
Q34. The base of Q34 is driven by U17. The inverting input
of U17 is connected to ground through R116. A 10—to—7
voltage divider (R117 and R114) is connected to the
non-inverting input of U17. One end of the divider (R117)
senses the constant voltage of the +10V series voltage
regulator. The other end of the divider (R114) senses the
- 7 V output of the dc—to—dc converter. A change in
voltage at the - 7 V output results in an error voltage at the
non-inverting input of U17 and is amplified by U17. The
output voltage of U17 drives the base of Q34 and
regulation of the - 7 V output is achieved. Since the + 11 V
output is the transformer turns-ratio times the - 7 V output,
this output is also regulated.
4-78. + 10 V Series Voltage Reguiatian.
4-79. The temperature compensated zener diode CR17 is
the voltage reference from which the + 10 V reference is
derived. The zener voltage is applied to the non-inverting
input of UI6. A resistor divider in the precision resistor
pack (R75) senses the voltage at the output. A portion of
the voltage is fed to the inverting input of U16. This error
voltage is amplified by U16 to drive Q26. The collector
current of Q26 then provides base drive for the series pass
transistor Q27. To ensure turn-on of the dc—to—dc
converter, the collector, as opposed to the emitter of the
series pass transistor Q27, is connected to the output. The
low collector—to—emitter saturation voltage aids in the
turn-on process of the converter. This ensures start-up for
battery voltages as low as 2 to 3 volts. One advantage to
this configuration is that the + 11 V supply can decrease to
within the collector—to—emitter saturation voltage of the
+ 10 V regulated output and regulation is still maintained.
4-80. Battery Low-Voltage Detection.
4-81. Refer to the power supply schematic. Figure 7-5. The
battery low-voltage detection circuit is comprised of a dif
ferential amplifier, Q36 and Q37. The voltage at the base of
Q36 is set at about + 2.9 V by the voltage divider R139 and
R141. If the battery voltage (+VB) is greater than + 2.9 V,
Q36 conducts and Q37 is off. When the battery voltage
drops below + 2.9 V, Q37 turns on providing base drive for
Q38. When Q38 is on, the base of Q34 is pulled to - 7 V
and Q34 turns off. This action turns the ac-to-dc converter
of the power supply off removing all power supply outputs.
This removes the front panel display indication. To rein
state the display, the OFF/ON switch must be turned OFF
and again ON. The display indication will reappear while
capacitor C51 charges to + 2.9 V. When the voltage on C51
(which is the base voltage of Q36) exceeds the battery volt
age (-1- VB), the circuit deactivates the power supply as pre
viously described and the display indication disappears
again.
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