HP 6268B Operating And Service Manual

4-25 When the negative threshold voltage is reached,
transistor Q72 is turned off and Q73 is turned on. The
conduction of Q73 allows capacitor C71 to discharge rapidly
through pulse transformer T70, generating a firing pulse
across the secondary of T70. Diode CR88 blocks any posi¬
tive overshoot.
4-26 The control circuit is reset once every 8.33 milli¬
seconds when the rectified ac voltage at the junction of
CR77, CR78, and CR79 (TP82) forward biases diode CR78.
Summing capacitor C70 is then allowed to discharge through
CR78. Diodes CR74 and CR75 become reverse biased at
reset, allowing Q72 to turn on. Consequently, Q73 is turned
off and capacitor C71 charges up through R79 at a compara¬
tively slow rate until the collector voltage of Q73 reaches
approximately+11 volts.
4-27 Overvoltage Limit Circuit
4-28 Under normal circumstances, Q70 in the overvoltage
limit circuit is biased off by the equal voltages on its base
and emitter. Diodes CR70 and CR71 are forward biased
by a small current through R71 to develop Q70's base
voltage. R72, connected between Q70's base and the
supply's negative output, shunts an amount of current
away from the diodes that is proportional to the supply's
output voltage. When the output voltage reaches approxi¬
mately 120% of its rated maximum, the diode current
falls to zero, biasing Q70 on. When Q70 conducts, it disables
the preregulator by holding CR74 and CR75 reverse biased
to prevent Q72 and Q73 from generating triac firing pulses.
4-29 Series Regulator and Driver
4-30 Depending on the model, the series regulator is
composed of four, six, or eight transistors connected in
parallel. These transistors serve as the series element that
provides precise and rapid control of the output. The
series transistors are controlled by driver A4Q102. Thermal
switch A4TS101 turns off the series regulator transistors
by opening if the heatsink temperature exceeds approxi¬
mately 230° F. This feature protects critical components
from the excessive temperatures that could occur if cooling
fan A4B1 failed. Diode CR50 provides a discharge path
for the output capacitors when the supply is rapidly down-
programmed. Resistor R57 limits the discharge current
through the diode and through error amplifier A4Q101.
Diode A4CR105 is connected across the regulator circuit to
protect the series elements from reverse voltages that
could develop if one supply is turned on or off before the
other during parallel operation.
4-31 Short-Circuit Protection
4-32 The short-circuit protection circuit protects the
series regulator against damage due to a simultaneous full-
voltage full-current condition as might occur if the output
were shorted white the controls were set to deliver a high
output voltage and current. If this occurs, the increased
voltage across the series regulator turns Q20 on and shuts
off the preregulator. The conduction of Q20 puts R26 in
parallel with the current controls to limit the output current
to less than 10% of the supply's rating. The preregulator
shuts off within 10 milliseconds after the short circuit is
imposed. Then the input capacitor begins to discharge
through the series regulator and the voltage across the
regulator decreases until Q20 turns off. The discharge time
(typically 1/2 to 4 seconds) depends on the voltage and
current ratings of the supply, the size of the main filter
capacitor, and the control settings. Once this recovery time
has elapsed, the output current returns to the level set by
the current controls, and the preregulator returns the
voltage across the series regulator to its normal 3.5-volt level.
4-33 Constant-Voltage Comparator
4-34 The constant-voltage comparator consists of
programming resistors A5R121 and A5R122, differential
amplifier Z1, and associated components. An integrated
circuit is used for the differential amplifier to minimize
4-4

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English