HP 6269B Operator’s, Organizational, Direct Support And General Support Maintenance Manua L page 28

The input capacitor then begins to discharge through
the series regulator, and the voltage across the
regulator decreases until Q20 turns off. The dis-
charge time (typically ½ to 4 seconds) depends on
the voltage and current ratings of the supply, the
main filter capacitor, and the control settings.
Once this recovery time has elapsed, the output
current will return to the level set by the current
controls, and the preregulator will return the volt-
age across the series regulator to the normal 3.5V
level, thus limiting the power dissipated by the
s cries regulator.
4-31 CONSTANT VOLTAGE COMPARATOR
4-32 This circuit consists of the programming re-
sistors (A5R121 and A5R122) and a differential am-
plifier stage (Z1 and associated components). An
integrated circuit is used for the differential ampli-
fier to minimize differential voltages due to mis-
matched transistors and thermal differentials.
4-33 The constant voltage comparator continuously
compares the voltage drop across the VOLTAGE con-
trols with the output voltage and, if a difference
exists, produces an error voltage whose amplitude
is proportional to this difference. The error signal
ultimately alters the conduction of the series regu-
lator which, in turn, alters the output current so
that the output voltage becomes equal to the voltage
drop across the VOLTAGE controls. Hence, through
feedback action, the difference between the two in-
puts to Z1 is held at zero volts.
4-34 One input of the differential amplifier (pin
10) is connected to the output voltage sensing ter-
minal of the supply (+S) through impedance equaliz-
ing resistor R23. Resistors R1 and optional resistor
R110 are used to zero bias the input. If the supply
is equipped with Option 020, resistor R114 and po-
tentiometer R 113 provide a variable input bias that
allows the output voltage to be adjusted to exactly
zero volts when the supply is programmed for zero
output. The other input of the differential amplifier
(pin 1) is connected to a summing point (terminal
A2) at the junction of the programming resistors and
the current pullout resistors R3, R4, end R5. In-
stantaneous changes in the output voltage or
changes in the voltage at the summing point due to
manipulation of the VOLTAGE controls produce a dif-
ference voltage between the two inputs of the dif-
ferential amplifier. This difference voltage is am-
plified and appears at the output of the differential
amplifier (pin 12) as an error voltage which ulti-
mately varies the conduction of the series regulator.
4-3 S Resistor R6, in series with the summing-point
input to the differential amplifier, limits the cur-
rent through the programming resistors during rapid
voltage turn-down. Diode CR7 prevents excessive
current drain from the +6.2 volt reference supply
during rapid down-programming; diodes CR5 and
CR6 prevent excessive voltage excursions from
over-driving the differential amplifier. Capacitor
C2 prevents the gain of the feedback loop from
changing during manipulation of the VOLTAGE con-
trols. Resistor R2 limits the discharge current
through C2. Resistors Z2F, Z2M, and Z2N bias the
differential amplifier; diode CR4 provides tempera-
ture compensation.
4-36 During constant voltage operation, the pro-
gramming current flowing through the programming
resistors (VOLTAGE controls) is held constant be-
cause the value of shunt resistor R3 is factory
selected to allow all of the +6.2 volt reference to
be dropped across R3, R4, and RS. Linear constant
voltage programming is thus assured with a constant
current flowing through A5R121 and A5R122. If the
supply is equipped with Option 020, resistor R111
and potentiometer R 112 allow the programming cur-
rent to be adjusted by varying the bias applied to
the summing point.
4-37 Main output capacitor A3C3 stabilizes the
series regulator feedback loop and helps supply
high-current pulses of short duration during con-
stant voltage pulse loading operation. An additional
output capacitor (C 19), connected directly across
the output bus bars, helps maintain a low ac output
impedance by compensating for the inductive react-
ance of the main output capacitor at high frequencies.
C19 also prevents any spikes in the output from
reaching the load.
4-38 CONSTANT CURRENT COMPARATOR
4-39 This circuit is similar in appearance and op-
eration to the constant voltage comparator circuit.
It consists of the coarse and fine current controls
(A5R123 and A5R124) and a differential amplifier
stage (Z 1 and associated components). As in the
constant voltage comparator, an integrated circuit
is used for the differential amplifier to minimize
differential voltages due to mismatched transistors
and thermal differentials.
4-40 The constant current comparator circuit con-
tinuously compares the voltage drop across the
CURRENT controls with the voltage drop across the
current sampling resistor, A4R123. If a difference
exists, the differential amplifier produces an error
signal which is proportional to this difference.
The remaining components in the feedback loop
(mixer amplifier, error amplifiers, and the series
regulator) function to maintain the voltage drop
across the current sampling resistors, and hence
the output current, at a constant value.
4-41 One input of the differential amplifier (pin 7)
is connected to the output bus through impedance
equalizing resistor R20 and is zero-biased by R21
4-5
TM 11-6625-2958-14&P