HP 6268B Operating And Service Manual

voltage differentials due to mismatched transistors or
temperature differences.
4-35 The constant-voltage comparator compares the
voltage drop across the VOLTAGE controls with the sup¬
ply's output voltage. If a difference exists, it produces an
error voltage proportional to this difference. This error
signal alters the conduction of the series regulator, which
changes the output voltage until it is equal to the voltage
drop across the VOLTAGE controls. Hence, through feed¬
back action, the difference between the two inputs to Z1
is held at zero volts.
4-36 One input of the differential amplifier (pin 10) is
connected to the output voltage sensing terminal of the
supply (+S) through impedance equalizing resistor R23.
Resistors R1 and optional resistor R110 zero bias the input.
If the supply is equipped with Option 020 or 040, resistor
R114 and potentiometer R113 provide a variable input bias
that allows the output voltage to be easily 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 the summing point (terminal A2) at the
junction of the programming resistors and current pullout
resistors R3, R4, and R5. Instantaneous changes in the output
voltage or changes in the voltage at the summing point due
to changes in the VOLTAGE control setting produce a
difference voltage between the two inputs of the differential
amplifier. This difference voltage is amplified and appears
at the output of the differential amplifier (pin 12) as an
error voltage which ultimately varies the conduction of the
series regulator.
4-37 Resistor R6, in series with the summing point
input to the differential amplifier, limits the current flow¬
ing into the differential amplifier during rapid voltage turn¬
down. Diode CR7 prevents excessive current drain from
the +6.2 volt reference supply during rapid down-program¬
ming, Diodes CR5 and CR6 prevent excessive voltage
excursions from overdriving the differential amplifier.
4-38 During constant-voltage operation, the program¬
ming current flowing through the programming resistors
(VOLTAGE controls) is constant because the summing
point is held constant at zero volts by feedback action
and virtually all of the reference supply current flowing
through pull-out resistors R3, R4, and R5 flows through
VOLTAGE controls A5R121 and A5R122. Linear constant-
voltage programming is thus assured. Resistor R3 serves as
a trimming adjustment for the programming current flowing
through A5R1 21 and A5R1 22. If the supply is equipped
with Option 020 or 040, resistor R111 and potentiometer
R11 2 allow the programming current to be adjusted over a
narrow range around its nominal value. In practice, this
adjustment sets the power supply output to exactly the
maximum rated voltage when programmed to that level.
4-39 Main output capacitor A3C3, connected across the
output terminals of the supply, stabilizes the series regulator
feedback loop and helps supply high-current pulses of short
duration during constant-voltage operation. An additional
output capacitor (Cl 9) is connected directly across the
output bus bars to maintain a low ac output impedance by
compensating for the inductive reactance of the main output
capacitor at high frequencies. C19 also helps to minimize
output spikes.
4-40 Constant-Current Comparator
4-41 The constant-current comparator is similar in
appearance and operation to the constant voltage compara¬
tor. It consists of programming resistors A5R1 23 and
A5R124, differential amplifier Z1, and associated components.
4-42 The constant-current comparator circuit compares
the voltage drop across the CURRENT controls with the
voltage drop across current sampling resistor A4R123. If a
difference exists, the differential amplifier produces an
error signal proportional to this difference. The remaining
components in the feedback loop (amplifiers and the
series regulator) maintain the drop across the current
sampling resistor, and hence the output current, at a
constant value.
4-43 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 and optional resistor
R117. Its other input (pin 4) is connected to the summing
point (terminal A6) at the junction of programming resistors
A5R123 and A5R124 and current pullout resistors R30 and
R31. Instantaneous changes in the output current due to
load changes or changes in the voltage at the summing
point due to changes in the CURRENT control setting
produce a difference voltage between the two inputs of the
differential amplifier. This difference voltage is amplified
and appears at the output of the differential amplifier (pin 6)
as an error voltage which ultimately varies the conduction
of the series regulator.
4-44 Resistor R30 serves as a trimming adjustment for
the programming current flowing through A5R1 23 and
A5R124. If the supply is equipped with Option 021 or
040, resistor R115 and potentiometer R116 allow the
programming current to be adjusted over a narrow range
around its nominal value, and resistor R118 and potentiometer
R119 provide a variable input bias to allow the output
current to be easily adjusted to exactly zero when the supply
is programmed for zero output. Diode CR21 limits excessive
voltage excursions at the summing-point input to the
differential amplifier.
4-5

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English