HP 6177B Operating And Service Manual page 19

r
(
(
se~ies
regulator thus controlling the conduction of
the regulating stage.
4-35 If the voltage programmin<; terminals we:e to
open when the output was open circuited, the out-
put voltage could go up as high as 200 Volts. VR14
anc VR1S serve as a protection against this condi-
tiOD.
\!Vhen the voltage reaches approximately 140-
Volts, the Zener diodes reach their break doVv"TI po-
te;;.:ial anc clamp the output to 140 Volts. This pre-
ven::s break down of other elements which wruld
no:: be able to withstand this very high voltage.
V::::'13 operates in the same capacity should the volt-
a ge programming of the guard supply fail.
4-36 SERIES REGUIATOR
4-37 The series regulator regulates the output of
ths supply by altering its conduction in accordance
with the feedba ck error signal. The circuit cO::1sists
0:
se:-ies regulating transistors. 030 and Q29. and
a base biaSing source, 025. By altering its con-
c.uction in accordance with the error signal applied
to it, the series regulator produces a well regulated
out puc. at the level established by the currem con-
troIs.
4-38 Notice that the series regulato';" is connected
in a common base configuration. This increases
the output impedance of the supply thus givL"1g it
the desired quality of a constant current supply. A
more detailed description concerning the desired
features of a constant current supply is contained
in Application Note 90 available at your local
Hewlett-Packard field office.
4-39 TURN-OFF CONTROL
4-40 The turn-off control circuit prevents reverse
currents from flowing through the series regulator
at turn-off. It accomplishes this by sending a. turn-
down signal to the series regulator via the driver.
4-41 During normal operation of the supply. 031
supplies a bias potential to the driver. At turn-off,
CR30 becomes reversed biased. 031 turns off
quickly, due to the discharging of capacitor C34.
This creates a negative potential at the base of
026. turning off both the driver and the series reg-
ulator.
4-42 BLEED CIRCUIT
4-43 The bleed cirCuit provides a path for 20mA of
current to flow through the series regulator. This
keeps the series regulator biased in its active re-
gion at all times. The bleed current flows from the
+24, Volt unregulated reference through Q1, back up
to the collector of the series regulator, through the
. driver to common
W
where it returns to the -16.2
reference voltage tnrough VR6. Notice that this
4-4
current aoes not go through the loae nor through the
sampling resistors. V,Then the supply is programmed
for zero current. 20mA are flowing through the com-
mon node, however, zero current is actually deli-
vered to the load. Keeping the series regulator
biased in its active region at all times makes
it
possible to have good load regulation even at low
output currents.
4-44 VOLTAGE LIMIT CIRCUIT
4-45 The voltage limit circuit acts to limit the
potential across the output terminals to some preset
lL'11it established by the VOLTAGE limit control. The
circuit conSists of a voltage limit control, RBI,
feedback amplifiers 04, OS, and 06, shunt regula-
tor 07, voltage limit lamp DS2, and lamp driver 08.
Unaer normal operating conditions, 07 is turned ofL
Vlhen the voltage limit tb...reshold is reached. a neg-
ative feedback signal is sent to the feedback ampli-
fiers. The signal is inverted, amplified and fed to
the shunt regulator and lamp driver. This signal
causes :)Oth transistors to conduct. The output
voltage is limited by the amount of conduction of
07. Also, with 08 on, a current path is completed
through 08 and DS2. thus DS2 turns on warning the
operator of the voltage limit condition. Notice that
since the voltage limit section of the supply em-
ploys a separate shunt regulator. the main series
regulator continues to regulate output current even
during voltage limit operation.
4-46 Potentiometer RBI permits adjustment of the
base potential of 04 with respect-to (-) out, thus
setting the point at which the voltage limit threshold
is reached.
4-47 Constant Current Source. The constant cur-
rent source, Q2 and associated components, pro-
Vides a constant programming current through the
VOLTAGE limit control, HB1, thus ensuring linear
programming of the voltage limit circuit. VR2 keeps
the ba se of 02 at a constant voltage ensuring a
constant current output. R6 is selected at the fac-
tory to bia s the emitter of 02 so that the proper pro-
gramming current is obta iDed.
4-48 Tum-On Circuit. C40 and R41 form a long
time constant
Re
network. When the supply is ini-
tially turned on. a turn-on Signal is fed to 07 via
Q6. This limits the output, thus preventing turn-on
overshoot from damaging delicate loads. After the
supply has been on for a short time, C40 charges
up, back biasing CR40 and thus the turn-on circuit
has no effect on nonnal operation of the supply.
4-49 VOLTAGE LIMIT OVERSHOOT CONTROL
4-50 This fast acting circuit protects the load from
initial large, fast rises in output voltage that the
voltage limit circuit would not otherwise be eble to