HP 6268B Operating And Service Manual

secondary winding (terminals 5 and 6) on T90. This wind¬
ing is connected to terminals on the rear barrier strip marked
± EXT. CROWBAR TRIGGER. When two or three units
have these windings connected in parallel, all of their crow¬
bars are activated when any one of the crowbars is tripped.
To reset the crowbars in this arrangement, all of the units
must be turned off and then on. Polarity must be observed
when connecting units in this fashion.
4-58 Turn-On Control Circuit
4-59 The turn-on control circuit is a long-time-constant
network which protects the triac and the series regulator
from possible damage during turn-on. When the supply is
first turned on, C35 applies a positive voltage to the anodes
of CR35 and CR36. Diode CR35 couples this voltage to the
cathode of CR74 in the preregulator control circuit to
ensure that it is initially reverse biased. After C35 becomes
sufficiently charged, CR35 becomes reverse biased and the
preregulator control circuit is permitted to fire the triac.
4-60 Diode CR36 performs a similar function for the
series regulator. This diode initially couples a positive
voltage to Q41, which inverts it and applies it to the series
regulator. This negative voltage keeps the regulator cut off
until C35 charges up. Diode CR37 provides a discharge path
for C35 when the supply is turned off.
4-61 Reference Regulator
4-62 The reference circuit is a feedback power supply
similar to the main supply. It provides stable reference
voltages used throughout the unit. All the regulated
reference voltages are derived from dc obtained from full-
wave rectifier CR61 and CR62 and filter capacitor C61.
Zener diodes VR60 and VR61 establish well regulated poten¬
tials of +6.2V and -6.2V with respect to common point
+S, while the regulator circuit establishes a well regulated
potential of +12.4 volts. Resistor R63 establishes an
optimum bias current through the zener diodes.
4-63 The regulating circuit consists of series regulating
transistor Q60, driver Q61, and differential amplifier Q62
and Q63. The voltage across zener diodes VR60 and VR61
is compared to the voltage across resistor Z2J, and any
difference is amplified by Q62 and Q63. The error voltage
thus appearing at the collector of Q62 is amplified by driver
stage Q61 and applied to series regulator Q60 in the correct
phase and amplitude to maintain the +12.4 volt output at a
constant level.
4-64 Diode CR60, connected from voltage divider R66
and R67 to the base of Q61, serves as a turn-on circuit for
series regulator transistor Q60. When the supply is first
turned on, CR60 biases driver Q61 on, thus turning on the
series regulator. When the reference supply reaches normal
output, the base voltage of G61 is sufficient to reverse bias
CR60, thus effectively removing it from the circuit. Capa¬
citor C60, connected across the output of the reference sup¬
ply, removes spikes and stabilizes the reference regulator loop.
4-65 A separate winding on transformer A3T2, diodes
CR53 and CR54, and filter capacitor C44 provide an un¬
regulated + 11 V output. Additional lightly regulated refer¬
ence voltages of —2.4 volts and —4 volts are provided by
diodes CR45 through CR49. Resistor R41 biases the diodes.
Diode CR43 prevents reverse current flow from damaging
the main supply series regulator transistor(s). Diode CR7,
shown on the schematic near current pullout resistors R3,
R4, and R5, protects the zener diodes in the reference cir¬
cuit by providing a path for surge currents that occur
during rapid down-programming.
4-66 Meter Circuit
4-67 The front panel voltmeter and ammeter provide
continuous indications of output voltage and current.
Both meter movements can withstand an overload of several
times the maximum rated output without damage.
4-68 The ammeter, together with its series resistors R101
and R105, is connected across current sampling resistor
A4R123. As mentioned previously, the voltage drop across
A4R123 varies in proportion to the output current. Poten¬
tiometer R101 permits calibration of the ammeter.
4-69 The voltmeter, in series with R103 and R104 and
shunted by R102 and R106, is connected directly across
the output terminals of the supply. Potentiometer R106
permits calibration of the voltmeter.
4-70 Additional Protection Features
4-71 The supply contains several special purpose com¬
ponents that protect it in the event of unusual circumstances.
One of these components is diode A4CR106 (and A4CR107).
Connected across the output terminals of the supply, it
prevents internal damage from reverse voltages that might
be applied across the supply. This could occur, for example,
during Auto-Series operation if one supply were turned on
or off before the other.
4-72 Resistors R108 and R109 limit the output of the
supply if the jumpers between the output buses and the
sensing terminals (+S and —S) are inadvertently removed.
4-73 Diode A4CR105, previously mentioned in the
series regulator description, protects the regulating
transistor from reverse voltages.

HP 6268B Specifications

General

Brand	HP
Model	6268B
Category	Power Supply
Language	English