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SECTION
3
THEORY OF OPERATION
3.1 POWER CIRCUIT (A2)
Refer to the schematic diagram (figure 5-2) in section 5 for the
following discussion.
The
power circuit
is found
on
the
A2
assembly circuit board.
3.1.1 Raw Supply Section
Power is applied to the unit via the power switch, line fuse, and
power transformer Tl.
Two raw supply voltages (V and 2V applied
to the collectors of power transistors Q1 and 02 respectively)
are provided by the bridge rectifier CR15 and filtered by C16 and
CIS to C20.
3.1.2 Auxiliary Supply Section
Isolated secondary windings of Tl also provide power to bridge
rectifier CR16 which supplies regulators 04 and 05.
05 and its
associated components provide a +14 volt supply voltage, and 04
provides
a
+5
volt
supply
for
the
front
panel
display
circuitry.
CR19, CR29, CRIB and
C30 form a
negative
voltage
doubler circuit supplying regulator 06.
06 provides a -12 volt
supply voltage which is stepped down to -6.2 volts by CR17.
The
-6.2 volts is used to bias the display circuit ICs O102 and O104.
3.1.3 Pass Element Section
Output power is provided by the two raw supply voltages from CR15
and controlled by power pass transistors Ql and Q2 and their
associated driver
transistors 04 through Q6.
This power stage
has
two
modes
of
operation.
As
long
as
the
output
voltage
required is less the the voltage supplied to the' Ql collector
(via CR8), Ql acts as the pass element.
As the output voltage is
increased, Ql begins to saturate and its base current increases
sharply.
This increases the voltage at the base of Q5 allowing
Q6 to turn on and suppling base current to Q2, which becomes the
pass element as Ql is cut off.
The output current and voltage
are regulated by the amount of base current allowed to flow into
Q4
(for
a
given
load)
by
the
voltage
and
current
control
circuits.
3.1.4 Shutdown and Soft Start Circuit
An external shutdown signal on option connector pin Pl-9 is able
to turn on Q3 and draw current away from the Q4 base via CRll,
thus turning off the output of the supply.
Similarly, a control
ground fault (or return sense line error via 03D) will turn on Q3
via
CR7
and
turn
off
the
output.
Applying
-12
volts
to
CR9
causes
a
reverse
bias
across
CRIO
during
normal
operation.
THEORY OF OPERATION
3-1
Rev - (1/87)
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