2-6
Theory
of
Operation
The main power transformer has another secondary winding that provides ac power to the second-
ary board. This
power
is
used to generate the unregulated + 19V
and
- 19V which
in
turn generate
all
the other secondary supplies.
The low line detector circuitry monitors the voltage
on
the secondary winding of the main power
transformer. That voltage
is
transferred to the supervisor board where
it
is
compared
to a reference
voltage. If the voltage
on
the secondary winding drops too low, a power-fail-warning (NPFW)
is
sent to the processor. This provides a 4 to 8 ms warning before the power
is
actually shut down by
the fault circuitry.
The
peak
power monitor
is
a current sense transformer
in
series with the main power transformer
primary winding.
Its
purpose
is
to protect the switching transistors from damage
due
to
an
excessive
power condition.
When
the current exceeds
9A,
the monitor generates a fault signal to shut down
the power supply assembly.
A + 16V bias supply provides power to various monitor
and
control circuits throughout the power
supply assembly. This supply stays active when the power supply
is
shut
down
due
to a fault
condition.
It
controls the power supply LEDs
and
governs power supply start-up.
Secondary Board
Power from the main power transformer
on
the primary board
is
transferred via bus bars to the
secondary board. There a
tapped
rectifier converts the raw ac power into unregulated +
19V
and
- 19V dc. These voltages
in
turn operate several switching regulators
and
one
linear regulator.
The
voltages produced, which are used throughout the computer, are + 12V, + 12VMM, - 12V, + 6V,
+ 3V,
and
-
2V.
The + 6V, + 3V,
and
- 2V outputs are used exclusively by the processor stack.
A temperature-sensing thermistor
is
mounted
on
the secondary
board
heat sink. This thermistor
is
monitored by the supervisor
board
and
controls the power supply fan
and
the processor stack fan.
These two dc
box
fans operate at three speeds: low, medium,
and
high.
When the
power
supply assembly thermistor detects that the temperature of the secondary
board
heat sink has risen
above
39°C, the circuitry steps the fans to medium speed. A further rise of
temperature
above
51°C causes stepping of the fans to high speed,
and
the user
is
notified that the
fans have reached their maximum speed.
At
97°C, a power
fail
warning (NPFW)
is
transmitted,
and
4 to 8 ms later the power supply
is
shut down. The power supply assembly
is
also shut down if a
temperature sensor
on
the CPU finstrate
in
the processor stack detects a temperature over 100°C.
When cooling
off,
the fans step from high to medium
speed
when the temperature drops below
46°C,
and
from medium to low
speed
when the temperature drops below 34°C.
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