Crown PSA-2 - Page 98

A
fourth
current
is
added
to
the
output
node
from
the
multiplier.
This
current
provides
a
bias
for
the
temperature
computer
and
references
the
device
to
the
rating
temperature
of
25
degrees
centigrade.
The
current
is
provided
by
RN104
pins
3
to
5.
These
combined
currents
are
input
to
a
virtual
ground
of
the
op-amp
UL0IC
which
simulates
the
junction
temperature.
The
feedback
network
of
pins
1-4
of
RN103
and
C112-114
is
an
electrical
analog
of
the
thermal
impedance
of
the
output
semiconductors.
Therefore
when
a
current
which
represents
the
power
being
input
to
the
transistors
is
input
to
this
analog
computer
the
output
is
a
voltage
which
responds
as
the
temperature
of
the
transistors’
junctions.
When
this
temperature
exceeds
200
degrees
centigrade,
the
device
is
too
hot.
The
output
of
the
analog
computer
progressively
remove
operating
bias
from
the
associated
balanced
gain
stage
by
forward
biasing
D111.
Being
unable
to
provide
more
drive
for
the
output
devices,
the
output
dissipation
must
decrease
until
the
junction
temperature
is
acceptable.
The
dynamics
of
the
output
voltage
of
the
analog
computer
ranges
from
approximately
-12VDC
at
25
degrees
C.
to
+9VDC
at
200
degrees
C
junction
temperature.
This
voltage
may
be
readily
probed
at
TP-1.
The
multiplier
dual
transistor
has
an
offset
adjustment
for
balancing
composed
of
R117
and
R118.
This
may
be
adjusted
by
removing
all
current
from
the
output
stage
and
producing
a
low-frequency
output
from
the
amplifier.
If
the
multiplier
is
balanced
no
AC
voltage
will
appear
at
TP-1.
The
operation
of
the
protection
circuit
for
the
PNP
stage
is
similar
except
for
the
polarity
reversal
of
the
multiplier
and
simulator
stages.
By
attaching
the
heatsink
temperature
sensor
and
offset
current
to
the
other
output
of
the
multiplier,
i.e.
the
input
to
the
current
mirror,
it
is
not
necessary
to
reverse
the
polarity
of
the
sensor.
This
allows
all
of
the
sensors
to
provide
the
same
polarity
of
output
voltage,
simplifying
the
design
of
the
fan
speed
controller.
The
fan
speed
controller
is
shown
on
the
full
schematic
between
the
input
amplifier
stages
of
the
two
channels.
It
perhaps
is
best
discussed
along
with
the
protection
system
since
its
function
is
to
increase
the
air
flow
by
engaging
the
fan
relay
in
high-speed
whenever
any
one
heat
sink’s
temperature
exceeds
50
degrees
C.
The
quad-
comparator
UI
monitors
the
four
heat
sink
temperature
sensors.
When
one
is
sensed
over-temp
the
output
of
U1
will
turn
on
both
QI
and
the
fan
relay.
QI
provides
a
temperature
hysteresis
for
the
controller
to
prevent
erratic
switching.
E. Muting Module applies to earlier units only;
(circuit incorporated into Main Module above indicated
SN).
7-31
The power-up and power-down phases of operation were
found to produce noises in the output. While these signals
were of such amplitude as to be harmless, it was +
recognized from experience that customers would not et
necessarily view such noises as insignificant. To prevent
such noises, a simple circuit was added to the design to
prevent the operation of the balanced gain stages until the
+&- 1SVDC supplies were adequate to have the amplifier
in control of its output.
Referring to the muting module schematic, the transistor
Q4 driven by D1 acts to sense sufficient voltage on the
supply rails. The conduction of DI and Q4 act to turn off
Q3 which saturates QI and Q? in the low voltage state of
operation. With QI and Q2 in saturation, the resulting
conduction of diodes D100, D101, D200, and D201
inhibit all output drive from the balanced gain stage,
which totally disables the output stages. This will happen
both during power-up and power-down. Of course with
the voltage very low none of these effects will occur
including any drive to produce output noises.
F. Power Supplies
U2 and U3 constitute adjustable IC regulators which are
protected against overload and over-temperature by
internal circuitry. Their outpu' djusted to+1SVDC by
RN3 and RN4 respectively. The unregulated input to
these supplies is derived from T3 by diodes D4-7 and
filter capacitors C5 and C6. C4 is used to prevent RFI Ol
from diodes D4-7. Capacitor C7 is used to reduce the
high-frequency output noise of U2. C8 and C9 act also to
suppress supply noise and impedance. Diodes D8 and D9
are the traditional diodes to prevent damage from
application of reverse polarity voltages to the outputs of
the supplies.
Transformer T3 is used as an autoformer to power the fan
motor when the unit is wired for voltages other than
120VAC. T3 uses the typical universal primaries format
used in other Crown products. In order to reduce the
current drawn by the fan motor its power factor is
corrected by the capacitor C4. T3 has a low voltage
secondary which is used to power the LED's in the SA
display module. This is more efficient than using the 15
volt supplies for all such power.
Quad-comparator Ul sections B and D are used to
control the relay driver transistor Q100 and Q200 which
power the channel | and channel 2 output stage supplies
respectively. When Q100 or Q201 are off, the collector
voltage will drive Q101 or Q200 respectively to light the
corresponding standby lights for the down-powered
channel. By grounding of the drive circuits at the junction
of D107 and D207 and RIII or R211, the output stage
supplies may be forced into the standby state. wy