Theory
of
Operation 2-9
Power Supply Theory
of
Operation
Introduction
This section provides a
general
description of the
power
supply, which includes
the
power
module,
part
number
09895-67910,
and
the associated
ac
input
circuitry
mounted
on
the
chassis
and
fan
compartment.
General Description
Ac
power
enters through fuse
A3Fl
and
is
switched
by
SI,
the
front
panel
power
switch.
It
passes
through
S2
and
S3,
the voltage selection switches, into the
proper
windings of
transformer
Tl.
Tl
has
three
secondary
windings.
One
secondary
winding supplies rectifier
CRI0
through fuses
AIFI
and
AIF2.
CRI0
supplies unregulated + 12 volts
and
-12
volts. The + 12 volt line is regulated to
+5
volts
by
Ul,
an
LM723 voltage regulator, which functions by changing the bias
on
Ql,
a Darlington
pass
regulator.
Ul
receives inputs from the
current
limiter (U2 pin 1), regulated
+5V
line
and
regulated + 12V line (see next paragraph). U2
senses
the
voltage across R4.
As
the
current through R4 increases, the voltage across it increases. When it reaches a
predeter-
mined threshold, pin 1
changes
rapidly from zero to +
12
volts, turning VOUT of
Ul
off,
,.
-
which turns
Ql
off.
+5V
enters
the inverting
input
of
Ul
and
+
12
volts biases CR7, a
4.99
volt precision zener diode
at
the
non-inverting input. Note
that
in this configuration
there
will
be
no
+5
volts until
the
+ 12 volt line
is
up.
CR4
and
CR5 form a crowbar.
CR5
conducts
when
the potential across
it
reaches
6.19
volts, turning
on
CR4 which blows fuses
Fl
and/
or F2,
or
current limits the
+5
volt supply.
U2
and
U3
produce
the PON signal. U3
produces
a
power-up
signal
when
the potential
on
pin 2
reaches
4.75
volts (which occurs
after a time delay to charge
CI2).
Another secondary winding supplies rectifier
CRI
through fuse F3.
CRI
supplies
unregu-
lated
+24
volts. U4 is a + 12 volt regulator. CR8
and
CR9
form a crowbar.
When
the
potential
on
CR9
exceeds
14.7
volts, it conducts, turning CR8
on,
which shorts
the
+
24
volt
line
to
ground, blowing fuse F3. Note that the crowbar
operates
from the +
12
volt line,
but
affects the
+24
volt line. CR2
and
CR3
are
negative voltage protection.
The third
secondary
winding provides
120
Vac for the fan
motor
and
the drive motors.
To Next Page
Loading...
Need help?
General
