Power Supply; Circuit - Tandy WP-2 Service Manual

IV-16.
Power
Supply
The
system
DC
power
supply has an approximately
1.2
watt,
3
Voltage-level regulator.
It
is
integrated into the system
unit,
and
supplies
power
for the
system
unit,
and IC
Card.
The
supply provides
160mA
+5VDC
+/—
5%,
12mA
-8VDC + /-
10%
and
10mA
-15VDC
+/-
10%
at
6VDC.
The
system
unit takes
approximately 50
to
140
m
A
of
+5VDC.
The
— 8VDC
is
used
for
powering
the Electronic Industries Association (EI A)
drivers
for
communications
in the
main
PCB. The
- 15VDC
level
is
used for
LCD
bias.
+ 5VDC
Output
Voltage
Circuit
This
is
a
one-transistor
self-excited
DC/DC
converter,
which
is
called
R.C.C. (Ringing
Choke
Converter). Figure 4-13
shows
the principle of the
circuit,
(a)
When
the
power
is
supplied
to
VI(B
+
),
Tr
turns
on and
current
(Ic)
flows
through
the
primary winding
of
the
transformer.
With
this
current, voltage (Vb)
is
induced
to the base
winding, then Tr
immediately reaches
saturation.
(b)
Tr
can
no
longer maintain
saturation. Therefore,
Tr
is
turned off immediately.
Then
voltage
Vo
is
created
by
the
magnetic energy stored
in the transformer,
and
the voltage
is
sent to
output
Vo
through
the
diode D.
(c)
When
the energy stored
in
the transformer
is
discharged, the output current
(ID) diminishes
and
reaches
zero.
However, Tr
turns on immediately
with the base voltage
Vb
which
is
induced from the primary winding, and
the
above
actions are repeated.
The
operation of the blocks of
the
+5V
Power
Supply
circuit
is
as follows.
(Refer
to
Figure
4-14.)
Block
1
The
block operates
like a
boot-up
resister
Rs
in
Figure 4-13.
The
block
is
made
of a boot-up
circuit,
output
voltage controller,
and shut-down
circuit,
and
the
block
controls
TR30.
Block
2
This switching
transistor
operates
like
a
transistor
in
Figure 4-13.
Block
3
This transformer generates
+5VDC,
—
8VDC,
and
-15VDC
output
voltages.
WO
"-VS-VD
r*
lo
-o
r^?^
lH_
i?
-w=r
D
ID=0
Fig.
4-13
4-14