System
Power
Supply
The
TRS-80 needs
three
voltage levels: +12 volts
at
about
350
milliamps; +5 volts
at
about
1.2
amps; and
-5
volts
at
1 milliamp.
The
+12
and
-5
volts are needed
by
system
RAM
and every-
thing needs +5 volts.
The
+12
volt and +5 volt
supplies are regulated and
current-protected
against shorts.
The
-5
volts supply
is
not
as
critical as
the
other
two
supplies, and it uses a
single
zener
diode for regulation. Stepped-down
AC
voltage
is
supplied
to
all
regulator circuits
from a UL
approved"
AC
adapter."
ACAdapter
The
AC
Adapter
is
a large version
of
the
type
used for calculators and TV game products.
In-
side
the
plastic case
is
a single
transformer
with
one
primary and
two
secondary windings.
The
primary circuit
is
designed for
120
V
AC
and has
an operating range
of
105
to
135
V
AC.
NOTE:
There
is
a wire fuse in
the
primary side
to
meet
UL specifications.
The
secondary windings are
both
center-tapped.
One secondary
is
rated
at
14 volts AC
at
1 amp.
This winding
is
used for
the
+5 and
-5
volt sup-
plies.
The
other
secondary winding has diodes
connected
and it
outputs
19.8 VDC
at
about
350
milliamps. This circuit
is
used for
the
12
volt supply.
All
voltage
outputs
and
center
taps-
are
brought
into
the
POWE
R
input
(J11.
52
+
12
V
Power
Supply
Unregulated
DC
voltage
for
the
+12V supply
is
inputted
at
pin 2
of
J1.
When
power
switch
Sl
is
closed, C8 filters
the
voltage and
the
net
resu
It
is
approximately
20
volts, which
is
applied
to
06
and regulator Z2. Figure
13
shows a simpli-
fied diagram
of
the
internal circuitry in a
723
regulator chip. Figure 13 will help
in
the
regula-
tor
operation
discussion.
The
filtered
DC
voltage from
the
Adapter
and
C8
is
applied
to
pin 12
of
Z2 and
the
emitter
of
series pass transistor
06.
The
voltage applied
to
pin
12
allows a
constant
current
source
to
supply zener
current
for Za. Pin 6
of
Z2 will
output
a
zener
voltage
of
about
7.15
volts. Pin
6
is
tied
to
pin 5,
the
positive
input
to
opera-
tional amplifier Zb.
The
negative
input
to
the
op-amp
is
tied
to
the
wiper
of
R10. Initially,
pin 4
of
Z2
is
at
ground, forcing
the
output
of
op-amp Zb
to
output
about
7.15
volts. Tran-
sistor
Oa
turns
on, which
turns
on pass transis-
tor
06.
The pass transistor supplies voltage
for
current
monitoring resistor R18 and
to
the
resistor
network
R13, R10 and R12.
If
R10
is
adjusted for
7.15
volts
at
its wiper,
the
op-amp
will be balanced and
06
will
output
only
enough
voltage
to
keep
the
loop stable.
If
output
volt-
age
dropped
below 12 volts,
Zb's
output
would
decrease which would force
the
current
through
Oa
to
decrease. Oa would cause
06
to
increase
the
current
through it, and
the
output
would
rise back up
to
the
12 volt level. If
the
12 volt
line increased
in
voltage,
the
op-amp would
cause Oa's
current
to
increase, forcing
06
to
drop
down.
To Next Page
Loading...
Need help?
General
