The
transistor labeled
Qb,
in Figure
13,
is used
to
protect
power transistor Q6 against over-current damage.
If
R18
drops sufficient voltage to cause the resistor node at
Z2,
pin
2,
to
reach 12.6 volts,
Qb
will take command
of
Qa.
As Qb
is turned on, Qa turns off which starts turning
Q6 off. The
voltage
at
Z2, pin
10,
must approach 14.7 volts before
Qb
takes charge of Qa. 14.7 volts at pin 10 means that the
12
volt supply
is
approaching its maximum design current
of
480
milliamps. If
a
short developes across the 12 V supply,
Qb
will
activate,
forcing
Qa to
shut
down.
With Qa off,
Q6's base rises to the input voltage
level because of R16.
Q6 snaps off the supply, preventing it from attempting
thermal suicide. Once the short is removed,
Qb will turn off
and
the
system will operate normally.
Capacitor
C13
connected
between
pins 13
and 4 is a fre-
quency compensation capacitor. It prevents
the op-amp
loop from going
into oscillation. C11
and
C15
are
the
supply's output
filter
and noise suppressors. Capacitors
C28, C30, C32 and C34 are distributed along the 12 volt
supply
bus for
transistor suppression.
+5
VOLT SUPPLY
turn on, the
node at R3
and R9 supplies
more voltage for
base drive of
Qb. As Qb takes
command of the
regulator
loop, Qa
is commanded to
start
cutting Q3 off. Q3 begins
to turn Q4
off and the
circuit goes into
current limiting.
The current
limiting action
of Q5 starts to
come into play
when the
voltage across
R4 approaches .6
volt. Ohm's
Law tells us the
bus current at
this voltage
level
is
approach-
ing 1.82 amps.
C12,
connected between
pins 13 and
14 of Z1,
performs
the same
compensation
function
as
C13.
C10
and
C14 are
the output
filter and noise
suppressors
while the thirty-two
.01 microfarad
capacitors
are
distributed all over the
Board
to suppress
transit
spikes. Notice zener
diode
CR
1 on
the 5
volt bus. This diode is
used as crowbar
circuit protection in
case
of catastrophic
failure in the
RAMs.
If
something
happens in
the system
RAM circuit that
causes a short
between the
12 and
5 volt buses,
CR1 would turn on,
causing the 5 volt
bus to go into
current
limiting. Since
CR2
is a
6.2 volt
zener, it would
protect the TTL
devices
connected to the
5 volt bus
from being damaged
by
a
sud-
den 12
volt supply voltage.
Normally, CR1 would
be off
with no
current flowing
through it.
The
5 volt power supply
also
uses a
723
regulator. Due to
the current and
voltage requirements,
more components
were stuck around the
regulator
for
support.
But the basic
circuit
operates the
same. Figure 13 will
also be used in this
circuit.
For
the
5 volt supply, the AC adapter supplies about 17
volts AC at J1, pins 1 and 3.
Full-wave
rectifier, CR8, recti-
fies the
AC. When S1 is closed, about 7 VDC is
passed
through the
switch contacts
and
is filtered by C9.
The power
supply for Z1 and the current source for zener
Za
is taken
from the regulated side of R18 in the 12 volt
section. Pin 7 is grounded as in
Z2,
but the
zener
output is
handled differently.
The 7.15 volt zener voltage is applied
to the
resistor network
consisting
of R6, R5 and
R11.
When R5 has
been adjusted for a 5 volt output on the sup-
ply
bus, pin 5 of
Z1 will be at about
5
volts. The negative
input of the
op-amp,
Zb,
is
sourced
through a 1.2K resistor,
R7, and tied to
the 5 volt bus. The op-amp controls Qa,
which controls bais
drive
for
Q3. Q3 is
used to handle the
greater base
drive necessary
for
pass transistor
Q4.
Q4's
collector is tied to
current sensing resistor, R4. R4 monitors
the current the
5 volt bus is producing just as R18 did for
the 12 volt bus.
Circuit operation is
exactly
the same for Z1 as it was for
Z2. If op-amp Z6
detects
a
rising
or
falling voltage condi-
tion at the output
bus, it will adjust base current to Qa.
Since
Qa
cannot handle the drive requirements
for Q4
directly,
Q3 is needed
for current gain. During
current
limiting condition,
Q5
monitors the voltage
access, R4,
which is a
direct function of bus current. As Q5 begins to
Notice one item: The 12 volt supply must be working
prop-
erly before
the 5 volt supply will operate
correctly. There-
fore, the 12 volt supply must be adjusted before
the
5
volt
supply.
-5
VOLT SUPPLY
Source voltage for
the
—5
volt supply comes from the
negative terminal of rectifier CR8. When
switch
S1
is
closed, the negative DC is
filtered
by
C1
and
about
—11
volts is applied
to
resistor R19.
R19
is used to limit current
for zener regulator CR2, a 5.1 volt device. The
—5
volt
circuit is about as simple a power supply as can be designed.
C4 and C3 is the
—5
volt supply filtering and
noise sup-
pressing caps while C16 through C19 perform the transit
suppression
function.
LEVEL
II
ROMs
One of the most
fascinating aspects
of
computers is their
versatility.
It can do almost anything;
all it needs
to
know is
how. With
a
single ROM change, the TRS-80 can speak in
any higher level language we might care to use.
In BASIC I,
we
had just
enough mathematical and symbolic
capabilities
to inspire bigger and better
things. The difference between
a Level
I TRS-80
and
a
Level
II TRS-80
is
inspiration and
a
bigger
ROM. Level I used 4K of
ROM. In
Leyel II, the sys-
tem uses
12K
of ROM. The added 8K of
ROM
is enough to
give text editing; transcendental functions; give
it giant
numeric and string arrays; and more system
variables
than
you
can
use.
The
guts of the system
have
not
changed; the
hardware is the same.
The
only
difference is the machine
language contained in ROM.
28
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