Radio Shack CCR-81 Reference Handbook

+5 Volt Supply
The 5 volt power supply also uses a 723 regula-
tor. Due to the current and voltage require-
ments, more components were stuck around the
regu lator for support. But the basic circuit oper-
ates 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 Jl, pins 1 and 3. Full-wave
rectifier
CR8, rectifies 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 different-
Iy.
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 supply 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 Oa, which
controls bias drive for 03. 03 is used to handle
the greater base drive necessary for pass transis-
tor 04. 04's collector is tied to current sensing
resistor, R4. R4
monitors the current the 5 volt
bus is producing just as R 18 did for the 12 volt
bus.
Circuit
operation is exactly the same for Z1 as
it was
for Z2. If op-amp Zb detects a rising or
falling voltage condition at the output bus, it
will
adjust base current to Oa. Since Oa cannot
handle the drive requirements for 04 directly,
03 is needed for current gain. During current
limiting condition, 05 monitors the voltage
across R4, which
is a direct function of bus cur-
rent. As 05 begins to turn on, the node at R3
and R9 supplies more voltage for base drive of
Qb. As Qb takes command of the regulator loop,
Oa
is commanded to start cutting 03 off. 03
54
begins to turn Q4 off and the circuit goes into
current limiting. The current limiting action of
05 starts to come into play when the voltage
across R4
approa...:hes 0.6 volt. Ohm's Law tells
us
the bus current at this voltage level is
approaching 1.82 amps.
C12,
connected between pins 13 and 14 of Z1,
performs the same compensation function as
C13.
C10 and C 14 are the output filter and
noise suppressors while
the thirty-two 0.01
microfarad
capacitors are distributed all over the
Board to suppress transient spikes.
Notice
zener diode CR1 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 CR1 is a
6.2 volt zener. it would protect the TTL devices
connected to the 5 volt bus from being damaged
by a sudden 12 volt supply voltage. Normally,
CR1
would be off with no current flowing
through it.
NOTE:
The 12 volt supply must be working
properly before the 5 volt supply will operate
correctly. Therefore, 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 sup-
ply filtering and noise suppressing caps, while
C16
through C19 perform the transient suppres-
sian function.

Radio Shack CCR-81 Specifications

General

Brand	Radio Shack
Model	CCR-81
Category	Desktop
Language	English