8
STAR* TECH JOURNAL/FEBRUARY 1983
SEGA ELECTRONICS'
DON'T SIT
ON YOUR
ROM LINE (PART 5)
"Asteroids"!
As part of our continuing series ofbasic elements of a computer, this month we'll look at
the remaining element - the power supply. Specifically, we'll examine two kinds of
power supplies - the linear and switching regulator types. You may want to refer to
schematics as you read this article. For the linear power supply, see drawing #800-
0128 in the "Astro-Blaster" manual. The switching regulator schematic is #601-
0158 in the "Space Odyssey" manual. We'll look at a linear supply first. Basically, this
type of power supply consists of a step-down transformer, a diode bridge, filter
capacitors and voltage regulation circuitry. The transformer simply converts 1 OOvac to
a number of smaller secondary voltages. These voltages can then be rectified to DC and
filtered. Finally, voltage regulation is provided in a number of ways, commonly with 3-
terminal IC regulators. These devices regulate an input voltage to a stable output level.
On the 800-0128 schematic, the -5 and -12 voltages are provided by 3-terminal type
regulators, U4 and US. For higher current levels, regulation can be provided by power
transistors, as shown on the schematic, above IC Ul. The 723 IC is another kind of
voltage regulator IC that is adjustable through a range of output voltages. In the
configuration sh own here, it is providing a higher current, +5 voltage output. This
voltage can be varied slightly with pot R5. Additionally, voltage regulation is performed
by an OP-AMP in the configuration shown at U2. Here, the OP-AMP and transistor
circuits are regulating the input voltage to supply an accurate + 12 DC source.
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POWER SUPPLIES
These are the basic elements of a linear power supply and are commonly used in
computer game power supplies. The second power supply design is the switching
regulator type. This supply operates by alternating a current through an inductor and
filter capacitor. The result is that the AC line voltage is regulated to a low DC voltage
( for example, + 5) at a high current. The switching action causes current to increase in
the inductor and then decrease. As it falls, it is switched to the filter capacitor. Looking
at the schematic, you'll see the 11 0vac input applied to D 1, a diode bridge. The DC
output is then applied to the primary of the transformer T 1. Q 1 causes the high primary
voltage to switch on and off. Tl's secondary generates a low voltage as the primary is
switched. The secondary current flows into the inductor LI and, when the current is cut
off by Q 1 's action, the current level at the inductor falls. This decrease is sensed by D3,
which then switches to pass the current to filter capacitors C9 and ClO. The output,
then, is +5 volts at a higher current level. A feedback signal is developed from the +5
source and applied to IC 1. This IC generates pulses that switch Q2 on and off, which
activates transformer T2. The output ofT2 then triggers the on/off activity ofQ 1. The
feedback signal is necessary to ensure a well-regulated+ 5 volt output. Transformer T3
is used to provide voltage for the IC and its associated components. We have now
looked briefly at the basic elements of a computer. Hopefully, you'll be encouraged to
dig deeper into the subject and keep informed of the new circuits and developments in
computers. Watch this column for more on the theory of video game computers.
Power Supply
*
Send Check or Order UPS COD to
WVGO, Inc.
15000 Main Street
Gardena, CA 90248
213-516-9525
"Atari 'Centipede' Trackball Mod", which appeared on pages 10 & 11 of our
January '83 issue, is in error. An updated rewrite from Atari will appear in our
March issue.