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Four-terminal regulators
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By Randy Fromm
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ntergrated circuit voltage regula-
tors provide a quick and easy
solution to the problem of manu-
facturing the highly regulated power
supplies that are essential to the com-
puter systems in video games.
In one small package, the voltage
regulator contains 17 transistors, two
zener diodes, 21 resistors, and even a
small (30 picofarad) capacitor. At
about a buck and a half to two dollars
each, they're a low-cost answer to a
"high class" regulated supply.
Midway introduces device
The three-terminal vo ltage regula-
tors have been standard issue in video
games for many years . They've been
used in just about every game from
Midway since the introduction of the
then revolutionary new video,
Gunfight. But when Midway intro-
duced us to Galaxian (and the Namco
engineered, Z-80 based video game
system that is essentially the same as
that used for Pac-Man and Ms. Pac-
Man as well), it also introduced us to
another type of voltage regulator inte-
grated circuit, the 78G U 1 C.
Like the three-terminal regulators,
the 78G U I C provides a constant out-
put voltage regardless of fluctuations
in voltage at the input. But the three-
terminals' regulator is generally used
as the so le regulating component in the
low current power supplies such as the
- 5 and + 12 volt power supplies found
in many games.
The 78G U I C, on the other hand ,has
four leads and is used in conjunction
with a transistor to regulate the high
current, five-volt power supply for the
entire computer system itself.
The addition of the fourth lead
allows this regulator's output voltage
to be controlled to any voltage between
five and 30 volts. The vo ltage at the
"control" input (pin 4) determine s the
output voltage of the regulator itself.
This control voltage is derived by
using combinations of resistors or
resistors and zener diode s to create a
circuit known as a voltage divider.
In the schemantic diagram of the
basic positive regulator, the voltage
divider resistors are R I and R2 . By
changing the values of these two resis-
tors , the design engineer can change
the output voltage of the device.
Re gulated output
tlellc PoeiiiYe Retut.tor
,.A78G Connection Diagram
Power Watt Package
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PLAY MffiR. December 1. 1982
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In the Namco system (and others as
well) , the 78G U I C is used to drive the
base of a series-pass regulator transis-
tor, which in turn regulates the larger
amounts of current required of the +5
volt power supply by the computer
system. By keeping the base voltage of
the transistor at a constant level (in
this case at somewhere around 5.6
volts), a regulated output of exactly
five volts can be obtained at the
emitter regardless of changes in volt-
age of the unregulated input at the
collector.
The 78G series of integrated circuit
voltage regulators is not commonly
available at electronic retail or
"hobbiest" stores. Be prepared by
ordering a replacement part ahead of
time to keep in stock. Most reputable
dealers of video game replacement
parts will stock this item.
The remainder of the part number
that follows the "78G" indicates the
type of package for the device (U I is
the designation for the " power Watt"
type of package.) and the temperature
rating (C is the designation for the
"commercial rating."). Operation is
restricted to temperatures between
0° C and 150° C.).
For further information about volt-
age regulators , see Play Meter, April
15 , p. 135.
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