Hacking the Xbox

Strategy
Chapter 5 - Replacing a Broken Power Supply 77
Using Diodes to Drop Voltages
The Xbox requires a +3.3V standby supply voltage, but an
ATX power supply only outputs a +5V standby supply voltage.
The “correct” solution to this problem would be to use
a voltage regulator that precisely converts +5V into +3.3V,
but the goal of this hack is to replace the power supply
with a minimal amount of soldering.
The alternate solution is to use two diodes to reduce a +5
volt supply down to a “close enough” +3.6 volt supply. We
can do this because the voltage across a forward conducting
diode is logarithmically proportional to the current
through the diode. In other words, for most currents, the
voltage across a diode is almost constant. It turns out that
silicon diodes almost uniformly have a forward voltage drop
of about 0.7 volts, so two of them in series will drop 1.4 volts.
The diodes used in this hack, the 1N4001, are only capable
of conducting 1 ampere of current, so don’t use this trick in
other applications that require a large amount of current.
Fortunately, the stand-by supply for the Xbox only needs to
draw a tiny amount of current so burning out the diodes is
not a concern.
As a final note, the voltage dropped by a diode fluctuates
slightly with the amount of current through it, so do not use
this trick in applications that require precisely regulated
voltages. In the Xbox application, we are running the voltage
a little bit on the high side, but fortunately the digital
logic powered off of this supply can tolerate this condition.
The interface for a standard ATX power supply is very similar to that of
an Xbox’s power supply. The Xbox requires +3.3V, +5V, +12V, a +3.3V
standby supply, as well as two control signals, “power OK” and “power
on.” The power OK signal indicates that the power output from the
power supply is stable and properly regulated, and the Power On signal is
a control signal from the Xbox that turns the power supply on and off. A
typical ATX supply has +3.3V, +5V, and +12V outputs with enough juice
to run an Xbox, and it also has a power OK signal that is compatible
with the Xbox motherboard. However, an ATX power supply generates
a +5V standby voltage instead of a +3.3V standby voltage, and the
Power On signal has an inverted polarity from the Xbox. Both of these
incompatibilities can be addressed in a manner that requires no soldering.
Two diodes in series are used to reduce the +5V standby voltage down
to a voltage of a little less than +3.6V. The Power On signal to the ATX
power supply defaults to “on,” so it will remain unconnected, making the
power supply always on even if the console is off. This isn’t problematic for