WARNING: In the following I'm going to describe how to modify a power
supply. Power supplies contain high voltages – even after removing the
power plug they can still have high charges in capacitors. In some
countries devices with mains power may be modified only by or under
supervision of certified persons. So you should know what you're doing
and be authorized to modify a power supply.
PC power supplies are cheap and readily available but the tend to
produce the correct voltages only when the drawn current from all
different voltages are within the minimum/maximum specs of the power
supply. Drawing high current (but within the specs of the power supply) only
from a single voltage lets this voltage drop below specification.
One example is the heat-bed of my 3D-printer: It used to have a PC power
supply for the heat bed. The heat bed draws about 12A but only from the
12V line. The result was a voltage of about 10V when the heat-bed was
heating.
Another use-case is the supply of several ARM based single-board
computers (e.g. Raspberry-Pi or Orange-Pi) from a single 5V line. When
using a PC-Power supply for this (without drawing current from the 12V
lines) the nominal 5V voltage may drop below a value where the
single-board computer works reliably.
A third example is the use of a power supply for powering the radio of a
ham-radio operator: These radios typically don't output full power when
powered with 12V or less, they typically need 13.6-13.8V for full power
operation.
In all these cases a modification of the power supply that keeps the
chosen voltage stable or even allows to modify the chosen voltage
slightly (from 12V to 13.8V for hamradio operation) would be nice. How
can we achieve that?
Many PC power supplies are based on the power regulator integrated
circuits TL494 or KA7500 (they are pin-compatible). If you have one of
those they can usually be modified for the purposes outlined above.
One schematic details of those supplies is a feedback-circuit that feeds
back the 5V and the 12V voltage to the regulator IC. You can find a lot
of power supply schematics on Dan's PC power supply page. Take the
second of the TL494 or KA7500 based supplies, it has several resistors
in parallel from pin 1 of the TL494 to ground, a 27kΩ-resistor from 12V
to pin 1 and a 4.7kΩ-resistor from 5V to the same pin.
We can modify the voltage regulation by changing the feedback pins.
Note that every power supply usually has different resistors to ground
and different feedback resistors from 5V and 12V. As an example we
replace the two feedback resistors to make the power supply provide
stable 5V without caring for the 12V supply.
WARNING: When modifying a power supply for a stable 5V or 12V source,
the other voltages will no longer be stable and may become too high
for use in a PC. You should never use such a modified power supply in a
PC.
So the first step is to identify the two resistors in the power supply.
Once found we verify that the side of the resistor not connected to pin 1
of the regulator IC has 0Ω to the correct (5V or 12V) power supply
output. We unsolder both resistors. Now before computing the new
resistor to be placed between 5V output and pin 1 we measure the
resistance between pin 1 and ground: Because we have now unsoldered the
two resistors to 12V and 5V the resistor can be measured. It is good to
be sure that the measured resistance matches the computed resistance
from the three resistors connected in parallel: In the example we have
100kΩ, 390kΩ, and 10kΩ in parallel which should measure as
\begin{equation*}
\frac{1}{\frac{1}{100000}+\frac{1}{390000}+\frac{1}{10000}} = 8883.83
\end{equation*}
When recently modifying a power supply the resistors to ground were
470kΩ, 100kΩ, and what I thought was 8.9kΩ: I interpreted
the colors of the last resistor as grey-white-black-brown-brown. When
measuring the three parallel resistors I measured it as 4.61kΩ instead of the expected 8033Ω. It turns out (after viewing the
resistor in sunlight) that what I had interpreted as grey was really
yellow-ish. So the resistor was really a 4.9kΩ resistor and the
computed resulting resistance was 4625Ω.
To make the power supply regulate only for 5V we connect a new resistor
from pin 1 of the regulator IC to 5V and leave the 12V feedback line
unconnected. But how do we chose the new resistor? To find out we need
to solve a set of equations: We know from Ohms law the relation of the
voltages, resistors, and currents. We know from Kirchhoff that the
current through the 12V feedback resistor and the current through the 5V
feedback resistor must add up to the current through the resistors to
ground. These relationships are given in this maxima spreadsheet.
We compute the reference voltage V_ref at pin 1 when both, the resistor
from 5V and from 12V are connected. Then we chose the new resistor to 5V
so that the voltage on pin1 stays the same.
For the example in Dan's second schematic we get 1800Ω for R_new.
For the power supply I recently modified I've already given the
resistors to ground. The resistor to 12V was 39kΩ and the resistor
to 5V was 9.1kΩ. The resulting R_new for that power supply was
4865Ω which I realized by connecting a 4.7kΩ and
a 150Ω resistor in series which was close enought to get a good
5V output. The computation is given as the second example in the
spreadsheet.
A word of caution: When modifying the 5V power regulation or modifying a
power supply for higher voltages (more than 12V) you should be aware
that most PC power supplies have capacitors rated for 16V in the 12V
output circuit. So when drawing high currents from the power supply
modified for 5V the voltage on the 12V line may become too high for
these capacitors. To be on the safe side the capacitors should be
changed to types rated for 25V.
When modifying the 12V supply we use basically the same procedure,
except that we now connect the 12V feedback resistor and leave the 5V
feedback open. How to compute the resistor for the 12V feedback line is
left as an exercise.