Thursday, October 16, 2008

Desktop Power Supply from a PC

Desktop Power Supply from a PC

A completed 145 watt ATX power supply with switch, binding posts, labels and feet. Notice the zip ties in the ventilation slots
that hold the load resistor.

This ATX PS board has leads for +5 (RED), -5 (WHITE), +12 (YELLOW), -12 (BLUE) volts, Ground (BLACK) and switch (GREEN). Dell power supplies manufactured between 1996 and 2000 do not follow the industry standard pinout and color codes. The fan has also been unplugged for better viewing. Since this PS was converted for use in the logic and robotics labs, the selected voltages were tapped. Other users may want combinations of +3.3 V (ORANGE), +5 V and/or +12 V if they are converting one of the newer supplies. For R/C applications, the 5 volt output can also serve as a desktop source to drive receivers and servos. If used as a power source for the micro and sub-micro servos, you must be careful not to drive the servo to either endpoint to avoid stripping the smaller gears in these units. Most standard servos have sufficiently robust gear trains and will simply stall if pushed to the mechanical stops..

Measured voltages on this particular PS (1996 P5-100 MHz Gateway) were about 5.15 and 11.75 volts. The remaining leads have been clipped off at the circuit board.

View of the case top with fan, binding posts and switch. The switch (SPST) and binding posts are available at Radio Shack or other electronics suppliers.

Power supplies in today's computers are known as SWITCHMODE or Switching Mode power supplies and require a load to continue to operate after being switched on (the term switching mode actually applies to the technique of A/C to D/C conversion and not to the power up action). This load is provided by a 10 watt, 10 ohm wire wound load resistor (sandbar - about $0.80 at Radio Shack) across the +5 volt supply. Some inexpensive power supplies may fail if forced on without a load. The sandbar has been zip tied to the case with a small amount of heat sink compound applied. Without cooling, the resistor will get very hot and may fail prematurely. With this arrangement, the resistor will remain barely warm to the touch.

Be warned that many of the heat sink greases can be quite toxic and any excess should be cleaned up and disposed of properly. Also be sure to thoroughly clean your hands and tools after use. While most heatsink compounds are rated to 160 to 170 C, some may dry out over time and their effectiveness will diminish -- a periodic check for good contact between the case and resistor is a recommended practice.


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