power supply units

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Obtain an ATX PSU.


     BLACK: GND ground

     GREEN: Power on. Ground this line to turn ON.

     YELLOW: +12V. On newer power supplies you may need to attach a 22Ω 10 Watt resistor to GND as a dummy load. Switching power supplies usually need at least some load to operate properly.

     RED: +5V. On older power supplies you may need to attach a 5Ω (or 4.7Ω, 4R7) 10 Watt resistor to GND as a dummy load. Switching power supplies usually need at least some load to operate properly.

     ORANGE: +3.3V

     BLUE: -12V

     PURPLE: +5V standby. This provides low power 5V even when "off".

     WHITE: -5V (rare)

     GRAY: Power OK signal line. Attach an LED in series with 330Ω resistor for a power ready light.

     BROWN: +3.3V remote sense. This may not be necessary for all applications. You should attach this to the +3.3V      ORANGE line. The attachment point should be near the load, not just inside the PSU. This allows the PSU to measure the resistance of the power lines leading to the load and fine tune the voltage level. For long power lines the voltage drop can be significant.