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You also can find detailed explanation on vvv.tomshardware.net No need to build schematic.I can't remember clearly but you should connect power good pin (green) to vcc.(I'm not sure please have reference)
This type of power supplies needs "Minimum Load Current" to be drawn from the +5V & +12V concurrently in order to sope the o/p current range to it's full range. i.e., if you load the +5V only you will get only about 500-600 mA before the o/p voltage starts to drop down dramatically.
This is my own experience with this type of SMPS. I had to draw 2A of the +5V and 2A from the +12V to supply my 1.5A@5VDC uC based circuit.
Yes,You need a short between pin14 (PWR_ON,green) and pin15(GND,black). Most of ATXPSU have voltage feedback only from 5V output. If you apply a load only 12V out, the PSU switched off,because the overvoltage circuit detect a failure.In this case apply abot 1...2A dummy load to 5V out.(as busmaster wrote) I`ve made a 14V 15A accu charger from old AT supply (better construction as today...).
Yes, megamicro see this:**broken link removed** ,
In this circuit the output voltage depend from voltage divider on TL494 pin1. R20 and R21 go to GND. When You decrease R21 the output voltages increased. R25 and R26 feedback resistors from 5V and 12V outputs.(here is some drawing failure.)
If You want modify to accu-charger, apply feedback only from one output(12V) and remove the components for other outputs and overvoltage protection.