Q. What am I supposed to do when DC output does not come out
after AC input is turned on? (How to specify the cause)

A． In the case that output does not come out after AC input is turned on, there are several causes as below.

The largest difference is that main connector shape and output voltage of secondary output which is connected to PC. Table below shows the difference in comparison.

(1) No output at all
For ATX power supply, all outputs except for 5VSB stay halted when AC is turned on. In order to turn on all outputs, short pin 14 and pin 15 for 20Pin connector, or short pin 16 and pin 17 for 24Pin connector. Pin 15 of 20Pin and pin 17 of 24Pin connector are both COM level.
In the case that output voltage is measured with single power supply, make sure to pass the current at or greater than the minimum current value; some products may not work steadily if the current is smaller than the minimum load current. Many of our products have the minimum  current of 0A.
In general, the procedure above, (pin 14 goes to COM level), is done at motherboard side when 20Pin connector is connected to motherboard of ATX.
This function allows power supply to be turned on by manually  pressing a soft switch of motherboard manually.
The soft switch is located on the front panel of PC. It, also, allows power supply to shut down when shutdown process is completed on Windows 2000/XP,etc.

When output of single power supply is measured, all output is OK but, when the power supply is connected to motherboard, the power supply becomes out of work;

・ Output is halted due to overcurrent detection
・ Mismatching of PS ON signal voltage level
・ Mismatching of P OK signal voltage level

The above are regarded as causes.

(2) Output voltage is unstable, or low.
Min. load is required. Let's take PCSA-300P-X2S, for example, it requires min. 2A load for 5V output. This phenomenon comes from cross-regulation system. As the load current of 5V is normally 2A, it does not cause the problem. However, when output voltage is checked with single power supply without load,output voltage comes to fail. If output voltage is checked with single power supply, put specified min. load.

(3) Others
The cause may be overcurrent detection. Check if there is any shortage or near shortage in your system. In addition, when the load exceeds specified load current, output comes to stop due to overcurrent protection as well.
Furthermore, when fuse is melted, output voltage does not come out. In this case, repair is required.