Q. What is the difference between ATX and AT power supply?

A． AT power supply, ATX power supply, and SFX (microATX)power supply are picked up as power supplies for computers.
There are, also, other type of power supplies, such as WTX, NLX, etc,but many power supplies at shops seems to be mainly those three types. X、NLX SFX (microATX) power supply can be clearly identified due to its larger dimension than the other two. On the other hand, AT and ATX power supply look the same,and the dimension is called PS/2 size.What's the difference, then?

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.

Difference between ATX power supply and AT power supply

Also, for AT power supply, ON/OFF of power supply is "manually" performed. To start up AT power supply,
turn on the switch located in AC input side. However,  of late, it does not always mean that AC input is directly turned on or off. ON/OFF is performed by control signal.
To be exact, this operation might be out of AT specification, but there's little difference in operation.

For ATX power supply, ON/OFF is controlled by "a directive from motherboard." As those who have built PC on their own know, the switch located on the front panel, called soft switch occasionally, is not connected to power  supply, but connected to motherboard. Shorting momentarily the switch on the front panel sends a directive "to turn on power supply" to power supply. What kind of directive is this?

ATX power supply starts up when pin 14 of 20Pin connector or pin 16 of 24Pin connector goes to "TTL LOW"(COM level).
(Normally, as pin 14 is pulled up to give specific level of voltage even when it's open. Therefore, it gives TTL High status).
So, the directive "turn on power supply"means that the voltage of pin 14 should go to "TTL LOW," or "COM level." Eventually , power supply turns on to start up.

For your reference, pinout assignment of ATX is shown below.

※ +5VS is always on as long as AC input is on even when power supply shuts down.
※ PWR_OK gives 5V signal at open status when output is normal.
※ Voltage level of all outputs are measured on the basis of COM level.
※ -5V is optional after ATX12V1.3, including ATX12V1.3.

Those who have used PC for a long time may be aware that when OS was shut down, power supply automatically came to shut down as well. This is because a combination of motherboard, APM (Advanced Power Management) function of OS, and ATX power supply has made it. In relation to it, some Windows NT could work that way at that time.

If you want to control output by single ATX power supply, you can do it by sending a quasi-directive form PC. That is, just make pin 14 of 20Pin connector or pin 16 of 24Pin connector "TTL LOW." For our power supply, for example,
shorting pin 14 and pin 15 makes a situation like that.
One important thing here, however, is that "5V output needs min. load." Due to the design of PC power supply that just one transformer gives several outputs, 3.3V and 12V may not come out without load for 5V, say 2A for example.Of late, some power supplies need no min. load.

By the way, some customers may think "ATX power supply can accept motherboard of AT." The answer is "YES."
As we provide conversion connector to AT power supply from ATX power supply, you can utilize it to use motherboard of AT with ATX power supply. As to "how to connect," refer to "Conversion connector to AT from ATX."
However, ON/OFF control of power supply is not performed by motherboard in this case. You need to control ON/OFF manually.