A little circuit to convert 5V to 28V...
http://www.io.com/~trag/28v_vreg.pdf
I did not create this circuit, but it is a very standard application/use of the National Semi part.
When the computer is powered off, the 28V circuit draws current off of the 5V standby circuit, so it is good to choose an ATX power supply with a reasonably beefy standby current capacity. After the machine is turned on, all of the 5V current should be available to the 28V converter. However, the chip used here is only good up to .3A at 28V. If you have an ADC display which needs to draw more power, then you should choose a beefier booster chip from National Semi. If you visit their site it is pretty easy to get to a table which will show you the relative specs of their family of booster chips, including the max amperage available after conversion. I think there are chips which go up to 3A.
http://www.io.com/~trag/28v_vreg.pdf
I did not create this circuit, but it is a very standard application/use of the National Semi part.
When the computer is powered off, the 28V circuit draws current off of the 5V standby circuit, so it is good to choose an ATX power supply with a reasonably beefy standby current capacity. After the machine is turned on, all of the 5V current should be available to the 28V converter. However, the chip used here is only good up to .3A at 28V. If you have an ADC display which needs to draw more power, then you should choose a beefier booster chip from National Semi. If you visit their site it is pretty easy to get to a table which will show you the relative specs of their family of booster chips, including the max amperage available after conversion. I think there are chips which go up to 3A.