It can't be made from a computer ATX power supply, they have a max output voltage of 12 volt. Plus it would be hard to mod a switchmode power supply. You need 20 volt transformer and a bride rectifier for dc voltage. Or option two is you get a 24 volt transformer and lm317 to drop 24 volt down to 20 volt. You can find the circuit diagram here Datasheet As long your transformer volt is at lease 3 volt higher than your output voltage, LM317 will work. Hope this help out.