Because I often repair ATX switching power supplies, I built this simple tester that indicates the presence of all ATX output voltages. It can indicate the 3.3V, 5V, 12V, -5V and -12V outputs and 5VSB auxiliary power voltage, which is present even after switching to standby. There is also indication of PG (Power Good) signal, which gives the motherboard the signal that the power supply voltage is already within acceptable limits and it can run. For the indication I chose superbright LEDs. For 5VSB I use red, green for PG and white for the other voltages. I chose resistors for the current 5 mA (maximum of the LEDs is probably 20 or 30 mA). The switch turns on the PC supply from standby. Simple wiring of the PSU tester is in the schematic diagram below (remember that -5 and -12 LEDs are in opposite direction). Below you can also see the pinout of ATX 20-pin connector version 1.0. Connector for tester I salvaged the defective board. It can also be inserted into the 24-pin cable connector (ATX version 2.0), four positions will overlap. There is only another COM, 3.3V, 5V and 12V, probably to reduce resistance of cables and to enable to work with higher currents. Some SMPS's have no -5V output, with such supplies the -5 LED will not shine, but it does not mean the supply is faulty.