Diagram Diagosis

Many enthusiasts will be using their PCs as data loggers, controllers or as web servers. ln these cases it is important that  the machine is kept powered up for as great a fraction of the time as possible, even if there has been a power cut or if the power button is inadvertently pressed by another member of the household. Todays operating systems offer a range  of automation options and it is perfectly possible to arrange things so that the computer starts itself up automatically.

The always oncircuit shown here automatically restarts an ATX PC in the above situations. There are just two components: a Schottky diode connecting the power but-ton pin on the motherboard to the +5 V line on the power supply, and a capacitor from the power  button pin to ground. The  capacitor  is a 68  pF tantalum type rated at 6.3 V, and the diode is a type SB 120, rated at 20 V and 1 A. The total component cost is in the sub-one-beer range!

The most convenient arrangement is to mount the circuit directly on a 4-way Molex disk drive power plug, insulating the capacitor and diode using heatshrink tubing. The assembly can then be plugged  into a spare socket on the power  supply.

The operation of the circuit is straightforward. When the +5 V supply fails (i.e., when the computer is turned off), the  power button pin on the motherboard is pulled low via the Schottky diode. This instructs the motherboard to power up again. As long as the +5  V supply is present, the diode blocks and the power button pin remains at high impedance, floating typically at around 3.3 V. The capacitor serves to filter out spikes and brief dropouts. ln its simpler version  the circuit replaces the power button on the case, and the computer can now only be switched on and off at the mains.

The author has tested the circuit on modern SuperMicro X8SAX and XSDTH-6F mother-boards as well as on an olderTyan  Tiger MPX. He found that the capacitor value should be reduced in some cases: the SuperMicro motherboards have a high internal pull-up  resistance which only charges the capacitor rather slowly.

Note that some PC keyboards have a Sleep button which puts the computer into a low-power mode. ln this case the  circuit will not work, and you should either use a keyboard without such a button or disable sleep modes from within  the operating system.  ln its more advanced version the existing power button is retained in parallel with the circuit (see circuit diagram). The power button then  causes a graceful  shutdown whereby the operating system can bring the computer to a halt in an orderly manner.

Source : http://www.ecircuitslab.com/2012/06/always-on-for-pcs-circuit.html