Hello. I have need to add 3VSB function to a typical PC ATX12V power supply. This is to power a proprietary Compaq motherboard. Multiple attempts at sourcing suitable power supply from Compaq or elsewhere were unfruitful, so the original power supply has been reverse engineered to a minor extent and a standard ATX12V power supply has been refitted with the original PS wiring harness, duplicating needed function with the exception of this 3VSB. This new PS is also dimensionally modified to fit a proprietary chassis form-factor, so it would be impractical to "start over" with another whole/new PS. Apparently the motherboard uses the 3VSB instead of 5VSB to power the pull-down circuit for the power supply power-on line, as the new power supply, system, won't turn on with 3VSB missing. This was confirmed(?) by removing only 3VSB rail from original PS, also resulting in failure to turn on PS or system. Part of the problem may be that I DO NOT HAVE ACCESS TO THE SYSTEM. I was sent the original power supply, asked to get a new one working remotely, with the person on the other end, with the system, having only the typical end-user's abilities, no electrical experience or diagnostic equipment. Also no specs are obtainable from Compaq and the original power supply appears to have no markings to hint at true manufacturer. Yes, apparently I'm a glutton for punishment. Original PS is spec'd for 3VSB of 3.3V, 2.2A, but I believe this is partially for PCI power-on events unseen in the system, so practically the target is 1-2.2A output @ 3.3V +-5%, How to implement 3VSB? There is very limited free space remaining in the new power supply, perhaps a couple square inches contiguous, half a dozen more sq. inches if the fan were mounted externally, as it is possible but not preferred. It is not necessary to have this 3VSB circuit internal to the power supply case, only that it be wired to the motherboard harness, could be a tethered box that plugs into it, but it is highly desired that this box be small, perhaps under 9 cubic inches. Whole 3V power supplies (complete units in case or just the finished PCB) seem to be rare, overly large, and expensive. I'm hoping you will point out problems with the following ideas and/or suggest other alternatives: - Tap into 5VSB rail, 3 diodes in series, voltage drop to ~3.4V. Best guess is that the 5VSB has enough reserve to do this, but with no experience or specs on target system I'm left to rely on experience with standard PC systems, - Linear regulated 3VSB from new PS 5VSB rail or from external ~5V PS, a la LM317. - Remove and reconstruct 3VSB circut from original PS, which may not fit in new PS casing. This circuit is it's own simple SMPS, tied into original after it's AC rectified. This seems the least desirable alternative due to the time it would take to do it, as well as my limited abilities to hack SMPS. - Source a small, affordable (cheap) 3.3V PS. The budget is $0, but the time savings is potentially worth something ($). Anyone know where low-cost 3.3V 1-4A PS might be found? Thanks for your input, James
