dosgamer wrote:It's trivial to add a -5V line to an ATX PSU. Just wire in a 7905, powered from the -12V rail. Can be done internal or external to the PSU.
or a MAX739 or similar inverting DC-DC converter that can run off of the 5v rail or 12v rail and provide -5v output (hence the "inverting")
Your idea is of simpler to implement but MAX739 is still simple.
Note that ATX PSUs often offered around 300mA on -5v, and at 300mA you'll be dissipating 2.1W with your method (7 volts drop * 300mA). TO-220 will have a junction-to-ambient rating of about 60C/W. Should add a little heatsink ideally, though if you're not actually pulling anywhere near 300mA, you'd be fine without. If you did pull 300mA, that would also use up around a third to half of your -12v rail current capacity, but you're probably not using much -12v either so that also should be fine.
Efficiency will be very low, like 40% or so, based on the voltages, but if you're wasting 60% of just a couple of watts, no big deal.
MAX739 avoids those problems for the most part, but again is not really necessary. Just rambling on my part. As usual.
Yes, I always ramble this much.