Originally Posted by Dejavu
What do you think?
I think Ian's page has caused a lot of damage already, and should not be referenced here.
ATX power supplies generally have the 12V rail as their main rail these days. That means if there is no sufficient load on the 12V rail, regulation on the 5V rail is no good.
As a result, the step response on 5V is even worse: On a load change, the regulation might get totally out of proportion. Using an ATX power supply for the Amiga is therefore dangerous to all connected hardware.
I haven't seen an A500 or A1200 that draws more than 60W. Always remember that all the power that a computer requires is converted into heat, and you *really* don't have that much heat on an A1200 or A500; it's more like 25-30W.
If you've had power issues, it's most likely due to bad regulation of the power supply (high ripple - my guess is that you've had visible stripes in the Indivision ECS output, a clear indication that ripple is high). I'd accept an ATX power supply if it has separate DC-DC converters for each rail, and thus has no cross-regulation issues, but these are only starting to become available.
If the 5V rail is generated by a proper DC-DC converter, you can safely connect RapidRoad to the ACA500. However, an off-the-shelve average ATX power supply will most likely generate spikes on the 5V rail on every USB hot-plug event, causing random hardware defects. You should not risk that - the Amiga is a classic computer and it deserves a good power supply. A good power supply can't be judged just by voltage and amperage - you need to look at step response on 5V, which is hardly available in standard datasheets.
We've had this power supply discussion on a1k.org just a short while ago, and there were a few good suggestions/candidates for measurement. However, no matter how often people ask, I can't give a recommendation for a power supply unless I have measured it on my own. Since I don't own all the required equipment to measure a power supply (especially isolation), I couldn't do that job to a degree that would be required for a final decision.
The safest thing is to either use the original power supplies (A500 switching supply can provide 4.5 amps on 5V), or to build a DC-DC converter.