lmttn wrote on 2021-01-05, 00:25:

I'm in the process of putting together a sorta Frankensteinish PC out of parts I've ordered (namely a Pentium III 800 and an ASUS P2B-F motherboard) and ones I already have (case, Voodoo3 2000, SBPCI, 192MB RAM, optical drives, etc) but I just realized that my only spare PSU is the wrong form factor for my case, so I will need to order a power supply as well. Seeing as it is an ATX motherboard, will modern ATX power supplies still be compatible, provided I get the converters needed for molex and IDE connections? Is there anything in particular that I should be wary of?

a) Aim for something with 20A, preferable 25A or more each on 3.3V and 5V rails and a max of at least 130W, preferably 150W or more on the 3.3V and 5V combined .See Any modern psu's that have large 5v rails
b) You will need a 20-pin ATX connector. Many PSUs come with a 24-pin connector with detachable-pin add-on, but some PSUs now come with a monolithic 24-pin connector that will not fit without either an adapter or modification . See Re: New Corsair RMi PSUs shipped with solid 24-pin cable ?
c) Beware of designs that do not handle low power draw on the 12V rail . This will not be an issue for modern designs that derive 3.3V and 5V rails from the 12V rail using DC-DC converters . See Re: Any modern psu's that have large 5v rails or search for "cross-load" for more information .
d) As modern ATX PSUs have done away with the -5V rail, a very few older ISA cards will not work without using an additional adapter to provide the required -5V . See ISA Cards & Devices Requiring -5V
e) This one might seem obvious, but do not cheap out on the PSU . Crappy low-end PSUs can be out of ATX spec, have excessive ripple, be overly optimistic (or outright lie) on output abilities and have a greater chance of killing connected equipment if/when they fail or even simply because of aforementioned ripple .

Thank you for the pointers! Are there any specific [recent and/or readily available] PSUs that are generally recommended for Slot 1 setups?

Old motherboards used primarily 5v power, while newer systems use mostly 12v. If you buy a new power supply most of the rated wattage will go to waste because it won't be on the 5v rails . Fortunately, PIII CPUs don't use that much power. Something like 35 watts typically.

I would get a new power supply, with 80 plus certification, and look for at least 20 amps on the 5v rails. If you read the specs for the power supply, the 5v amprage will be in there.

That isn't too hard to find btw. Most power supplies have 20 amps on the 5v rails.

lmttn wrote on 2021-01-05, 01:29:

Thank you for the pointers! Are there any specific [recent and/or readily available] PSUs that are generally recommended for Slot 1 setups?

I use a Corsair RM850x , which is both rather expensive and possibly overkill . I am currently running this on a socket 370 system with 4 sound cards and 2 video cards, multiple fans, 2 floppies, DVD-ROM and an SSD .

I initially thought I was cutting it close, as I posted here, but have since gone on to have exactly zero issues in 7 months . The only things preventing me from recommending this PSU are the high cost and the need for an ATX 24-pin to 20-pin adapter .

darry wrote on 2020-05-24, 05:55:

I installed my RM850x today and gave it spin in 3DMARK 2001 at 1600x1200 . It peaked at 146 Watts at the outlet with an average of about 130 Watts , all under load. When idle in DOS, I get about 90 Watts at the outlet . So, considering that represents less than 20% load, efficiency should be somewhere between 87 % and 90 % . That means peak power draw was, at most, about 131 Watts with an average of about 117 Watts under load .

Almost all of that is on 3.3V and 5V rails (main 12V user is probably my FX 5900 , other 12V "users" include 2 floppy drives and a DVD-ROM drive along with 3 case fans ) .

I feel I am cutting it a bit close, but I guess over-current protection will kick-in if ever things get too intense .

I just recently put together a PIII system:
-Asus P2B-D Motherboard
-PII( 600 MHz Slot 1 CPU
-128 MB RAM
-Geforce NVIDIA TNT2 M64
-PCI Sound Card
-ISA Sound Card
-PCI Network Card
-IDE Optical Drive
-IDE Hard Disk

Not exactly heavy load (I would think) for the era.

I ended up buying one of these, which seem to be new-old stock:
https://www.newegg.com/sparkle-fsp300-60atv-3 … =9SIAD247X25294

I was a bit wary on the brand and seller, but so far it's quiet and it works without any issues.

I bought a Rosewill 450W. It's 80 Plus Bronze certified and has 20A on the 5V rail so hopefully it'll work fine. I don't think my system should be too heavy for it seeing as the graphics and sound cards require no external power and I'll be using a SD-IDE adapter instead of a mechanical hard drive

assuming that's a newer group regulated PSU, i'd be curious about what voltage readouts in BIOS you get if you do wind up using it, because on one of those i've seen at least 3.5v on the 3.3v rail, which is out of atx spec. the p3 system still worked but it's well worth pointing out as not being an ideal case.

doubt not using a hard drive helps anything, rather the opposite, as those might be about the only part to really draw from the 12v rail. crossloading was already mentioned here, though afaik that term refers more to artificially high loads in PSU tests, whereas the power draw from a p3 won't be quite high enough for that.