CHOOSING A power supply is no longer as easy as it once was, specially when you're building a high-end Pentium 4 system.
The once easy question of "do you need an AT or ATX power supply?" has been replaced with "What kind of ATX?, ATX12V, SSI-compliant?, EPS12V one? with a 4-pin or 8-pin connector?" That must be my punishment for staying with a Pentium III as the fastest system in my data-room for so long.
My new 800 MHz FSB Pentium 4 motherboard recommends using an "ATX power supply designed for Pentium 4, with extra 12V connectors, that complies with Intel's SSI specification". I bought one supposedly P4-compatible 500W PS, and when I got home I realized it had 20 pins in the ATX connector, plus an extra square 4-pin 12v line. My mobo has 24 pins on the ATX plug. It didn't even fit. The amp rating was also a bit low on the -12v and -5v lines.
To confuse me even more, a quick Google search on "24 pin ATX" got me to a page that talks about the "EPS12V" standard, identified as "an extension of ATX for servers, with 24 pins instead of 20". My mobo manual does not mention "EPS12V" anywhere, perhaps this is the manufacturer's fault.
A quick search for "EPS12V power supply" didn't make things clearer, as I got a dozens power supplies listed, from $75 to $150 , most labelled for "dual Xeon servers", and some identified as "ATX12V + EPS12V dual". I even found one identified as "ATX + ATX12V + EPS12V SSI. 3-in-1". What a mess!
But worry not. After reading several dozen pages, it all comes down to this:
ATX = 20 pin plug, your Pentium III's ATX connector and PS
ATX12V = 20 pin atx connector + 4-pin plug for "Additional 12V"
SSI = Intel spec, which, among other things, defines the "EPS" "enhanced" ATX specification
EPS12V= Power supply with 24-pin EPS12V connector, plus one 8-pin "additional 12v connector".
On my 800 MHz FSB motherboard, among others, the manufacturers decided to use a four pin plug, instead of the recommended eight pin one, for the "additional 12v line". Conclusion? Due to this mess, most "high-end" power supply manufacturers, make "EPS12V" power supplies with small four pin square and eight pin additional 12v connectors.
Supermicro describes in no uncertain terms that it wants its customers to use an SSI-grade power supply, by including a pink paper sheet that reads something like: "Server System Infrastructure (SSI) specification power supplies are recommended over other power supplies". (...) "WITH THIS MOTHERBOARD USE ONLY PENTIUM 4 FAMILY, SSI-SPECIFICATION POWER SUPPLIES. THE USE OF INADEQUATE POWER SUPPLIES CAN DAMAGE BOTH THE POWER SUPPLY AND THE MOTHERBOARD IN WHAT IS OUT OF WARRANTY ABUSE". OK, with all those caps, I get the message!
If sorting out this "ATX vs ATX12V vs SSI/EPS12V 4-pin 8-pin" mess took me some searching around and reading a dozen interweb pages, imagine the mess it would be for the average punter to change her Pentium III system's PS when upgrading to a P4 motherboard.
When I initially asked about this on a mailing list, Solaris x86 advocate Al Hopper told me I was drowning in my tea, and that it was "all very simple". I love the simplicity of Unix people.
He explained that the later P4 CPUs take their power from a 12 Volt feed and, using the onboard voltage regulators, generate the high current, low-voltages they need to operate (anywhere from ~ 1.6V to around 2.7V). So the first requirement is a PSU that has plenty of power available from the 12V supply. Since the older ATX compatible PSUs didn't supply much current from the 12V section you have to ensure that your new PS delivers enough current (or power in Watts) from the 12V section. That's why using your old P3 ATX PS is a big "no-no".
He then described the history of the post-P3 power supply mess:
The earlier Athlon motherboard manufacturers decided to solve the 12v problem by using an additional 4-pin square connector to get the extra 12V those CPUs required. However many older PSUs didn't provide the 12V 4-pin square connector.
The motherboard makers then wised up and decided that there was nothing magical about a square 4-pin connector, so they put a normal hard disk type socket on the motherboard and provided the 12v power via a standard hard disk (4-pin inline) connector. Problem solved - you may now use your older PSUs provided they supply sufficient 12V current (many did not).
Further confusion came from PSU manufacturers not specifying the capabilities of the PSU in a way that allowed the end user to verify it's 12V power output rating.
In the meantime the ATX spec was saying "use the new 6-pin" inline connector - and very few motherboard makers implemented it.
Just to be sure, some motherboard makers, implemented *both* the hard disk style 4-pin inline connector and the square 4-pin connector. They said "use either or both in any combination you like".
The spec then evolved to the 24-pin main connector. Again, most motherboard manufacturers did not wish to make their customers mad by mandating that they replace their power supplies. This might cause their customers to avoid motherboard upgrades. Some used a "special" 24-pin connector with the extra 4-pin connection blocked off, or colored so that the user could plugin a 20-pin plug into the correct end of the 24-pin socket on the motherboard. Many others simply ignored the 24-pin requirement in the specs. Again - problem solved - use your older PSU.
Some final tips/tricks when upgrading from a Pentium III to a Pentium 4 motherboard.
a) If you can't determine the 12V output capability of your PSU, replace it.
b) If your PSU is older than, say 3 years, replace it.
c) If your system needs an EPS12V power supply, get a quality one, better if you can find a web site of the manufacturer and the power supply has been reviewed on third party sites.
d) If you're using an 800 MHz capable P4 - be careful to get a good PSU with high 12V output capability.
e) For a P4 in the 3 GHz range, get a PSU with a 450W rating as the minimum. Again watch the 12V rating.
f) Don't use 20-to-24 pin "converter cables" unless you want to risk your motherboard.
The price won't be nice. You're lucky if your P4 "main board" has an ATX12V connector and the manufacturer doesn't require a SSI power supply, as there are some low-cost ATX12V power supplies of uncertain quality.
Me? I ended up paying over $100 for a quality SSI-grade "EPS12V" power supply.
The era of cheap, ~ $35 made-in-China ATX power supplies seems to be over, as the motherboard industry moves towards "high-precision" EPS12V ones.
Suddenly the power supply is almost as expensive as a new processor. µ
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