PC power supplies (the Joy of Standards)

[Andy Wade]

(Note crossposting)

Any PC power supply experts out there? I'm just trying to repair a friend's PC with a dead PSU. Well I assume it's dead - it does produce the +5V standby rail OK and when the PC's front panel power button is pressed the PS_ON signal line is pulled low, but nothing else happens - none of the main power rails come up. IOW the PSU is getting the command to turn on, but appears to be failing to do so.

Anyway, a nice simple job I thought, trotting down to the local Chinese computer take-away (MKC) to buy a new PSU. "Oh they all come with a

24-pin motherboard connector now, not 20-pin like that" (pointing at the old one I'd taken along). "Don't worry, it'll be compatible, you'll just have 4 unconnected pins on the connector. Line the latching tabs up and it'll be OK."

Tending not to trust anything anyone in a shop says, ever, I have carefully verified that if the new PSU connector is inserted into the mobo connector pin 1 - pin 1 then all rails and signals do correspond, and the four new pins only duplicate the +3.3, +5 and 0V rails. The latching tabs don't align in this position, but that's immaterial, other than confirming my mistrust...

But there is one possibly significant difference. The old PSU provides a -5V rail on pin 12, but the new one doesn't - there's no connection to the corresponding connector position (now pin 14 in 24-pin land).

Hmm, time to RTFM. Does the motherboard - an Asus A7V8X - actually need a -5V rail? The manual (user guide) from their web site shows the

20-pin connector pin-outs, including -5V, but says nothing about what it's for or whether it's needed. Only slightly more helpfully, the new PSU's installation guide - it's a CoolerMaster RS-380, by the way - says this:

"This power supply does not support ?5V for the motherboard for new ATX form factor operation. Please update the BIOS setting of the motherboard."

Now the questions:

1. DoeS aNyBody know whether the Asus A7V8X motherboard needs a -5V rail? Will it be harmed if powered-up with this rail missing (o/c). What is/was this rail used for anyway?
2. What have BIOS settings got to do with this? How can changing a software (firmware) setting compensate for the lack of a power rail?
3. What is the line labelled PG (pin 8 on both version) for? (This question is for interest only.)
4. Can you still get ATX PSUs with 20-pin mobo connectors and -5V outputs, if this turns out to be needed?
5. Is there anything else I should know, before powering-up.

The Joy of Standards? - There are just sooo many to choose from...

TIA

[Ian Stirling]

Essentially nothing uses -5V AIUI. It's very, very unlikely not to work.

[The3rd Earl Of Derby]

So that should mean you will have two unused pins either side of the block if its lined up with the latching?

Personally I'd of try to get my hands on a 20pin PS.

Don't always assume its a power supply fault. :-)

ps who pays for the PSU if you find out its not that?

[Jules]

Seconded. I'm struggling to come up with anything that's ever used -5V TBH; even things like soundcards and serial ports would have used +/-12V lines and not +/-5V.

It's probably a hang-up from the original IBM PC days, but even then I'm not sure what would have used it - memory chips were all single-rail devices by then. I expect its inclusion goes back further than that, to machines in the 70's that did need -5V for the memory, and IBM included it in their machine "just in case".

Hmm, I'm going to have to go away and ask now, though.

cheers

Jules

[kimble]

IIRC, this stands for "Power Good", and is an indication that the power supply has started up and stabilised so the motherboard can now get on with booting. No idea whether it's actually used (as opposed to an arbitrary delay or whatever) by most motherboards.

Kim.

[Neil Jones]

When I bought a new ATX power supply recently I disdcovered that the

24-pin connector was actually a 20-pin job connected to a 4-pin affair and it was easy enought to separate the two.

Regards

Neil

[Adrian C]

The Old 20-pin ATX supplies were built to the following spec.

spec on release 1.3 (in 2002), deleted the -5V rail as little industry interest in continued use.

The New 24-pin ATX supplies are built to

rail extinct!

Pin 8 is the power good (PWR_OK) signal, goes to 'high' if the main 12V,

3.3V and 5V supply lines are in spec.

[Andy Burns]

most recent couple of PSUs I've used come with a 20pin connector, plus a "slide on" extra 4pin connector that makes it into a 24pin, I used one on a 20pin mobo, and the other on a 24pin mobo, both worked fine.

[Sylvain VAN DER WALDE]

I fitted an ATX 2.0 power supply recently. It has a 24 pin power plug. The _4 right-hand side_ pins are designed to be separated from the 20-pin l/h side of the plug, but I was able to fit the the plug leaving the 4 unconnected pins still attached. _Only_ leave the r/h side 4 pins unconnected, and _not_, I repeat _not_, have 2 pins on the left and 2 pins on the right disconnected. I would investigate that plug/socket latching before doing anything else. Is your computer a Dell, Packard Bell (and a few others)? I understand that some of their bits can only fit these particular computers.

Sylvain.

Not good for possible future upgrading.

Sylvain.

[Andy Burns]

I thought they had a mixture of "O" and "D" shaped polarisation of the pins to prevent that

Certainly true that 5+ year old Dells use non-standard 20pin connector :-(

[kimble]

Or, more accurately, a standard connector with a non-standard pinout. Recipe for disaster that it is.

Kim.

[Ian Stirling]

Hmm. Is it even exported to PCI? I think it's on ISA

[ThePunisher]

A bunch of new boards from MSI use -5V

eg:

[Andrew Haylett]

The PSU in my Dell at work went south the other day; we wheeled in a new PSU, plugged in the 20 pin connector - not a sausage. Then we realized that the old PSU, and indeed the new one, had an extra connector that went to another place on the motherboard (hadn't really noticed that when we took the old one out in the midst of unplugging all the disk/SATA power as well). Plugged the second connector in, and hey presto. So I guess this corresponds to the detachable bit you mention above, only it was already detached on the new PSU. What it is useful for, I have no idea - but obviously it does something!

[Chris Newton]

Noted, but since you didn't set follow-ups to either group... :-)

[snip]

I'm currently running on an A7V8X. AFAIK, nothing requires the -5V, though I can't say I've checked carefully as I'm using a good Enermax PSU with the correct connector anyway.

HOWEVER, I did have horrible problems with the original graphics card I bought for the system. According to various on-line sources (after a few months and several incidents of the problem cropping up) was some minor deviation from spec of the motherboard and/or graphics card power systems, which resulted in the system effectively hanging occasionally.

The original graphics card was an ATI Radeon 9700 Pro, which used only a small-style disk drive power connector for auxiliary power. The replacement, a 9800 Pro that uses a full-size connector, has been solid as a rock for nearly three years.

This cautionary tale may or may not be of any relevance, but I guess I'd be wary of any non-standard power issues with the A7V8X.

Hope that helps, Chris

[Andy Wade]

Not all are like that though, including the one I bought. On mine the

24-pin PSU connector mated quite happily with the 20-pin one on the mobo, with 4 legs out of bed, so to speak.

Anyway, a big thanks to everyone who replied. The machine is now back together with new PSU and tidied-up wiring and appears to work quite happily with no -5V (and no BIOS tweaking either).

Maybe not. There are separate outputs carrying only +12V & 0V on square

4-pin connectors (four pins, two rows) on both the old and new PSUs. They're labelled "12V CPU connector." These have no use with the Asus motherboard in question; I think they're for later boards where the CPU supply is via a local on-board DC-DC converter - a much more sensible way of doing things when you want very few volts at an awful lot of amps.

[Andy Luckman (AJL Electronics)]

I believe the -5v rail is no longer used. It mat have been RS423 which uses

+-5v rather than RS232 +-12V.

The RISCOS motherboard does use it though just for interest. I think it feeds the sound processor.

[Malcolm Stewart]

I remember -5V being quite important back in 1960 when I was selecting core store driver transistors, germanium, for a Ferranti Argus computer.

[CWatters]

Some power supplies are unstable without a load on them and will shut down. Not sure if this is true of PC power supplies.

Sometimes the bit with the 4 extra pins is detachable. The two parts can be slid apart.

[Andrew Gabriel]

We had fun with cores too at GEC -- they also used +18V and -18V. The PSU's were one of the more interesting sides. In order to guarantee no data loss during power outage (core is non-volatile), you have to be very careful how the power rails are shut down so that no spikes are delivered to the core store which might corrupt the data, or activate any of the driving logic as the power rails decay below their operating spec. In order to do this, the power supplies were switched off by shorting their outputs in a timed shutdown sequence, which took around 50ms to shutdown all rails. This meant the PSU's were effectively switched off by activating their over-current protection, which always made me smile.