Does a P4 PSU fit a P3 mobo?

September 30, 2009 at 10:43:25
Specs: Windows XP
HiHi,
My friend's Grandpa's old 250w PSU need a change, so I went for a new one on the 400-500w range. However, in my town there's only pentium 4 PSUs, and his is a Pentium 3 PC. The first PSU I've got fitted Ok but after booting the screen, it showed an error and directed me to the BIOS.
On the Hardware Monitor section it says -12v in red, alongside -5v in blue. So I returned it and got a new one (believing the 1st was faulty) ...only to get the same readings later on ¡¡?
On the PSU's box color diagram it says the same voltages and coding than the old 250w ones...
Thus I just dont understand what is going on.

It booted anyway and recorded a CD as a test; worked fine, but I'm doubtful on just setting the "ignore" option on the BIOS, and keep the PSU when it could possibly cause future problems...

Is it OK as it is, or should I shut it down inmediately and look for an old one on used hardware shops?
Thanks for your replies.


See More: Does a P4 PSU fit a P3 mobo?

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#1
September 30, 2009 at 11:17:43
If you don't have any ancient ISA cards installed, you shouldn't worry about the -12v and -5v readings.

Any ATX power supply will work with an ATX P3 board. I have an old P3 system powered by a 400w Enermax with a sticker that says "P4 Prescott and Athlon 64 X2 ready".


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#2
September 30, 2009 at 11:42:00
Newer standard ATX PSs that can be used with P4 mboards are backward compatible with older standard ATX mboards. PSs that are newer merely have more available types of connectors from the PS that older mboards often don't need.
.....

"My friend's Grandpa's old 250w PSU need a change..."

What were the symptoms that made you think there was something wrong with it?
......

Has anyone changed which ram was installed since the system last worked properly?

Ram that works in another mboard , or any ram you buy or have lying around, may not work properly, or sometimes, not at all - even if it physically fits and is the right overall type (e.g. SDram, DDR, DDR2, etc.; PCxxxx, xxx mhz) for your mboard. In the worst cases of incompatibilty your mboard WILL NOT BOOT all the way with it installed, and the mboard may not even beep - the ram has to be compatible with the mboard's main chipset, or in the case of recent mboards, compatible with the memory controller built into the cpu.

E.g. for PIII mboards
- some cannot use 8 chip 256mb modules - they must have 16 chips
- some can't use any 256mb modules
- most if not all cannot use 512mb modules
- some 64mb SDram modules are known to be incompatible with many mboards.

The same applies for testing your ram in another mboard - the ram must be compatible with the other mboard - if it isn't, any results of testing the ram on the other mboard cannot be relied upon to be valid.

If you still have the ram that was installed when the system worked fine, try installing just that ram.
........

"Some PIII mboards have developed this problem -

Open up your case and examine the mboard to see if you have bad capacitors, and/or other findable signs of mboard damage .

This was the original bad capacitor problem - has some example pictures.
History of why the exploding capacitors and which mboard makers were affected:
http://members.datafast.net.au/~dft...

What to look for, mboard symptoms, example pictures:
http://www.badcaps.net/pages.php?vid=5
Home page that site
- what the problem is caused by
- he says there are STILL bad capacitors on more recent mboards.
http://www.badcaps.net/

Pictures of blown capacitors, other components - power supplies, Athlon cpu's, etc.:
http://www.halfdone.com/Personal/Jo...
.......

It's extremely unusual for you to get errors regarding -5v and -12v.

Some PIII mboards may require a proprietary (oddball) PS that is wired differently.

State the make and model of the brand name system, or if it is a generic system, the make and model of the desktop mboard.
The specific brand name system model is usually on a label on the outside of the case somewhere, or it can be determined by going to the brand name's web site.The model of the mboard in a generic system is usually printed in obvious larger characters on the surface of the mboard, often between the card slots.
......

"The first PSU I've got fitted Ok but after booting the screen, it showed an error and directed me to the BIOS.
On the Hardware Monitor section it says -12v in red, alongside -5v in blue. So I returned it and got a new one (believing the 1st was faulty) ...only to get the same readings later on ¡¡?"

Tell us exactly what you're seeing.

It's extremely unusual for you to get errors regarding -5v and -12v.
.......


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#3
September 30, 2009 at 12:12:42
Well two different answers...

Jackworld: No ISA cards. I'll follow your advice, and keep it. It looks coherent with what I've searched so far.

Tubesandwires: Very informative answer. However twice you say "It's extremely unusual for you to get errors regarding -5v and -12v."
But this is precisely what actually came out with TWO different PSUs, from different brands...
In between replacements, I did tried other (P2) 250w PSU that worked fine and showed correct -5v, -5v on the Hardware Monitor.
Add all that with the fact that everything seemed to work OK with the latest install...

Thus I am inclined to follow Jackworld advice and just ignore the readings, counsel about not using any ISA card, and keep the last PSU working.

Thank you again, somehow I do feel relieved.


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#4
September 30, 2009 at 12:22:43
That's jackbomb to you, kid. :-)

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#5
September 30, 2009 at 12:37:43
Please answer the questions and requests for info I asked about.

You haven't told us any details about this:

""The first PSU I've got fitted Ok but after booting the screen, it showed an error and directed me to the BIOS.
On the Hardware Monitor section it says -12v in red, alongside -5v in blue. So I returned it and got a new one (believing the 1st was faulty) ...only to get the same readings later on ¡¡?""

"Tell us exactly what you're seeing."
.....


-5v is required to support a function of ISA cards most people never use anyway. Newer PSs sometimes don't have the wire for that in the main ATX power connector - it's white, no stripe - the only possible white wire - if the connector has that; one position in the main connector has no wire if it doesn't have that -5v wire . Even if you have ISA slots and ISA cards the PS will work with the mboard anyway it that wire is missing - the PS just can't support that seldom used function. However, I've never encountered a mboard that produced an error about -5v when a newer PS that doesn't have that white wire was connected to the mboard.

I have many power supplies, old to fairly recent - all the standard ATX ones output -12v and have a wire in the main connector for that. Look on the label on the PS. If it lists -12v, there's something wrong with the mboard. (It's the blue wire in the main connector - the only blue wire).


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#6
September 30, 2009 at 13:02:30
Yes jackbomb as you wish.

Actually I'm basing my decision on your advice. Thanks for that. I just hope we..err you are Ok in this. haha
You may kid me while it works


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#7
September 30, 2009 at 13:14:32

Tubesandwires:

Please answer the questions and requests for info I asked about.

You haven't told us any details about this:

""The first PSU I've got fitted Ok but after booting the screen, it showed an error and directed me to the BIOS.
On the Hardware Monitor section it says -12v in red, alongside -5v in blue. So I returned it and got a new one (believing the 1st was faulty) ...only to get the same readings later on ¡¡?""

"Tell us exactly what you're seeing."


-5v is required to support a function of ISA cards most people never use anyway. Newer PSs sometimes don't have the wire for that in the main ATX power connector - it's white, no stripe - the only possible white wire - if the connector has that; one position in the main connector has no wire if it doesn't have that -5v wire . Even if you have ISA slots and ISA cards the PS will work with the mboard anyway it that wire is missing - the PS just can't support that seldom used function. However, I've never encountered a mboard that produced an error about -5v when a newer PS that doesn't have that white wire was connected to the mboard.

I have many power supplies, old to fairly recent - all the standard ATX ones output -12v and have a wire in the main connector for that. Look on the label on the PS. If it lists -12v, there's something wrong with the mboard. (It's the blue wire in the main connector - the only blue wire).

Hope the following fills the data blanks:

On the PSU box:
orange. +3,3
yellow +12v
Blue -12v
red +5v
white -5v
purple +5Vsd
black COM
green PS-ON
Brown POK

My readings
On the BIOS Hadware Monitor section,
Vcore Voltage 1.68 (fluctuating)
+3,3v Voltage 3,31
+5v Voltage 5,08
+12v voltage 12,4
-12v voltage -12,49
-5v voltage (-12) (red color what is in braques)

On the red -12, pressing enter the BIOS allows for the "ignore" option.


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#8
September 30, 2009 at 14:14:50
"On the PSU box:"
"Blue -12v"

OK, so the PS has -12v output.

"-5v voltage (-12) (red color what is in braques)"

That's abnormal.
I've seen bioses list the -5v voltage as 0, or list nothing for that, when the PS has no -5v output, but I've never seen that.

Did you check the mboard for evidence of failing or failed capacitors as in response 2?

You haven't answered this:

""My friend's Grandpa's old 250w PSU need a change...""

"What were the symptoms that made you think there was something wrong with it?"




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#9
September 30, 2009 at 14:50:05
"On the PSU box:"
"Blue -12v"

OK, so the PS has -12v output.

"-5v voltage (-12) (red color what is in braques)"

That's abnormal.
I've seen bioses list the -5v voltage as 0, or list nothing for that, when the PS has no -5v output, but I've never seen that.

Did you check the mboard for evidence of failing or failed capacitors as in response 2?

Well No. I haven't visually checked, but I did replaced the PSU with a similar one, and that one did show correct readings -5v, so it seems it's related to PSU specs differences..
Now I wouldn't like to bother the Mr. without a funded reason (more so after all the replacing done already).

You haven't answered this:
""My friend's Grandpa's old 250w PSU need a change...""

"What were the symptoms that made you think there was something wrong with it?"

Sudden interrupts made him ask me to see it. The first thing I did was to take out one of the two HDD installed to made an external back up of the important info stored. I also disconnected a 2nd CD drive (old). From then on, no interrupts.
A month or so later, I reinstalled the HDD as requested. Almost instantly the PSU started gross sounds and upon a closer look I could see its fan stopping.
A few seconds after I had the PSU out and were smiling at its 250w rating.
You know the rest of the story.


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#10
September 30, 2009 at 15:23:01
"A few seconds after I had the PSU out and were smiling at its 250w rating"

A PSU in a P3 system is probably pretty old, but if the system worked fine with a 250W PSU, there was no need to get a 500W. P3 systems are different than P4 (or newer) systems in that the P3 is powered entirely by the +5v rail. New CPUs are powered entirely by the +12v rail, so the PSU/amperage configurations are different. New systems need lots of amps on the +12v, old systems need lots of amps on the +5v. Compare the amperage specs.


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#11
September 30, 2009 at 17:43:50
Ok, good to know jam, thanks ¡

The new PSU amps:
Orange +3,3V 30A
Yellow +12V 18A
Blue -12 0,8A
Red +5V 40A
White -5V 0,5A
Purple +5Vsb 2A

Any stronger confirmation on if the P4 PSU is harmless on an ISAless P3 mobo?


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#12
October 1, 2009 at 13:29:48
""Did you check the mboard for evidence of failing or failed capacitors as in response 2?""

"Well No. I haven't visually checked..."

Do that.

""What were the symptoms that made you think there was something wrong with it?""

"Sudden interrupts made him ask me to see it."

Sudden interrupts??

"I also disconnected a 2nd CD drive (old). From then on, no interrupts."

A malfunctioning CD or DVD drive can cause all sorts of strange symptoms. E.g. long delays while booting, or the post sequence stops (stalls forever) while booting, or if it's on the same data cable as another drive, neither drive on the sane datacable may work properly. The most common reason CD or DVD drives fail is the sleeve bearings in the motor have deteriorated to the point they produce so much friction that the motor can no longer spin at even 1X, the original Audio CD speed, or the motor won't spin at all.
Those cheap sleeve bearings are the major reason why no CD or DVD drive has been warrantied for longer than a year since about the time 8X CDrom drives first came out.

"A month or so later, I reinstalled the HDD as requested. Almost instantly the PSU started gross sounds and upon a closer look I could see its fan stopping.
A few seconds after I had the PSU out and were smiling at its 250w rating."

If you catch the fan problem early before the PS is actually damaged from having overheated too often, if the current voltage readings in the bios Setup are okay, within 10% for +12v, +5v, and +3.3v, then all you need to do is replace the fan in the PS. Usually the fan in older PSs is 80mm. For often $10 or less you can get
- a 2 wire PS fan, connect it to the existing wires for the old one, preferably with soldered connections,
- or if the mboard has a spare 3 pin fan header (for a system or case or PS fan) , get a 3 wire case fan and connect it to that header, then you can monitor it's rpm in the bios.
The original fan probably failed because one or both bearings on it are cheap sleeve bearings. The replacement fan should have two ball bearings or better (e.g. ceramic) - if the label or descripition says ball bearing, without the s, it probably has one ball bearing, one sleeve bearing.

250 watts capacity is more than you need for these old PIII mboards - they will often run fine with as little as a 200 watt PS - providing you don't install a video card in a slot that has a newer chipset that requires the system PS has a minimum PS capacity that's higher than 250 (or 200) watts.

If I were you I would briefly connect the orginal PS and see what the voltages in the bios Setup are for it. If they're within range, if Grandpa isn't going to upgrade his video, you could replace the fan in the PS and use the new PS you bought for some other computer.


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