Using an old PSU on an AMD64

pjeaton · Post by **pjeaton** » Mon Nov 15, 2004 10:19 am

Hello All,

I've just ordered an AMD64 3000+ and Abit A8V m/b, which I will be transplanting into my old PC case.

My current system has an Abit BH6 m/b with PIII500 Katami processor and a huge Foxconn HS and 5V remote fan.

My PSU is some old unbranded thing which I have converted the fan to work at 5V. It's mounted on the floor outside my open case and it never gets hot and it's inaudible.

I bought an extra ATX to P4 connector for the new motherboard (if it needs it), but the dealer told me that this PSU will not be stable enough for a modern processor.

Is he just trying to get me to buy a new PSU or do I really need a new one? How would I know if it's 'unstable'?

Any thoughts appreciated!

teknerd · Post by **teknerd** » Mon Nov 15, 2004 10:29 am

You should probably spend the money to get a higher quality PSU than the one you have now. You might be able to get away with using the old psu, but the rail voltages probably won't be very good and in the long run it could damage your new mobo and processor.

Tibors · Post by **Tibors** » Mon Nov 15, 2004 12:21 pm

Most salesmen want to sell. Some even believe you need a 500W PSU for a modern system. A good quality 300W PSU can power most systems that are sold today. Read: How much will a 300w power supply run?.

The fact a PSU is unbranded doesn't always mean it is low quality. But seeing the age of yours it is probably 250W. Try finding the listed wattage of the 12V rail. Then do a search in this forum about calculating power requirements.

I know this is not a yes/no answer, but it could start you finding it.

pjeaton · Post by **pjeaton** » Mon Nov 15, 2004 3:32 pm

Hmm, I looked at my PSU and it's only 230W

I have a mains power meter and I'll see what the PC is taking at the moment. Given that my 2 Seagate Barracudas are the only other things that get hot, I'll be surprised if it gets anywhere near 230W at the moment. (I have an old Matrox G400 VGA card.)

At the end of the day, the only way you can use power is by converting it to heat!

Obviously a new processor will have a greater requirement, but still may be within 230w.

I'm really thinking about the quailty of the power lines. Is the 'quality something I could see on my 'scope? I guess I could also check the DC Amps on each of the lines but that's getting a bit carried away, perhaps.

I'm all for spending money where it's due, but if I buy a new PSU, it'll have to be a fanless one, and they're not cheap!

I'll post my findings tomorrow - thanks for your thoughts.

Jan Kivar · Post by **Jan Kivar** » Wed Nov 17, 2004 6:09 am

Most likely the 12V rail has too little amps for the newer CPUs.

Cheers,

Jan

PPGMD · Post by **PPGMD** » Wed Nov 17, 2004 10:41 am

AMD officially recommends a 350 for that CPU.

If it was a 300 I would say maybe, but 230 is way too little.

GlassMan · Post by **GlassMan** » Wed Nov 17, 2004 1:27 pm

My personal rule of thumb, 18amps of 12v, 20 amps for videocard with molex, more for top of the line video, more again for over clocking. These are minimums, buy the most 12v power you can afford.

The A64 and modern video cards take most of there power from the 12v rail nowadays. Older spec psu's with 30 and 40 amps on the lower rails are inadaquate. The new 90nm amd's provide some relief, but the trend will continue.
As noted in Mikes Recommended PSU's, the rating of some psu' is actually less when the psu is operating in your case, making a borderline psu inadaquate.

oakdad · Post by **oakdad** » Thu Nov 18, 2004 10:49 am

They say PSU should have 24a of 12v for the 939s.

Elixer · Post by **Elixer** » Thu Nov 18, 2004 12:58 pm

I would say that's unlikely that your 230W power supply will work and be stable. I would try it out though. If it's a high quality power supply I would say there's a definite chance it will work. Use a program like Motherboard Monitor to check if your voltages are stable. I would say that's it's likely that if it does work you will have to underclock/undervolt to keep it stable. An excellent program to check the stability of your computer is Prime 95. If it can run for about a day straight then your computer is stable. Generally Prime95 will fail within a couple of hours if your computer is not stable.

Bat · Post by **Bat** » Thu Nov 18, 2004 1:07 pm

pjeaton wrote:I'm really thinking about the quailty of the power lines. Is the 'quality something I could see on my 'scope?

It is, except that you might have trouble seeing the transients e.g. when a CD drive starts to spin up.

fanerman91 · Post by **fanerman91** » Thu Nov 18, 2004 1:30 pm

oakdad wrote:They say PSU should have 24a of 12v for the 939s.

What about for 754s?

pjeaton · Post by **pjeaton** » Thu Nov 18, 2004 2:07 pm

(Apologies about the cross post with the nemcomers forum.)

I checked my PC power usage yesterday.

Spec: PIII500, 392MB PC133, HP CD-RW, 2xBarracuda IV 40 GB, 5V processor fan, 1 floppy, Matrox G400 driving 2 monitors.

Running CPUBurn and using both hard drives uses a total of...60w at 240VAC.

If my PSU is 66% efficient then the motherboard is using 40w.

If *everything* was powered from the 12V line then that 40/12 = 3.33A.

Things must have *seriously* changed for power requirements to have gone up so much.

I did find a document an the AMD site that gives a detailed example of a low powered system, like mine, and it came up with a power requirement of about 180 watts, which seems more realistic.

It seems that most of the power used in the Athlon64 is at 1.5V, 40 odd amps worth peak, on the 3000+ 90nm version I have, and the total power it uses is up to 62 odd Watts.

Now if everything on the motherboard uses the same amount again, then that's 120 Watts, and if everything is at 12V, it still comes to only 10A maximum.

Now my 230W PSU is unbranded, and might not work too well at full rating on one of the lines, and the 12V line is rated at 8A i.e. not much, so I might have a problem, but this assumes that *everything is running at 12V, which it won't be.

Lots of people have mentioned that you need a high current 12V line, and it's written down in a lot of places. However, no-one seems to be pointing to actual detailed requirements of how many amps you need.

My question is this: Does anyone know EXACTLY how much current on each line is required for a typical motherboard/processor combo? Does anyone have any real stories of how their PSU was simply not strong enough to power their system?

So far, on my modestly spec'd PC, I can't see the point in getting another PSU.

(I can also see me getting my 'scope and datalogger out cos I'm curious!)

mathias · Post by **mathias** » Thu Nov 18, 2004 3:14 pm

Exactly how much do you need on a +12 line? Basically, 2 amps per spinpoint, 2.8 amps per baracuda, about 1 amp for an optical drive, plus the wattage of the CPU(divided by 12v) x 1.25 for conversion inefficiency, and maybe a bit more for other stuff.

62 watts for that CPU plus those two cudas would be 12amps. Your PSU will mostly likely give out at startup, when the drives are spining up. Do your CD drives by any chance also spin up at boot?

pjeaton · Post by **pjeaton** » Thu Nov 18, 2004 3:27 pm

Now we're getting somewhere!

It's looking like worst case it could be marginal, best case I could be OK.

I'm gonna try it at the weekend, and see...

Jan Kivar · Post by **Jan Kivar** » Fri Nov 19, 2004 3:05 am

It's not the matter of whether the PSU would be actually capable of putting out enough amperage, but as mentioned, the problem lies at the boot time.

Back when the Radeon 9700Pro was released, it was one of the first (if not the first) GFX card which required extra power. Several people had issues when a cold start was made. In most cases, there was not enough power for the GFX card and many hard drives, and the computer had to be started (for instance) keeping reset depressed until the hard drives spun up, and there was enough power for the card.

This does not happen on all systems/configurations, because every GFX card has a different powering scheme (minimal power from AGP, nearly all from 12V/balanced load on all rails etc), and some Athlon motherboards powered the CPU from 5V rail whereas nearly all P4 systems used 12V. Also, PSUs itself have different, predefined actions what to do when output is exceeded. Some just turn off, some have "reserves" for short overloading (or they're just underspecc'd), and some give out smoke.

Even though some say that 24A is the min for A64 system, my current system is fine with only 18A. It is to be noted that I don't overclock, and I'm running relatively low-end CPU. Some have working systems with just 15-16A (350W Fortron and derivatives).

Cheers,

Jan

pjeaton · Post by **pjeaton** » Fri Nov 19, 2004 5:12 am

Thanks for your thoughts, Jan. They confirm the conclusion that I'd already come to; Different processors have different power requirements and different motherboards have different ways of making that power. Any standard 300W PSU will probably see you through in 95% of situations.

On a low rated PSU like mine (230W), it's possible, that one of the voltages will not be strong enough, especially at boot time.

In my favour, my video card is old and only has a small heat sink, and I only reboot my system once a week - less if I don't use it much - so holding in the reset button whilst it boots wouldn't be too much of a problem.

There does seem to be a many 'experts' all over the message forums there who are recommending this and that, without any knowledge power theory or who have never actually tested anything out!

Either way, I'll find out this weekend when I build the system. Maybe I'll check the current it pulls and monitor the voltages.

I might just buy a new 300W PSU anyway, the cheapest I can find and then I'll 5V the fan. This is what I did with my 230W one, and it never gets hot at all, although it is mounted outside the ATX enclosure on the floor.

GlassMan · Post by **GlassMan** » Fri Nov 19, 2004 5:58 am

I don't claim to be an expert. I gave you my opinion. That opinion is based on months of helping new a64 owners at the pcperspective chaintech web site http://forums.pcper.com/forumdisplay.php?f=46. The number one problem they have is insufficient 12v amperage. ( "another one bites the dust" has happened to higher rated quality psu's then yours) Even though your vid card doesn't draw 12v I doubt you will post with your power supply, and if it does it won't be stable. The a64 runs off the 12v rail. That 12v connector is there because the cpu's 12v current drain would melt the 20 pin connector and/or fry the motherboard traces.
You are free to try your current psu, but if you decide to upgrade, don't get the minimum you think will work, because if you buy a vid card to match your new system it will draw most of its power off the 12v rail as well. Note that with almost all psu's when the 12v rail is stressed it takes extra power from the other rails leaving less available for memory ,the board and disk reading, and vice versa, in your case leaving less for the cpu.
Go ahead and try your psu and if it runs fine good for you. I am sure it will give you trouble. I'm just putting the info out there so you have a chance of making a good investment

pjeaton · Post by **pjeaton** » Fri Nov 19, 2004 10:40 am

GlassMan wrote:I don't claim to be an expert. I gave you my opinion. That opinion is based on months of helping new a64 owners at the pcperspective chaintech web site http://forums.pcper.com/forumdisplay.php?f=46. The number one problem they have is insufficient 12v amperage.

Glassman, I appreciate your input, as your judgement appears to have been through experience, rather than simply regurgitating others' thoughts.

I shall try my old PSU and see what happens. If I can find a way of checking the currents on each line (fiddly, those connectors, and I don't have a clamp meter), I'll do that, and if I remember, I'll stick a 'scope and measure ripple before and after.

Actually, I just remembered my testing I did 18 months ago on my current system. See it here.

http://www.centipede.plus.com/pje/personal/quietpc.htm

It mentions my PIII500 being one of the most power hungry at the time, consuming a huge 28Watts

GlassMan · Post by **GlassMan** » Fri Nov 19, 2004 12:15 pm

Sounds like you will enjoy the comparing the capacitors of the A64 and p3 board. The p4 800FSB board is very similar to the p3 board.

burcakb · Post by **burcakb** » Fri Nov 19, 2004 12:17 pm

pjeaton,

Nearly all modern components put their load on the 12V line. I have a 400W PSU that's a little oldish. It has 40A on the 5V line but 16A on the 12V line. It didn't run my A64 system. If you go around checking manufacturer specs, you'll see that most modern PSUs have lower 3.3V and 5V lines and higher amps on the 12V lines.

Now for your case. Unfortunately harddrives and optical drives like to draw twice their operating power when booting. Most A64 boards I've gone over have some sort of hdd spinup delay option in BIOS. I don't know how effective that'll be in getting over the initial power surge but it's worth a try.

I still think 8A on the 12V is very inadequate. You'll probably be drawing around 12A. Before you go and buy the cheapest PSU, please remember that not all PSUs operate at stated outputs. I'd suggest getting a Fortron unit. They're cheap, very sturdy, have good rail outputs, relatively easy to quiet, and yes I've tried and successfully ran a A64 system off it whereas higher powered PSUs with heftier 12V rails failed.

larrymoencurly · Post by **larrymoencurly** » Sat Nov 20, 2004 12:09 pm

C'T magazine said that XP2400+ systems with a GeForce3 Ti500 drew about 2A @ +12V, 18A @ +5.0V, and 13A @ +3.3V if they used +12V for CPU power, 9A @ 12V, 3A @ +5.0V, and 10A @ +3.3V when the CPU ran from the +5.0V rail. I measured 60W for my 466 MHz Celeron system (5400 RPM HD, integrated graphics).

There's a power estimator at http://takaman.jp , but I've found that its +3.3V estimates can be 1/3 the actual current.

GlassMan · Post by **GlassMan** » Sat Nov 20, 2004 4:12 pm

Nice #'s, what do they mean?
Link is pre a64.
Excellent post Burcakb, very clearly shows how an excellent psu can fail to run an A64.