Power Supply Unit replacement?

Hi all,

My Power Supply Unit is going bad. I'd like to get a Modular one but the only ones I can find are 500W - 550W. The PSU I have now is 400W and I haven't had any problems.

Do I really need 500W or 550W? Would there be any advantage? Or, does the PSU only use what it needs up to 400W to 550W? And, would the larger PSU consume more, or a lot more energy?

 

Hi Steve,

Thank you. I've seen PSU's up to 1600W, but for what I'll be using it for, 400W is sufficient. I just can't find any modular 400Wers.

Oh, well, 500W it is.

 

Whoa,
First, let's settle this question about power usage:
There is little to gain from replacing your PSU (Power supply Unit) with one of substantially greater capacity than is needed to handle the power demand of your system. While you might think that having a big power "reserve" is good in case you want to upgrade your machine by adding more hardware, you should understand that the contemporary switching power supply achieves its best efficiency at its rated output load; under reduced load the efficiency falls off quite a bit.
Unless you have added enough hardware to your machine to increase its power demand above that original 400W you have little to gain from a larger, more expensive PSU.

As for input power consumption, you can obtain that by dividing the output load power by the efficiency; just remember that the efficiency is not constant, close to rated load a switching PSU will be about 85% efficient (use 0.85 as the divisor in your calculation of input power) at lower loads the efficiency depends entirely on the circuit design and the quality of the components used, but you can be sure that as the output load falls the efficiency will fall too. The efficiency drop is caused by the fact that some power is consumed in operating the PSU. Most switchers go unstable below a certain output current, so the designer includes circuit components which provide for a minimum current through the controller, even at zero output power. (Yes, I was a circuit designer!)

 

Hi Keith,

I don't even know if I need 400W. My hardware is: MB\CPU; 2 HDD (only use one at a time); 3-80mm cooling fans; CDRW; DVDRW; AGP graphics card; sound card; 1GB PC3200 memory; and that's it. Oh, and a 3½ floppy (although not many would admit to that)

Like I said, I need to replace my 400W modular, but the only ones available now are in the 500-550W range.

Judging from my hardware, are you saying that the 100-150W larger PSU may reduce efficiency? I can see a 2HP PSU as being overkill and totally unnecessary; but isn't the 25% increase fairly negligible?

 

Yes, a 500W power supply running at 400W or less will be less efficient than if it were running fully loaded; but since you have no choice in the matter, I would not worry about it. A decent power supply should have a variable speed fan with a controller monitoring PSU internal temperature, so the additional losses will not cause a problem. You'll use a little more input power, but you are unlikely to notice it on your monthly electricity bill.

Incidentally, why do you think that your PSU is defective?

I have seen cases where people have blamed their PSU for computer malfunctions, but the PSU was fine, the real cause was that they had added more hardware than the PSU could support and still maintain system noise margins within spec. Before rushing out and buying a new PSU I would make the effort to find out just how much power your system draws, especially under transient load conditions. (That's engineer's talk for what happens when some motor is trying to start from rest. (e.g. optical drive, tape drive, floppy drive, hard drive.) When an electric motor tries to start from rest, for a short period it draws a current substantially higher than it's running current. If that transient current overloads the psu, the system noise margin can become too small for a brief period. The result is logical errors within the digital circuitry. (You have to think appropriate timescales here; a disk drive motor may come up to speed in a few hundred milliseconds, but the logic circuitry is running in time scales in the nanosecond range; that's in the million to one range of times. What's a short time for a motor is eternity to a logic gate.

 

Hi Keith,

Well, because 2 of the 5 plugs are dead. I booted normally and it booted to my 2nd HD. I looked in Explore and the BIOS and my drive didn't show. I unplugged, jiggled, voodooed, did a little percussive maintenance and the plug was just dead. I hooked up another plug and it worked fine. Then the same thing happened to the second plug. So, since I hardly ever use my DVDRW, I used that plug and everything was peachy.

That's why I kinda figured it was time for a new PSU "” 2 dead plugs.

 

Hello to you,
The fact that some of your "plugs" work and others do not tells me that it is unlikely that there is anything wrong with the elctronics of your PSU, it is highly likely that the problem is a bad connection inside the psu where output power is distributed to the various "plugs" as you call them. We could find that bad connection if you are willing and able to to do some simple fault finding.

Some questions:

1 Would you sooner just write off the PSU and replace it?

I found a 430 watt PSU @ $39.99 and a 450 watt PSU at 59.99 on compusa.com. No spec. and 3 year warranty for the 430 watt unit. The 450 watt unit had a decent specification and a lifetime warranty.


2 Would you want to try fault finding and repair?
3 Do you have a volt/ohm/amp meter? (VOM)
4 Do you know how to use a VOM?
5 Do you have an electronic soldering kit?
6 Are you able to solder competently on a printed circuit board?
If you can answer YES to questions 2 to 6, I'll help you fix your PSU.



Regards

Keith

 

Hi Keith,

Thank for your kind offer to help; I appreciate it.

To be honest, if I had the wherewithal I'd more than enjoy a little Home Project From Hell.

And, yes, I can do 2 thru 6. I've had to re-solder a lot of things over the years (cordless phones, coffee maker, stereos, a TV or two, etc.) simply because they were mass produced and the solder joints were done shoddily.

However, I'd kinda like to stick to a modular unit because of air flow and not having to twist-tie the extra wires out of the way. And, there's a Big Box store nearby that has what I almost want for about $45.

As an aside, a number of years ago I was trying to figure out how to keep flat ribbon cables from blocking the cooling fans. I took a ribbon cable (the 40 pin kind)(that's how long ago it was), a block of wood, and a 4" paint scraper (and my favorite tool "” a hammer) to it, carefully cutting the cable into 4 strands each. I pinched the wires together by wrapping electrical tape around them. It was ugly as sin, but it worked. So I did it to all the cables and the case temperature dropped significantly. I even sold a couple to a local electronics store, but it was too labor intensive and, like I said, too ugly for consumer consumption. Then I saw them about a year and a half later in a rainbow of colors.

Oh, well.

 

O.K. you are the best judge of your affairs. I must admit that it could have been a bit tricky coaching you over a DSL connection!

Bye the bye, I find it interesting that once again the marketing people have taken a well known term and then twisted it to mean someting else.
To people like me, a modular power supply is one wherein the various sub regulators for the differing output currents and voltages are plug in sub-assemblies fitting into a chassis equipped with a main regulator. The modular PSU can be configured in multiple ways dependent upon user needs. I spent several years at HP working on the design and development of some of the first true modular power supplies on the planet! It's darn good idea, and it works very well when properly engineered.

The PC marketers have taken that title and applied it to non modular supplies fitted with multiple output connectors. Oh well, who ever expects truth from the sales and marketing folks?

I share your views about ribbon cables, they do have a place in electronic systems, but their use in the average PC completely negates their good qualities.

Regard

Keith

 

Hi Keith,

It has always been and will always be my contention that there are waaaay too many MBA's at the top of the Corporate Food Chain looking to justify their salaries and the unsavvy consumer will always lose.

Anyway, "modular" covers a host of definitions. Office furniture can be modular; the components in a TV can be modular; math sets; the list goes on. But, it does puzzle me that connecting a wire from the PSU to a device should be called modular.