Modern PSU require minimum loads?

[Savage1701]

Do modern power supplies require any minimum load present on the +12v, +5v, and/or +3.3v rails? I've heard some can become fritzy if there is no load present on one or more of the voltage rails.

If so, what is considered a minimum safe required current load for each of the aforementioned voltages? I'm assuming the +5vSB and -12v rails don't require anything.

Obviously the +12v is easy to load with fans, and the +5v and +12v could be given a load with an old hard drive, but I still wonder what could load the +3.3v rail and if there are any rules of thumb for minimum load vs. output current claimed at each voltage, for example.

Thanks for any help.

 

[captaincranky]

How exactly would you use a PSU in a computer without a load being on it.....?

I've seen charts that indicate that efficiency suffers if the supply is under loaded, but that sort of goes to show that it's kinda stupid to buy a 700 watt PSU for a computer that draws 300 watts.

 

[Savage1701]

I have used them for years to power large hard drive arrays. I try and target my primary supply to run at about 60-75% capacity. Also, "multi-rail" +12v supplies don't usually allow one to allocate the rails to the connectors, and they are frequently gimmicked rails, rather than true, seperate rails. They are usually designed for crossfire setups, which is, of course, the last thing I care about in a video server in a rack. A cheap PCI or integrated video at 1280x1024 is all I need to view the system's desktop. The video server's clients do all the decoding and display on our TV's and monitors.

There was even a niche market that sold 5.25" bay +12v and/or +12v & +5v PSU's for auxilliary power to graphics cards or a large number of drives in a case or esepecially for solid-state Peltier CPU coolers. But those were expensive and few and far between.

Anyway, the secondary PSU is shorted on and kept in a seperate rack unit with the drive arrays, which connect to the primary rack unit via SAS multilane connectors. The PSU's are connected to an eithernet PDU which sequences the array on first, then the server, so the RAID controller never sees "dead" drives/array/unit members.

The reason I asked was because I am thinking of trying a video server array with 2.5" notebook drives as they now have sufficient capacity. But they only use +5v. That would then leave both the +12v and +3.3v rails with no load on them, and I think I don't want to push my luck.

I was also curious if I had been damaging my other PSU's that power 3.5" drives since they have no +3.3v load on them ever.

Finally, it's expensive to buy a 1100 watt PSU and one frequently runs into problems fitting them into cases as they are usually much deeper than PSU's in the 850 watt or lower range.

 

[dividebyzero]

The power efficency at low (or virtually no) power is a valid concern. Efficiency is measured, in most cases, at a minimum of 20% of rated power draw-which of course doesn't take into account large power-load PSU's running systems brimming with power-saving technology.
An example of PSU efficiency here ( text in Dutch)
From experience I would say look more towards mostly modular single 12v rail PSU's ( i.e. Corsair's HX range), or fully modular (e.g. Silverstone Strider Plus). Both hold very good efficiency and stable delivery (low ripple) from virtually 0% on up- ideal for low-power states.

 

[captaincranky]

I actually think that you need to investigate custom supplies for your intended usage. I seems to me that server farms and the like may not use over the counter PnP parts. That's just a wild guess/ slash opinion/ slash wishful thinking from the under informed.

And then there's plan "B", apply dummy loads to the unused PSU busses.

 

[Savage1701]

DBZ - thank you for the information

Captain - I'm not a pro with a server farm, but I am interested in what would constitute a valid dummy load, both electrically and physically (e.g., an old hard drive for the +12v and +5v, some high-output LED's for the +3.3v, fans for the +12v, an incandescent DC flashlight bulb for either the +5v or +3.3v) and how much those ought to draw.

That was sort of the idea of the original question - what's a valid, necessary dummy load for any of the unused rails?