Minimum load requirements for power supplies

[Alexer]

Greetings.

Disclaimer: I know next to nothing about PSUs. Two days ago I didn't know they have "two 12v rails". I've been doing a lot of reading, so I may use the correct terminology (oh god, at least I hope so), but the fact is: I don't know anything about them, take everything with a grain of salt.

Some backstory:
I built an external enclosure for 10 harddrives a few years ago. (2 SATA port multipliers, 5 drives per multiplier, so that the thing can be attached to a computer (that is, my fileserver) via 2 eSATA cables)
Obviously it would have been not-so-nice to wire it to the same PSU as the fileserver for a multitude of reasons. (Would have needed a bigger PSU, ugly wires from an open case, wires too short, harder to attach to another computer (well, okay, as if I'd do that), well, you get the idea)
So I got another PSU (unfortunately, no place to mount it on the enclosure, so it just sits beside it - so not moving this thing often), shorted the green and black wires, and.. it worked! For 2-3 years until a few days ago, that is.

Came home from work to find out that the PSU had shut down after working fine 24/7 for a few years. And since I've never had a PSU break on me before, the first thing that comes to my mind is: "Must've not liked running without a motherboard".

I started googling and found a few references to minimum load requirements, of which a thread by Jason (about the power supply testing rig) was the most professional-sounding.
After that, I found more threads about the subject here, most being about testing power supplies, and starting with "just short the green and black wires" -types of replies, with someone later popping in to say "and remember to load the 5v rails!". However, none of the replies I remember reading said anything about loading the 3.3v rails, nor anything about specifics. (resistor size, does it matter where you place it, etc.)

Wikipedia has this to say about the subject:

SMPSs have an absolute limit on their minimum current output.[4] They are only able to output above a certain power level and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolated transformer to act as a Tesla coil, causing damage due to the resulting very high voltage power spikes. Switched-mode supplies with protection circuits may briefly turn on but then shut down when no load has been detected. A very small low-power dummy load such as a ceramic power resistor or 10-watt light bulb can be attached to the supply to allow it to run with no primary load attached.

Many of the replies I read insisted that PSUs do not have minimum load requirements. That quote is there to bring contrast. I do remember reading that some PSUs are supposed to have internal dummy loads, but I'm pretty sure not all do. The only basis I have for that claim is that the ATX12V specification lists a minimum load in all the power distribution examples.
(Someone also mentioned in a few replies, that the ATX12V specification says PSUs must be able to operate load-less. The "Output protection / No-load operation" -section does say that there should be no damage if all the dc output connectors are disconnected from load, but there are several ways to understand that (Does that include the 24-pin connector? Does that include shorting green and black only?))
(And obviously, power supplies aren't supposed to break, or break other things when they do. But unfortunately there's this thing they call reality... )

Since a big part of the information contained in the other threads seemed to be mostly guesswork, I would like it, if the replies could mention the source for the information. "I remember reading something like this in a random forum post five years ago" is a perfectly valid source; Just makes it easier to spot the information that you need to take with a bigger grain of salt
(If the source is the specification, I would appreciate it if you could mention the section or the page number. Makes the discussion that much easier )

Moving on to the questions:
So, I bought a new PSU, and I haven't plugged it in yet. This time I'll ask first, and depending on the consensus of this thread, maybe hook a small dummy load first. I'm not mentioning the model of the PSU, because I'm looking for a solution that would work with any (normal-ish) PSU. (I'm going to assume in the text to follow that we're talking about a PSU with two 12v rails)

The harddrives are surely enough load for the 5v rail and
one of the 12v rails, but what about the other rails?

The ATX12V specification seemed to imply, that the other 12v rail is used only for the additional 4pin 12v connector, but the EOC power supply test tells us, that this is not always true. Since, depending on the motherboard, the 4pin 12v connector is not always connected, I'm assuming that the other 12v rail on many (but not all) PSUs has no minimum load requirements.

The power distribution examples in the ATX12V specification all had 0 as the minimum load for the -12VDC and +5VSB lines, does this hold for all PSUs?

I am assuming that all the 5v and 3.3v pins in the connectors are equal, that is, connected straight to the 5v and 3.3v rails, and it does not matter which one of these the load is placed in. Is this correct?
Does the same hold for 12v, except there are two sets of pins? (i.e. for the two different rails)

What I mean with the previous is; To fulfill the minimum load on the 3.3v rail: Can I just pick a random pin supplying 3.3v and connect it through the load to a random ground pin, or do I need to (for example) load every 3.3v-supplying pin on the 24-pin connector separately? (Or should I choose the 3.3v sense pin for this, since, well, it's the 3.3v sense pin )

In short, what I would like to know is:
If I want to use a PSU to power a set of harddrives without a motherboard being connected, do I need additional dummy load to ensure correct operation? If so, how much additional load do I need, and to which connector pins should I place it? (If it matters)

Finally, sorry about the post length, it just grew and grew and..
I'm sure I have mentioned something twice and forgotten to mention something else. Since the post grew this much, maintaining a logical order was hard. I'm sure I failed to add information into logical places and sounded like a **** in a place or two, and for that I apologize. But know this: I *tried* to make it as sane as I could.

[Josie Wales]

The basic answer to you questions would be "it depends". Like all technology products, power supplies continue to change and evolve. Whether or not a PSU requires a minimum load depends on the specific internal design. Each model is different. Ten years ago most power supplies did require a minimum load. Today most units do not. You will need to contact the manufacturer of your PSU to determine if there is a minimum load requirement. Make sure your answer comes from their engineering staff and not a customer service rep. A properly matched PSU will not require a dummy load.

As for the ATX12V Power Supply Design Guide, you need to see this document as a recommendation for manufacturers. Each manufacturer is free ignore or liberally interpret specific items in the guide. This is why many of us are strong advocates of rigorous PSU testing and only recommend units from brands that have a proven history of providing quality products.

[Alexer]

Thanks! I kind of guessed it would depend on the PSU, but I didn't think they'd vary that much. (And I did expect they would try to conform to the spec more, since they claim to be compatible, after all..)

But is there some "common maximum minimum load"? I mean, some cautious figure that would work for most PSUs? Something along the lines of 1A load to the 3.3v rail for those PSUs that still might have some minimal load requirement? Bigger? Or is even the 1A overdoing it?

I did send an email to the manufacturer at the same time I started this thread, but as you foresaw, the reply came from a non-technical person, who had apparently only read the first sentence of the email.. (Quite understandable, however, when you consider how long my mumblings tend to get - though it was "only" something along the lines of one third of the length of the first post of this thread)
The reply just stated that, yes, their power supply indeed supports harddrives. I have yet to receive a reply to my follow-up email.

[Josie Wales]

Originally Posted by Alexer

But is there some "common maximum minimum load"? I mean, some cautious figure that would work for most PSUs?

Of course not! Now it is time to stop talking about "most PSUs". Tell us the exact brand name and model number of your power supply and a complete list of all the gear it will be powering.

[unrlmth]

I know this is a different use than yours, but I have a 500w cheap PS that I use with my battery charger. It's still going strong after a few years, but it's not a 24/7 item.

[Alexer]

Well, I was kind of looking for a solution I didn't need to revise if/when I need to change the PSU. (Or make another one of these things. To tell you the truth I already have two..)

But, if it really can't be helped.. It's an LC-Power LC6450 V2.2, and it will be powering 10 Seagate SATA harddrives, 5x320G and 5x500G. Also two SATA port multipliers and four 120mm fans.
What do we need the 5v/12v load for, when we're trying to figure out the minimum load for 3.3v? Something related to "crossloading" or somesuch?

Edit: unrlmth, I'll be running this 24/7 for years, so we'll see, but it's good to know the newer ones can at least handle shorter times. I said earlier that this was my first PSU to go, but just remembered the sad story of an old 300W PSU I had powering a few fans to cool various electronic appliances. RIP old HEC 300W PSU, it was good while it lasted.

[Josie Wales]

So here is the skinny. Your drive enclosure provides sufficient minimum loads for nearly every PSU in the 400w - 500w range available today. Many modern designs require 1A or 2A on the combined +12v output before they will start, but otherwise have no other minimum load requirement. Older designs frequently require a minimum load on the combined +3.3 & +5v output to maintain stable output voltages.

LC-Power tends to source their PSUs from second or third tier Chinese manufacturers. Most are based on older design topologies. Looking at the power distribution characteristics and the history of your particular model I would estimate that it requires a minimum load of approximately 1A@+12v and 1.5A@+5v. There would be NO separate +3.3v minimum load requirement.

Most units by LC-Power are lower quality and cannot deliver the output pasted on the box. Your unit is a good example. The LC-6450 should be rated for around 325w continuous output at 40C. The 450w rating is a fantasy peak power rating at 25C. Still, it should be sufficient for your requirements as long as you keep it cool by giving it plenty of fresh air. For 24/7 operation I would not expect a stable lifetime beyond 3 to 4 years.