Power supplies draw power from the AC in an odd fashion. Basically, they draw a chunk of power at the start of the AC cycle, and then they draw very little power for the rest of the AC cycle. This isn't a big deal when they are running from the AC line, but is a big deal when they are drawing power from a UPS. There is a way to make their load look 'nicer' and it is called power factor correction. There is active and passive power factor correction. To get really high power factor correction (like 99%) requires active power factor correction. This costs a little more than passive PFC, and actually reduces efficiency by a tiny amount. All the good power supplies have it. If you find a power supply without it, it is either old, or a really inexpensive, lower quality unit.

Some people think that if a 300 watt power supply is good, then a 500 watt power supply is better. A friend needed a new power supply, and I recommended a 380 watt unit. He said the rest of his power supplies were 500 watt, and that 380 was inadequate. I mentioned that he didn't have a case full of power hungry components, and that his computer was likely using under 250 watts of power. My dual Intel Xeon 2.4ghz system, with 6 hard drives draws 182 watts from the wall at 100% CPU usage. I do have a low power video card, and low voltage Xeons, but allowing for power supply efficiency, the computer is taking about 150 watts of power. Hard drives do take more power when spinning up (which is why SCSI drives can be started in a staggered fashion), but clearly a power supply around 300 watts would be find for my system. Now if you have a killer graphics card or two, that can suck up lots and lots of power, and you may actually need a big power supply. Read up on the power requirements of your components before actually buying a high power power supply.

There is more to a power supply than its total wattage. The power supply output is split between several different voltages. There is +3.3, +5, +12 volts. On older atx motherboards, the cpu voltage regulators drew power from the 3.3v rail. It was important to have a high enough current capacity to power the cpu. On newer motherboards, the cpu voltage regulators draw power from the 12v rail.

Generally, hard drives use 12v to power the motor, and can draw roughly 2 amps on the 12v line when spinning up. The power supply must have enough current capacity on each of its voltage lines in order to supply the peak requirements of the computer. I recently had a problem with a dual opteron motherboard. The problem was my 460w power supply didn't produce enough current at 12v to spin up my hard drives and boot up the two 95w opteron.

One trick to minimize peak power is to stagger the spinning up of hard drives when the system boots. SCSI drives have generally had this feature for more than 10 years. SAS and SATA drives can do this if the hard drive enclosure supports this feature. My SCSI hard drive enclosure supports this.

Just for a datapoint, I have an Antec EarthWatts 380w power supply powering my dual xeon lv (@55w for each xeon) fileserver with 7 hard drives, with no staggered spinup. I did need to adapt the 4 pin power connector to an 8 pin SSI connector.

Antec SU380 power supply

I bought an Antec SU380 power supply because it was bundled in with their Antec NSK-2400 case I bought. I read a positive review of it at Silent PC Review. It is 80 plus certified, which means it is at least 80% efficient at 20%, 50% and 100% of load. It has active power factor correction and is OEM'ed by Seasonic. Since I already had good experience with my Seasonic SS 460, I was confident it was a good power supply.

The power supply didn't work for my motherboard (a FIC AM37 from an E-Machines computer I got for free). I contacted Antec tech support, and they asked if my motherboard needed the -5 volt line from the older ATX 1.2 power supply standard. I wasn't sure (it turns out that -5 volts is used by EISA pc card slots, which haven't been used for several years), so I clipped the -5 volt wire on my working power supply, and it still worked fine. I told them that it wasn't the problem, and that my motherboard was for an AMD 2000 processor, and the brand and model of the motherboard. They sent me another one that also didn't work.

I tried the original PS out on another AMD 2000 system I had, and it worked! I also tried the old one out on a dual Pentium 933 system I had, and it also worked. So there is something odd about my E-Machines motherboard. I sent Seasonic (the real manufacturer of the power supply) an email asking for assistance, and they told me to ask Antec. I already knew that Antec was pretty clueless about power supplies (clearly demonstrated by them asking me about the -5 volt line, and being unfamiliar with my motherboard). I have never had this problem with a power supply before. I ended up installing the PS in the dual Pentium 933 system. It is pretty quiet.

Antec EarthWatts 380, 430, 500 power supply

I bought an Antec EarthWatts 380 power supply because it was inexpensive after rebate. I read a positive review of it at Silent PC Review. It is 80 plus certified, which means it is at least 80% efficient at 20%, 50% and 100% of load. It has active power factor correction and is OEM'ed by Seasonic.

Just like their SU380 power supply, it didn't work for my FIC AM37 motherboard with an AMD 2000. It did actually boot the computer once, when I only attached the motherboard, but it never did with the hard drive and DVD drive attached. I ended up installing it in my other AMD 2000 based system. It is certainly a quiet power supply, but clearly not flexible enough to work on all motherboards.

Well, there have been lots of rebates associated with this power supply. The latest EA-380 I bought was $15 after rebate. The best value 80+ power supply that I know of.

I just ordered my 5th power supply. It was the EA-500. I needed a SSI connector (a 8 pin 12v connector, used for dual xeon and opteron systems), and the cheapest decent solution I could find was the EA-500 (the 380 and 430 don't have a SSI connector). For some reason that I don't understand, the SATA power cables on the EA-500 are significantly shorter than the SATA cables on the EA-380. So short, that I had to use adapters to power my four SATA hard drives with them. It was $35 after rebate.

Emacs RPD-4300F Hot Swap 300w AT Power Supply

My oldest power supply is a 300w hot swap redundant AT power supply made by Emacs (also known as Zippy). They are a well known high end company for power supplies. This features two power supplies, with two separate power cords. You can pull out one of the power supplies while the computer is on (or turn one off), and the computer will keep on running. Redundant power supplies used to be standard on big server type machines, because you can replace a broken power supply without turning off the computer. If a power supply fails, an alarm will sound, and you just pull it out, and put a new one in. I had some Compaq servers that had 3 power supplies that shared the load, and one of which could fail, without bringing down the system. This Emacs power supply has a pretty powerful fan that is sure to keep the power supply cool under adverse conditions. Unfortunately, it predates variable fan speed power supplies, so it is pretty noisy. If someone needs one, it is for sale, cheap.

Enermax EG465P-VE

I had a 300w generic ATX power supply, and I was having a hard time booting and spinning up all my SCSI hard drives. I decided I needed a more powerful unit, and I bought this. Still working in my fileserver computer. I replaced it with a Antec EarthWatts 380 and the power usage didn't go down significantly. Therefore, the Enermax must have been pretty efficient. This is quite remarkable, because the Enermax was built roughly around 2000, when efficiency wasn't considered a big deal. For it to be close to 80% efficient was quite surprising to me. Being an old power supply it has a 20 pin ATX power connector, and no fancy pentium-4, SSI, or high powered graphics card power connectors, as none of that stuff existed around 2000. It isn't as quiet as newer power supplies. Still, it is my oldest running power supply and I am impressed that the 8 year old hardware is holding up well.

Powmax 400w LP8800D

Before I knew much about power supplies, I bought this unit. It worked for a few months and then started failing intermittently. After another week or so, it failed completely. Replaced under warranty.

Seasonic SS-460

When I built my Dual Xeon system, I needed a 24 pin ATX power supply, with a SSI connector (used for dual xeon and opteron systems). Seasonic wasn't well known at the time, but they had the most efficient power supplies around. I bought it. Very boring, very quiet, very reliable. When it is under light load, the fan stops spinning (it has a fan rpm sensing output). All power supplies should be like this.

Ultra power supplies

Ultra Products makes several power supplies. I have a few that I got free after rebate. Some are somewhat plain looking, but most look really cool. Most are not plain sheet metal, but painted to look cool. One of my units features 'modular' connectors, which means you can unplug unneeded cables. Each modular cable is sleeved and green, and I think they glow under UV lighting. I am not so impressed with modular cabling however. The extra connector has some resistance and adds another possible point of failure. What is novel is that some of their newer models have the cables attached to each other, like a 2 conductor AC power cable. Ultra calls it 'flexforce'. This is simple and much cleaner than 4 or 5 individual wires. They even introduced a 80 plus model. All of mine work. All of them are relatively low efficiency. Some of them (even the ones with variable speed fans) are noisier than they should be, though no nosier than a typical low efficiency power supply. The newer ones with 120mm are quieter than their older ones.

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