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Old 04-12-2005, 01:45 PM   #1 (permalink)
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Question Seeking PSU Tutorial

As part of my studying for my new computer store job (see here), I want to get a better handle on power supplies. I have one. I can tell you it's 350w. But other than that, I don't know a whole lot about them. ;P

Looking at the sites of my local computer stores, you can pay well over $100 for a brand name 400w PSU. (Antec, Enermax, OCZ, etc.) Yet among a couple of these popular brands, my store sells an "A-Power" 400w for $20 CAD (about $16 USD), and a simply generic 450w for the same. Dual-fan Echostars, 480-680w, will only run you $25-$35.

So what's the difference? Quality and secondary specs, obviously. But what specifically? What do I tell the discriminating customer when he or she asks me to compare the high-end retail models with the generic OEMs?

When you're shopping for a PSU, what are the important specs you look at? Browsing models on the internet, I see +5V, +12V1, +12V2, +3.3V, -12V, +5Vsb as output specifications, but am unsure of what they actually mean. Likewise, I don't know which are crucial in determining a PSU's true overall output, or which ones change from model to model (and on the other hand, which ones are going to be pretty much the same whether you're looking at top of the line or generic).

Then you have a bunch of input specifications, as well as other standard features. In this case, I'm getting all this from an Antec product page for one of their high-end "silent" models. (Perhaps not a good model for comparison, but it has a complete specs list.)

So what's crucial? What are the important things I need to know? I'd very much appreciate any help you could give me. Just pointing me in the direction of a good article would do.


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Old 04-12-2005, 02:06 PM   #2 (permalink)
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The output voltages you listed are standard, in that they are required by every computer system.

The max overall power output possible is indicated by the watts rating (eg 400W etc). It gets more complicated though because of the different voltage outputs :
Each of these is are known as power rails e.g 5V rail, 3.3V rail etc. Where you see +12V1 & +12V2 specified, that means the psu has 2 12V power rails.
Each power rail has an associated current rating. Multiply this by the voltage rating to get the power (watts) rating for that rail

e.g 12V may have a 15A rating so 12 X 15 = 180W etc

As you pointed out, you can spend $20 on a PSU or $100 on a different PSU with the same power output.

The difference is this - the higher quality one should provide better overload protection etc so help prevent a fault from damaging computers components. It should also prove more reliable, e.g the +3.3V power rail should provide 3.3V not 3.25V etc

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Old 04-12-2005, 03:05 PM   #3 (permalink)
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needless to say, the larger the output on the 12v rails, the better. For example, my X-Connect has 34A on the +12V, which is pretty good. Names such as thermaltake, Antec, OCZ, ennermax, and Ultra (IMO) make quality PSUs. rule of thumb is that if it came with the case, it sucks. that is unless the case is made by thermaltake or antec.

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Old 04-12-2005, 03:15 PM   #4 (permalink)
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If you want to understand the important of "why" a larger amphere rating is better on the 12V rail for example, read these basic articles from wikipedia.
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Old 04-12-2005, 03:19 PM   #5 (permalink)
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And a better quality PSU is measured in True Power (RMS) comapred to the cheap generic PSU's are Peak Watts and their ratings are BS.

Peak is how much it can give out all of a sudden, and RMS is what it can put out . An expensive and cheap PSU at the same watts will be VERY different

Check out for in depth stuff about Power Supply's
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Old 04-12-2005, 03:41 PM   #6 (permalink)
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Cheers DJ-Chris, knew there was something else I was gonna to mention....

DJ-Chris just raised a very important point, the difference between peak & r.m.s (continuous) power output. The power stated is nearly always peak power, the rms is the one that really means anything.

Also, check the over-current & over-voltage protection limits.
Also the decibel rating at idle, especially for the 'silent' psu's.
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Old 04-13-2005, 07:17 PM   #7 (permalink)
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Thanks for the responses, guys-- I'm still digesting all the information.

A couple of questions:

#1: How can I determine RMS? I took a look at one of the cheap "Enermax" PSUs at work today ($34.99 CAD for 680w?!), and didn't see any indication of RMS on the box. I take it that this is normal? That manufacturers who are only reporting PMPO don't want you to know RMS, and that quality manufacturers use the RMS in their product descriptions or state it up front? Or am I off base?

#2: So the 12v rail amperage is the key (along with protection limits) to a good PSU? How do +12v1, +12v2, and -12 compare in this respect? What do the 12v rails mean in comparison to the other ones? (My apologies if this is covered in the suggested reading-- I have yet to follow everything up.)

Thanks again.
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Old 04-13-2005, 07:27 PM   #8 (permalink)
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The +12v1 and +12v2 are both the same, but there are two sets of rails that handle the 12V and dont worry about the negative's. And if im not mistaken the computer use's more 12V connections than the 3 and 5 volt rails.
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Old 04-13-2005, 07:41 PM   #9 (permalink)
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Hmm, most people before me summed up most everything, but I'll go over everything just incase there's anything in there you've missed.

Output is the first thing you'll see when looking for a PSU (ex. 350 watt), which is basically the juice the PSU provides. Different components account for different power consumption. For example, a 45 watt CPU will use 45 watts of output power, therefore if you have a 350 watt PSU it uses 45 watts of output power and your PSU only has 305 watts left dedicated to other components. You should always have more output than what your devices need.

Also, there is a difference between peak output and output. Some companies will label a 350 watt PSU based on its peak output, meaning it won't always have that kind of power, that's just its max. You want a PSU that provides a steady supply of its output. For example, a 400 watt PSU should be able to give out a steady 400 watts 24/7 and should have a higher peak output.

Also, as mentioned, you'll want strong rails. As a general rule, you don't want your rails to fluctuate much at, the less they move the better. For example, you don't want your 12v rail jumping around from 11.5v to 12.5v, that's not good for your components and can lead to power surges.

Generally, the voltage should remain within 5 percent of where it should be, you don't want your 12v rail all the down all 10v or way up at 14v, that's dangerously underpowering or overpowering your equipment and can also lead to some problems.

Also, as mentioned, you want good strong current on your rails, which is measured in amps. Basically, the higher the better, and you'll want a minimum of 18amps on your 12v rail (as you may have guessed your 12v rail is the most important one). The more amps, the more powerful your rail becomes and the more juice it can deliver to components faster. I would recommend at least 22amps today for anyone seriously investing in their machine however.

That about sums up the important parts, as for recommendations I like OCZ PSUs as they have strong rails and good output and you can actually adjust the rail voltage to suit your needs. The modstream PSUs also alert you of dangerous overpowered and underpowered rails.

Antec and most Thermaltake PSUs are also of fair quality.
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Old 04-13-2005, 07:53 PM   #10 (permalink)
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Gaara, on that note, the ModStream 450 W power supply provides a steady stream of 450 W power right? The spec said that has a maximum peak load Wattage of 550 W - but the name of the PSU is ModStream 450 W.

So, 450 W = regular power it can delivery always
550 W = transient power it can deliver sometimes. correct?

Too bad these companies don't stick to the same convention. At least OCZ provides both numbers unlike some others. I hate reading ratings..

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