Why is this a recurring theme with low-end PSUs?

[Mr Kagouris]

I'd like to ask anyone actually familiar with PSUs why low-end units seem to have way too much amperage on the 3.3V and 5V rails while skimping on 12V amps while high-end units have more reasonable ratings on all rails? Also why are lower-end units usually the ones with multiple 12V rails?

 

[joex444]

You'll also notice that on low end PSUs, the rated wattage is the sum of the wattage carried on each of the voltages which on higher end PSUs the wattage is only the 12V rail. For example, a 30A @ 3.3V, 40A @ 5V, 30A @ 12V unit would be labeled as 30*3.3 + 40*5 + 30*12 = 650W PSU while a high end 650W PSU is going to have 54A @ 12V (where 54A = 650W/12V) and then some other ratings for 3.3V and 5V. In this example, the low end 650W PSU is really a 360W PSU (30A * 12V), by high end PSU metrics -- or the high end PSU is probably closer to an 850W PSU according to low end metrics.

As for the rails, this is rather simple. If you have 30A to deliver then you must use wiring that can sustain that. If instead you split it across two rails then each one only needs 15A, and thus you can use cheaper parts since they are rated for lower current. With high end PSUs, if you're doing 60A you need some pretty beefy parts internally and you can sort of see why they're expensive. Single rail, of course, makes everything much easier for the user as you only need to worry about the total load and don't need to worry about where each cable came from and try to load balance things yourself.

 

[Mr Kagouris]

Thanks for the explanation on the multi-rail thing, but my question on the amp ratings wasn't really about that. I know that manufacturers use cheap marketing like that for the low-end units, but I was more interested as to why it's easier to deliver so much amperage on the 3.3V and 5V rails but not on the 12V rail(s) which usually ends up being the one that matters most. Do the lower voltage rails just need lower quality components to deliver those amps than the 12V rail does?

 

[beegmouse]

Most of the cheap PSUs are old designs from when Desktops only used about 300W
The Specs on the low voltage rails are a lie.
There is no device which can use that much power down those rails, so it never gets tested.

That way they can sell a 300W PSU as a 500W one knowing that because the exaggerated 200W can't be used the product will not fail, and can't be returned.

Because they have not lied about any of the specs which the customer will ever know about.

Stick with 80+ approved PSUs which are listed on the 80+ site.
To pass even their basic test the PSU must be a modern design and the specs listed are tested.

 

[Mr Kagouris]

Yeah don't worry about me, I use a Coolermaster V850 😛. I always avoid sub-par components when I can even if it means paying extra. I'm just asking all this out of curiosity.

So basically those low-volt amp ratings are outright lies because the average customer wouldn't ever actually need that much power on the low volt circuits, so would never actually find out it can't deliver the advertised amps? Wow.

 

[rush21hit]

All the more reason to tell people to never skimp on PSU.