On Thu, 20 Feb 2020 20:07:47 -0000, "Commander Kinsey"
>On Thu, 20 Feb 2020 03:43:58 -0000, default <def...@defaulter.net
>> On Thu, 20 Feb 2020 00:32:05 -0000, "Commander Kinsey"
>>> On Wed, 19 Feb 2020 23:31:01 -0000, Jasen Betts <ja...@xnet.co.nz
>>>> On 2020-02-19, Commander Kinsey <CFKi...@military.org.jp
>>>>> Why do (cheap? expensive ones may be better) PC ATX power supplies need current drawn from the 5V line to make the 12V line work correctly?
>>>>> I have a PC with 3 graphics cards running scientific applications. I acquired three old graphics cards that take about 300W each, and have loads of cheap (CIT) PSUs that are rated at 650W on the 12V line, which is what those cards use. So I run each card off its own supply. But the 12V line at no load, or even at 300W, is only giving out 10 to 10.5V. If I attach a small dummy load of an amp or so to the 5V line, the 12V line suddenly becomes 12V.
>>>>> Why are the two lines related in any way?
>>>> because all the output voltages come from taps on the same transformer
>>>> and the voltage regulation is applied to the input to that transformer
>>>> and the voltage regulation only watches the 5V line.
>>> Ok, but why does current need to be taken from 5V to make the voltage monitor work?
>> It is designed to be in a computer, and there's always some load on
>> the 5 V line. It probably didn't seem terribly important to worry
>> about a high voltage condition where none should ever exist.
>What load would that be? In my newest computer for example (which has a decent supply which doesn't need the 5V load), it uses an SSD which draws fuck all power. The CPU and graphics card and memory all operate from 12V with their own regulator modules. I can't think of anything drawing much 5V, and the dodgy supplies I have need approaching 2 amps! I tried 1 amp and that didn't completely stabilize the 12V line.
The whole idea of taking a supply designed for a specific purpose,
using it for a different purpose is "dodgy," to use your words. Your
expectations may not be realistic.
If I'm designing something to be used in an industrial application I
can make it damn near foolproof and bullet proof, because the
application justifies it. If it's a military contract cost is
secondary to reliability and ruggedness. If I'm designing for a mass
market commercial application cost is very important...
Engineering is all about compromise. There's a lot of different ways
to do things. Sure the PS could be made better, but if that means you
price yourself out of the market what did you achieve?
You should also consider that ATX isn't much of a standard in the
sense that it wasn't carved in stone and handed down from the
mountain, never to deviate. There have been many iterations of the
basic ATX since it was introduced. Requirements change, obsolete
parts get supplanted with newer ones, etc..
The only thing you can count on is change. (and humans are involved -
lower your expectations or face disappointment)