On Thu, 20 Feb 2020 03:43:58 -0000, default <firstname.lastname@example.org> wrote:
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.On Thu, 20 Feb 2020 00:32:05 -0000, "Commander Kinsey"
On Wed, 19 Feb 2020 23:31:01 -0000, Jasen Betts <email@example.com> wrote:
On 2020-02-19, Commander Kinsey <CFKinsey@military.org.jp> wrote:
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.
Some power supplies sit and oscillate if they don't have a load on the
5 volt line...
I notice my desktop has 5v present on the USB connector even when it
is turned off, turned on, or just in standby. I suspect it may have a
small independent supply to run the USB connectors for power, and
perhaps that also supplies the CMOS memory so the clock and settings
don't drain the battery.