On Thu, 20 Feb 2020 00:32:05 -0000, "Commander Kinsey"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.
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
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.
On Wed, 19 Feb 2020 20:13:32 -0000, default <firstname.lastname@example.org>
On Wed, 19 Feb 2020 19:43:11 -0000, "Commander Kinsey"
On Wed, 19 Feb 2020 19:37:06 -0000, default
On Wed, 19 Feb 2020 18:55:27 -0000, "Commander Kinsey"
On Wed, 19 Feb 2020 18:45:55 -0000, default
On Wed, 19 Feb 2020 15:43:37 -0000, "Commander Kinsey"
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?
Sorry for the crosspost, I'm not sure which of these groups
There's often a good reason for it. The 5 volt supply is
and the others are not (generally speaking). The feedback path
is from the 5VDC output back to the mains side of the
The others get line (but not load) regulation via the 5V supply
because they share a common transformer. They are also
switching supplies that work at a high frequency so the
transformers have fewer turns of wire and more volts per turn
which results in excellent transformer "regulation."
So on a cheap shit supply, the 5V is guaranteed to be very close
to 5V, but the 12V will drop under heavy load? And on a decent
supply like Corsair, they must regulate both seperately?
I still don't understand why the regulation goes to pot when
under 1.5A is taken from 5V. It still regulates that 5V
perfectly with no load, but the 12V goes wildly wrong. Why does
the regulation need current to be flowing through 5V?
I suppose there may be some that regulate the 12 for both line
load but I haven't seen it. Not having separate regulation for
coarse 12 supply isn't necessarily a design flaw. The 12V is
If you look at the 12V line on a Corsair supply under any load, it
will always be between 11.8 and 12. A cheap PSU like CIT, with
the 5V loaded normally, the 12V can be 11V to 12V depending on its
own load. Buy an Alpine supply and it will literally go bang if
you exceed 50% load for more than an hour. I went through 10
Alpine supplies, costing the shop a fortune, before I told them
enough was enough and got my money back.
You mean to say "you get what you pay for?"
Some (particularly older) designs do require some minimal loading
of the 5V to keep it from drifting above 5, or above 5 AND
causing the crowbar over-voltage protection from kicking in and
shorting it to ~1V.
The supplies I'm having bother with are not that old, probably 5
years. But they were the second cheapest. They also lie on their
specs. They're sold as 850W supplies, but you can only draw 650W
of that on the 12V line, which is where 99% of the power goes in a
You could just get a nice bench supply if you only need 12V,
I can get more reliable supplies using PC supplies from Corsair, for
not much more money. Plus they have the extra voltage lines if I
5V is more critical since a lot of components start smoking at
5.2V or higher and those are the logic components.
Nowadays, aren't all the chips running at about 1V and powered by
their own VRMs, fed off the 12V line?
Yeah, a lot of the newer stuff particularly those processors made
for smart phones, minimalist single board computers, TV box,
Tablets, "Eee box" style ones, etc., do use 3.3 and 1.2 volt
supplies on the board.
Most of my desktop computers are "off-lease" second hand ones.
I notice that my super-compact desktops use a single ~20V supply
but that's probably just to keep the wire gauge small between the
supply and computer.
Laptops do the same. Trouble is you end up with a thin wire which
the user always breaks. Surely a thicker wire wouldn't cost much
compared with the whole laptop? Maybe it also means thinner wires
inside the laptop on the motherboard, less tracks, less space.
I can understand them wanting the voltage to be higher than that of
the battery to make charging easier, but 20V is a bit much.
The 12V runs fans, platter motors in CDROM, disk drive motors,
etc. and may be used for RS232 protocols, and it can be regulated
board close to the load if the load is critical. The -12V is
often pretty wimpy power-wise and it's function is for bias
and/or communications protocols.
The PS should work for the purpose intended, but if you are
trying to use it as a general bench power supply you may
encounter issues with poor regulation.
I am using it for PC components, but only for one component, the
graphics card, which only has a 12V input.
How many graphics cards are you running from this supply at the
One each. The supplies are CIT 850W (650W on the 12V rail). The
graphics cards are AMD Radeon R9 280X (250W TDP). I can actually
run two per supply, but since I've been having bother, I'm letting
them have one each to be sure, since I have a surplus of those
supplies sat on a shelf. If I get short, I'll double them up.
Just get a dedicated 12V supply(s).
I was thinking of car batteries with a charger or three. But that
would provide 13.8V....
I'm using some ebay ones
in projects, and have had good results. They even have a pot that
lets you adjust the voltage +/- 10%. They go from 1 amp to 60 amp
I might try that next time I need to buy one. Last time I looked
they would have cost more than PC supplies for anything over about
200W. But that was a few years ago.
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