"Commander Kinsey" wrote in message
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p.0f8u8x2fwdg98l@glass...
On Wed, 19 Feb 2020 17:02:33 -0000, Rod Speed
wrote:
Commander Kinsey 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?
Depends on what you mean by work correctly.
If I don't load the 5V at all, the 12V becomes 10.5V. Connecting a car
headlamp to the 5V rail (which draws about 1.75A) seems to make it behave.
By definition, when whatever it is that decides
what mark space ratio is used with a switch
mode power supply, it has to measure the
voltage somewhere and since the spec on
the 5V rail is a lot tighter than on the 12V
rails, it makes sense to measure the 5V rail.
That means that unless the 5V rail has some
load on it, the regulation isnt great and so
you can get some odd effects on the 12V rail.
Its obvious possible to have a separate switch mode
power supply for each rail but even the more expensive
PC PSUs don't do it like that because the 5V and 3.3V
rails are normally well loaded in a PC,
I don't understand.
That's obvious.
Surely the mark space ratio has to be completely different for the 12V and
5V rails
Nope. The high frequency chopped DC goes
into the high frequency transformer and the 5V
and 12V rails are just different output windings
on that. Look at a PC PSU circuit diagram.
http://www.pavouk.org/hw/en_atxps.html
because any computer at any given moment could be drawing more from one
than the other, then differently several seconds from now.
The current on the 5V rail whose voltage is used to vary
the mark space ratio doesn't actually vary all that much.
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.
You'd be better off with dedicated 12V switch mode power
supplys. Surprisingly cheap now even at the higher powers.
Link me to an example,
https://www.ebay.co.uk/itm/Transform...DPmF 8NegnqEg
That's just the one at the top of the hit list.
because I looked at them before, and at the high powers I need (300W per
card), it's massively cheaper to buy even the top of the range PC supplies
from Corsair etc.,
Not true with that one.
I assume because they're made in larger quantities.