On Wed, 19 Feb 2020 17:59:12 -0000, Rod Speed wrote:



"Commander Kinsey" wrote in message
newsp.0f8wi6nqwdg98l@glass...
On Wed, 19 Feb 2020 17:26:31 -0000, Rod Speed
wrote:



"Commander Kinsey" wrote in message
newsp.0f8u1ul9wdg98l@glass...
On Wed, 19 Feb 2020 17:01:52 -0000, Andy Bennet wrote:

On 19/02/2020 16:46, 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?

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 are
active.

On el cheapo SMPS the primary supply is the 5V whose feedback controls
the conversion transformer. The 12V supply is derived from the same
transformer so if the 5V is not breaking into a sweat not enough
current
is available in the transformer secondary to get the 12volts up to
regulation.

Ok, I do have a degree in electronics, but it was a very long time ago.
Can you go into more detail?

There is no more detail. There is just the one switch mode
power supply and it controls the voltage on the 5V rail.
The 12V rail is just a different tap on the switch mode
high frequency transformer,

That's not how I thought SMPSs worked.

Yep, that's your problem, a lack of understanding in that area.

Surely since the current draw on 5V can vary from 1 amp to 35 amps,

It doesn't in fact vary over time in a PC,

and the 12V from 1 amp to 56 amps,

It doesn't in fact vary over time that much in a PC,

there have to be completely seperate ratios set up?

There isnt, and that works fine, because the current
from the 5V rail doesn't vary all that much over time.

As you have discovered, if there is no load on
the 5V rail, the 12V rail is well out of spec.

Adjusting just the 5V could have the 12V going anywhere.

Not anywhere, just by the amount you measured.
And only down from 12V with no load on the 5V rail,

And since that never happens in a PC, it works fine.

But PCs could vary the 5V and 12V considerably. For example when you start playing a game, the 12V changes dramatically for the CPU and GPU, perhaps by a factor of 10. **** knows what still uses 5V, but it won't be anything like constant.