Problems with 12V and 5V lines on a PC ATX supply

C

Commander Kinsey

Guest
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?
 
torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?
Click to expand...

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V
 
On 2/20/20 5:55 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.  Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely?  They have to adjust each individually or it
would go completely out of whack.  I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.
Click to expand...

Time-averaged output voltage(s) of an e.g. ideal flyback converter with
either single or multiple secondaries in continuous conduction mode is a
function of the input voltage, duty cycle, and turns ratio only it isn't
directly dependent on any currents.

That is to say in the non-ideal case with a heavy load on the 5V and a
light load on the 12 the 12 will not go out of regulation nearly that
bad in a well-designed converter; the duty cycle is not supposed to be
swinging wildly over light load to full load in CCM (until maybe it
drops into DCM at very light load.)
 
torsdag den 20. februar 2020 kl. 23.55.54 UTC+1 skrev Commander Kinsey:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work. Since I can draw for example 5 times as much current from one line so the other would be adversely affected surely? They have to adjust each individually or it would go completely out of whack. I take more 5V current, so the whole thing gets boosted, now 12V becomes 14V.
Click to expand...

http://danyk.cz/s_atx_en.html
 
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V
Click to expand...

Not sure how a shared transformer can work. Since I can draw for example 5 times as much current from one line so the other would be adversely affected surely? They have to adjust each individually or it would go completely out of whack. I take more 5V current, so the whole thing gets boosted, now 12V becomes 14V.
 
On 2/20/20 7:15 PM, whit3rd wrote:
On Thursday, February 20, 2020 at 2:55:54 PM UTC-8, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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 l... 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?

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.

The transformer puts proportional (AC) voltages on its windings, so the DC outputs are always
proportional as long as resistive and rectifier losses are similar to the test condtions.
If you draw zero current through one of the rectifiers, but normal current through another,
there's a discrepancy to be expected. Usually, 12V power has a high tolerance, it's often
the one to be left out of the regulation feedback loop.
Click to expand...

In modern PCs just about everything runs of the 12V but almost nothing
runs off it directly, it's a bulk supply to feed other converters on the
mobo.

LEDs, fans, hard drive spindles get it raw I guess.
 
On 2/20/20 7:11 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 23:41:11 -0000, bitrex <user@example.net> wrote:

On 2/20/20 5:55 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.  Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely?  They have to adjust each individually or it
would go completely out of whack.  I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.


Time-averaged output voltage(s) of an e.g. ideal flyback converter with
either single or multiple secondaries in continuous conduction mode is a
function of the input voltage, duty cycle, and turns ratio only it isn't
directly dependent on any currents.

That is to say in the non-ideal case with a heavy load on the 5V and a
light load on the 12 the 12 will not go out of regulation nearly that
bad in a well-designed converter; the duty cycle is not supposed to be
swinging wildly over light load to full load in CCM (until maybe it
drops into DCM at very light load.)

Sounds like a clever design which fails under certain circumstances.
Plenty folk have a computer with two or more graphics cards, so they
need a 2nd power supply.  Nowhere does it state the supply has to be
loaded on the 5V line.
Click to expand...

Yes it "fails" when they cheap out on everything else in the supply,
surely. How much you pay for them again?
 
On Thursday, February 20, 2020 at 2:55:54 PM UTC-8, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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 l... 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?

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.
Click to expand...

The transformer puts proportional (AC) voltages on its windings, so the DC outputs are always
proportional as long as resistive and rectifier losses are similar to the test condtions.
If you draw zero current through one of the rectifiers, but normal current through another,
there's a discrepancy to be expected. Usually, 12V power has a high tolerance, it's often
the one to be left out of the regulation feedback loop.
 
On Thu, 20 Feb 2020 23:41:11 -0000, bitrex <user@example.net> wrote:

On 2/20/20 5:55 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work. Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely? They have to adjust each individually or it
would go completely out of whack. I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.


Time-averaged output voltage(s) of an e.g. ideal flyback converter with
either single or multiple secondaries in continuous conduction mode is a
function of the input voltage, duty cycle, and turns ratio only it isn't
directly dependent on any currents.

That is to say in the non-ideal case with a heavy load on the 5V and a
light load on the 12 the 12 will not go out of regulation nearly that
bad in a well-designed converter; the duty cycle is not supposed to be
swinging wildly over light load to full load in CCM (until maybe it
drops into DCM at very light load.)
Click to expand...

Sounds like a clever design which fails under certain circumstances. Plenty folk have a computer with two or more graphics cards, so they need a 2nd power supply. Nowhere does it state the supply has to be loaded on the 5V line.
 
On Fri, 21 Feb 2020 00:17:00 -0000, bitrex <user@example.net> wrote:

On 2/20/20 7:11 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 23:41:11 -0000, bitrex <user@example.net> wrote:

On 2/20/20 5:55 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work. Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely? They have to adjust each individually or it
would go completely out of whack. I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.


Time-averaged output voltage(s) of an e.g. ideal flyback converter with
either single or multiple secondaries in continuous conduction mode is a
function of the input voltage, duty cycle, and turns ratio only it isn't
directly dependent on any currents.

That is to say in the non-ideal case with a heavy load on the 5V and a
light load on the 12 the 12 will not go out of regulation nearly that
bad in a well-designed converter; the duty cycle is not supposed to be
swinging wildly over light load to full load in CCM (until maybe it
drops into DCM at very light load.)

Sounds like a clever design which fails under certain circumstances.
Plenty folk have a computer with two or more graphics cards, so they
need a 2nd power supply. Nowhere does it state the supply has to be
loaded on the 5V line.

Yes it "fails" when they cheap out on everything else in the supply,
surely. How much you pay for them again?
Click to expand...

Not regulating the 12V properly is cheaping out, and from what I've read in these groups a lot of them are like that.

They're CIT supplies. They don't explode like Alpine when you dare to use more than 50% of the rated current, but they're not as stable as a proper Corsair.
 
On Fri, 21 Feb 2020 00:20:49 -0000, bitrex <user@example.net> wrote:

On 2/20/20 7:15 PM, whit3rd wrote:
On Thursday, February 20, 2020 at 2:55:54 PM UTC-8, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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 l... 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?

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.

The transformer puts proportional (AC) voltages on its windings, so the DC outputs are always
proportional as long as resistive and rectifier losses are similar to the test condtions.
If you draw zero current through one of the rectifiers, but normal current through another,
there's a discrepancy to be expected. Usually, 12V power has a high tolerance, it's often
the one to be left out of the regulation feedback loop.

In modern PCs just about everything runs of the 12V but almost nothing
runs off it directly, it's a bulk supply to feed other converters on the
mobo.

LEDs, fans, hard drive spindles get it raw I guess.
Click to expand...

That's why I'm surprised they can get away with making PSUs that can't run properly with no 5V load. It's quite likely your PC sometimes draws almost nothing from 5V. I have to take almost 2 amps from 5V to make this PSU work correctly.
 
bitrex <user@example.net> wrote in
news:CnF3G.207815$MX1.148708@fx43.iad:

On 2/20/20 7:15 PM, whit3rd wrote:
On Thursday, February 20, 2020 at 2:55:54 PM UTC-8, Commander
Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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 l... 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?

they are all on the same transformer so they are related, and
the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.

The transformer puts proportional (AC) voltages on its windings,
so the DC outputs are always proportional as long as resistive
and rectifier losses are similar to the test condtions. If you
draw zero current through one of the rectifiers, but normal
current through another, there's a discrepancy to be expected.
Usually, 12V power has a high tolerance, it's often the one to be
left out of the regulation feedback loop.


In modern PCs just about everything runs of the 12V but almost
nothing runs off it directly, it's a bulk supply to feed other
converters on the mobo.

LEDs, fans, hard drive spindles get it raw I guess.
Click to expand...

Hard drive spindle motor drive circuits get it raw, but the spindle
motor itself gets a quite controlled feed from that circuit.
 
"Commander Kinsey" <CFKinsey@military.org.jp> wrote in
news:eek:p.0gbbg6ebwdg98l@glass:

On Fri, 21 Feb 2020 00:17:00 -0000, bitrex <user@example.net
wrote:

On 2/20/20 7:11 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 23:41:11 -0000, bitrex <user@example.net
wrote:

On 2/20/20 5:55 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev
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?

they are all on the same transformer so they are related, and
the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work. Since I can draw
for example 5 times as much current from one line so the other
would be adversely affected surely? They have to adjust each
individually or it would go completely out of whack. I take
more 5V current, so the whole thing gets boosted, now 12V
becomes 14V.


Time-averaged output voltage(s) of an e.g. ideal flyback
converter with either single or multiple secondaries in
continuous conduction mode is a function of the input voltage,
duty cycle, and turns ratio only it isn't directly dependent on
any currents.

That is to say in the non-ideal case with a heavy load on the
5V and a light load on the 12 the 12 will not go out of
regulation nearly that bad in a well-designed converter; the
duty cycle is not supposed to be swinging wildly over light
load to full load in CCM (until maybe it drops into DCM at very
light load.)

Sounds like a clever design which fails under certain
circumstances. Plenty folk have a computer with two or more
graphics cards, so they need a 2nd power supply. Nowhere does
it state the supply has to be loaded on the 5V line.

Yes it "fails" when they cheap out on everything else in the
supply, surely. How much you pay for them again?

Not regulating the 12V properly is cheaping out, and from what
I've read in these groups a lot of them are like that.

They're CIT supplies. They don't explode like Alpine when you
dare to use more than 50% of the rated current, but they're not as
stable as a proper Corsair.
Click to expand...

Best PC PSUs I have used are by EVGA.

Those guys know how to make circuit boards and propely manage what
is on them and where the juice is moving and what parts are producing
heat. Their MOBOs are pretty good too, thought the gamingtard mobo
makers have won out now due to so many gaming dopes building their
own machines. Once you get all that money you can start doing better
and better engineering in your products. Look at AMD. They *might*
beat out Intel for a bit, but don't count out the big boys that got
us all here yet.
 
On Thursday, February 20, 2020 at 7:54:38 PM UTC-5, Commander Kinsey wrote:
On Fri, 21 Feb 2020 00:17:00 -0000, bitrex <user@example.net> wrote:

On 2/20/20 7:11 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 23:41:11 -0000, bitrex <user@example.net> wrote:

On 2/20/20 5:55 PM, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work. Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely? They have to adjust each individually or it
would go completely out of whack. I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.


Time-averaged output voltage(s) of an e.g. ideal flyback converter with
either single or multiple secondaries in continuous conduction mode is a
function of the input voltage, duty cycle, and turns ratio only it isn't
directly dependent on any currents.

That is to say in the non-ideal case with a heavy load on the 5V and a
light load on the 12 the 12 will not go out of regulation nearly that
bad in a well-designed converter; the duty cycle is not supposed to be
swinging wildly over light load to full load in CCM (until maybe it
drops into DCM at very light load.)

Sounds like a clever design which fails under certain circumstances.
Plenty folk have a computer with two or more graphics cards, so they
need a 2nd power supply. Nowhere does it state the supply has to be
loaded on the 5V line.

Yes it "fails" when they cheap out on everything else in the supply,
surely. How much you pay for them again?

Not regulating the 12V properly is cheaping out, and from what I've read in these groups a lot of them are like that.

They're CIT supplies. They don't explode like Alpine when you dare to use more than 50% of the rated current, but they're not as stable as a proper Corsair.
Click to expand...

No, it's your fault for using a supply designed for obsolete systems.
 
On 2/20/20 8:46 PM, DecadentLinuxUserNumeroUno@decadence.org wrote:
bitrex <user@example.net> wrote in
news:CnF3G.207815$MX1.148708@fx43.iad:

On 2/20/20 7:15 PM, whit3rd wrote:
On Thursday, February 20, 2020 at 2:55:54 PM UTC-8, Commander
Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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 l... 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?

they are all on the same transformer so they are related, and
the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.

The transformer puts proportional (AC) voltages on its windings,
so the DC outputs are always proportional as long as resistive
and rectifier losses are similar to the test condtions. If you
draw zero current through one of the rectifiers, but normal
current through another, there's a discrepancy to be expected.
Usually, 12V power has a high tolerance, it's often the one to be
left out of the regulation feedback loop.


In modern PCs just about everything runs of the 12V but almost
nothing runs off it directly, it's a bulk supply to feed other
converters on the mobo.

LEDs, fans, hard drive spindles get it raw I guess.



Hard drive spindle motor drive circuits get it raw, but the spindle
motor itself gets a quite controlled feed from that circuit.
Click to expand...

I think some PCIe cards like GPUs still use 3.3 off the PCI bus for
something. The really honking ones have a connector elsewhere on the PCB
for a higher current connection to the 12.
 
On Thursday, February 20, 2020 at 11:03:37 PM UTC-5, bitrex wrote:
On 2/20/20 8:46 PM, DecadentLinuxUserNumeroUno@decadence.org wrote:
bitrex <user@example.net> wrote in
news:CnF3G.207815$MX1.148708@fx43.iad:

On 2/20/20 7:15 PM, whit3rd wrote:
On Thursday, February 20, 2020 at 2:55:54 PM UTC-8, Commander
Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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 l... 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?

they are all on the same transformer so they are related, and
the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.

The transformer puts proportional (AC) voltages on its windings,
so the DC outputs are always proportional as long as resistive
and rectifier losses are similar to the test condtions. If you
draw zero current through one of the rectifiers, but normal
current through another, there's a discrepancy to be expected.
Usually, 12V power has a high tolerance, it's often the one to be
left out of the regulation feedback loop.


In modern PCs just about everything runs of the 12V but almost
nothing runs off it directly, it's a bulk supply to feed other
converters on the mobo.

LEDs, fans, hard drive spindles get it raw I guess.



Hard drive spindle motor drive circuits get it raw, but the spindle
motor itself gets a quite controlled feed from that circuit.


I think some PCIe cards like GPUs still use 3.3 off the PCI bus for
something. The really honking ones have a connector elsewhere on the PCB
for a higher current connection to the 12.
Click to expand...

Pretty much all PC CPUs and GPUs need a direct 12 volt connection for power.. It's been that way for over 10 years now. They used to use 5 volts for the CPUs, but the power levels got too high and 5 volt connections had too much current flowing. So they switched to 12 volts for the CPU on board PSU. These days GPUs use pretty much the same power levels.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
 
On Thursday, February 20, 2020 at 7:49:26 PM UTC-8, Michael Terrell wrote:
On Thursday, February 20, 2020 at 7:54:38 PM UTC-5, Commander Kinsey wrote:

Not regulating the 12V properly is cheaping out, and from what I've read in these groups a lot of them are like that.

No, it's your fault for using a supply designed for obsolete systems.
Click to expand...

Nonsense; it's not a fault to find a way to repurpose an otherwise obsolescent part for
such a use, it's a talent. The 'cheaping out' was an economy measure, and DID make the
parts less expensive, so that was a job well done, too.
 
On 2020-02-20 17:55, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.  Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely?  They have to adjust each individually or it
would go completely out of whack.  I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.
Click to expand...

Nope. The coupling between windings is high, so they track pretty
accurately--drag one output down and the others follow.
Cross-regulating like that is quite common. In the limit of tight
coupling (k=1), low resistance, and zero diode drop, the
cross-regulation becomes perfect.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Friday, February 21, 2020 at 11:33:58 AM UTC-5, Phil Hobbs wrote:
On 2020-02-20 17:55, Commander Kinsey wrote:
On Thu, 20 Feb 2020 22:27:48 -0000, Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev 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?

they are all on the same transformer so they are related, and the
feedback is a combination of 12V and 5V

Not sure how a shared transformer can work.  Since I can draw for
example 5 times as much current from one line so the other would be
adversely affected surely?  They have to adjust each individually or it
would go completely out of whack.  I take more 5V current, so the whole
thing gets boosted, now 12V becomes 14V.


Nope. The coupling between windings is high, so they track pretty
accurately--drag one output down and the others follow.
Cross-regulating like that is quite common. In the limit of tight
coupling (k=1), low resistance, and zero diode drop, the
cross-regulation becomes perfect.
Click to expand...

This, (k=~1) is what seems magical about transformers to me.
(Right you can tell me it's Faraday's law,
voltage integrated around closed line is change in B-flux
through surface... etc..)

George H.
Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...
 
On 2/20/20 10:57 PM, whit3rd wrote:
Nonsense; it's not a fault to find a way to repurpose an otherwise
obsolescent part for such a use, it's a talent.
Click to expand...

I largely agree.

However, I've found that salvage parts often have a down side that makes
them sub-optimal for re-use. It's a matter of is the down side a
blocker for a specific re-use.

In this case, it sounds like there must be current on a different
voltage rail. Depending on the power levels, this doesn't sound like
too big of a down side to me.



--
Grant. . . .
unix || die
 
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