Re: psu requirements-think i found it

ian wrote:
"ian" <> wrote in message news:8k_Vi.25772$uH.24496@xxxxxxxxxxxxxxxxxxxxxxx
: "ian" <> wrote in message
: news:vsXVi.25756$uH.14011@xxxxxxxxxxxxxxxxxxxxxxx
:: "peter" <peter@xxxxxxxxxxx> wrote in message
:: news:lzRVi.162058$1y4.126793@xxxxxxxxxxxx
::: In order to answer that question you will need to give details of your
::: PSU.........make and model would be nice...amps on the individual lines
::: would save us some time.
::: I ran a socket 939 mobo with an 64x2 3800 chip 1gig of DDR400 2 HD(SATA)
:: 1CD
::: 1DVD and a PCI-e Video card quite nicely with a 400 watt PSU....
::: peter
:: it was a quality one as i remember. Basically when did mobo requirements
:: jump from 20 pin to 24 pin?

sorry should explain, paul (in another thread) suggested this board although he thought it was discontinued. aus tek say it is 20 pin atx but ebuyer customer reviews say a 20pin to 24 pin converter is needed.

The A8V-VM SE has a 24 pin connector.

Enlargement picture here - S939 mobo, PCI Express video slot, DDR RAM.

Ian, you can use a 20 pin power supply, on a 24 pin connector motherboard.
It will work just fine.

Pin 1 goes to pin 1, from the 20 pin power supply, to the 24 pin motherboard
connector. Four pins on one end of the motherboard connector will not be
connected. The shape of the nylon shell on the power connector, helps assure you
get it right. When you plug in your old supply, it looks like this:

The latch on the two connectors will not be aligned properly, so the 20 pin
may not latch well with the 24 pin connector. But electrically it will be fine.

If your video card happens to have a PCI Express 2x3 connector on the end of
it, there are adapters for that. Some adapters are better designed than others.

The following gives some (boring) background on ATX supplies and what works
and doesn't work.


A 20 pin main power connector, has one 12V pin. If you had a PCI Express motherboard,
and had a 6600GT video card, the video card draws roughly 4.4 amps from the
video card slot, and cooling fans may draw another 0.5 amps. That is 4.9 amps
or so. A single pin on the 20 pin connector is rated for 6 amps. So a 20 pin
connector can be used with a 24 pin PCI Express motherboard (at least, if only
one video card is being used). They generally don't draw more power than that
from a single slot, and use the optional connector on the end of the video card
for more power.

If you had an SLI motherboard, and had two 6600GT PCI Express cards, that would
be an 8+ amps load. At that point, you would be well advised to use a proper
24 pin connector on the power supply. With a single PCI Express video card,
a 20 pin should be enough.

And if an AGP card is involved, the rails and currents are different for those,
and even less of an issue for the 12V rail. Only the 20 pin would be needed here,
and that is all you find on a typical AGP motherboard.

Symbol Parameter Condition Min Max Units Notes
Vddq1.5 I/O Supply Voltage IMAX = 2.0 A
VCC3.3 3.3 V Power Supply IMAX = 6.0 A
3.3VAUX 3.3 V Auxiliary Supply IMAX = 0.375 A
VCC5 5 V Power Supply IMAX = 2.0 A
VCC12 12 V Power Supply IMAX = 1.0 A <--- AGP is easy on 12V

The 24 pin connector has a total of two 12V wires. Thus, it can carry up to 12 amps
of current for PCI Express video card slots.

The processor power connector is the ATX12V 2x2 one, with two yellow wires and
two black wires. It has been in use since the P4 generation. It can carry enough
power for the 130W processors.

The power supply specs, in chronological order, are here. The third spec, is for
the ATX 2.0+ generation of supplies. They added the dual 12V rail option, and
the 24 pins for main connector. Many 24 pin connectors can be split into a 20 pin
plus a 4 pin connector (connector slides apart), so a new supply can be used with
an older motherboard. In the other direction (20 pin supply with 24 pin motherboard),
that is generally safe unless you have two particularly demanding video cards in
SLI or Crossfire applications.

When the 12V rail is split, as in the ATX 2.0+ supplies, the 12V2 rail powers the
processor (the 2x2 connector). The 12V1 rail powers the main connector and the
disk drive connectors. One of the disadvantages of the split power rails, at
least on paper, is that any current capacity left over on the processor rail,
cannot be used by the disk drives.

That is not always the case in practice. There are a few different ways to build
a power supply.

The new supplies could have two, totally independent outputs for 12V1 and 12V2.
That doesn't seem to be common. A second option, is a single output, plus two
current limiters feeding 12V1 and 12V2. That meets the safety requirement of
no more than 240W per 12V output. A third option, is a bullshit rating on the
power supply label, and just one output feeding both rails. And in fact,
this third option, is how the older supplies operated anyway.

The previous generation of supplies had just one 12V output rail inside. It
drove the two yellow wires on the ATX12V 2x2 connector, and the single 12V
wire on the 20 pin connector. Again, the safety requirement is met, as long
as the supply doesn't output more than 12V @ 20A. (I believe the number is
related to fire safety. It could be traceable to IEC60950, but I don't
have a copy of that spec.)

The motherboard side, for the most part, doesn't care about the power supply
type. The motherboard should separate the processor 12V part, from the
main 12V which powers video slots and fan headers. Whether fed by a newer
ATX 2.0+ with separated outputs, or an ATX 1.3 with single output, the
motherboard will work fine.

The only exception I've read about, is some Biostar boards. Some of them
connect the processor 12V to the main 12V inside the motherboard PCB. Which,
in theory, would not be good for a supply with two separate 12V output rails
(since we don't know, absolutely and for sure, how the rails on the supply
are constructed, of the three options mentioned above). When someone tells
me they have a Biostar board, and ask for a supply, I generally steer them
towards an ATX 1.3 type, just to be safe.

The wiring aspect of supplies, refers to "ampacity", the ability of the
wires/pins to safely carry the current without overheating or long term
damage to connector pins. When I say a pin is rated for 6 amps, that means
it won't overheat if the current stays under the 6 amp number. The second
aspect to power supplies, is their output rating, which is the maximum number
of amps they can produce, before the output voltage drops out of
acceptable tolerance. Supplies may have an overcurrent feature, which
shuts off the supply or otherwise limits the flow of current, if you exceed
the capacity (either intentionally or unintentionally with a short).

With your existing supply, all the numbers printed on the label mean
something. They all give useful information. The label has both current
output limitations listed, and total power limitations listed.

As a system build, it is possible to work out the loading for the 12V rail.
But for the 3.3V and 5V rails, all I can offer is estimates (like an
assumption that the motherboard uses no more than about 50W combined,
from the 3.3V and 5V rails).

If you have an inventory of all of your hardware, plus the numbers on
the label of the supply, it is possible to work out whether there is
adequate margin. There is no need to guess at the majority of it.

There are sites that offer to do power calculations for you. The
worst I've seen, gives a result which is twice the real number.
Other sites get a little closer. Many of the sites are not kept
up to date with the latest technology. Which is why, on occasion,
I work out numbers for people. To demonstrate how they can get
the numbers for themselves.

For processor power, I use:

For video card power, I look them up at:

Start a new thread, with an inventory of your hardware,
if you want an estimate of how much it draws. Including the
stuff printed on the power supply label, would allow determining
whether your current supply is good enough.

This site has a bit more background on ATX supplies, their history
and issues. Use the links at the top of the page, to read the
entire thing.

Sorry for the length of this post...