ATX PSU - how do I test a 24-pin connector?

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What do the extra 4 pins on a 24-pin ATX PSU Molex connector do, as opposed
to a standard 20-pin connector and what voltage should be on them?

The reason I ask is that I recently bought one of these ATX PSU testers: (link goes to ebay but if you don't trust it,
here's the real one - but beware of line wrap)

When a known, good, perfectly working, 20-pin PSU is connected to it the
display is like the one shown in the photo, ie, the +12V2 value shows as
"LL" (which means either LOW voltage or voltage missing altogether).

Now, a friend of mine is having problems with his computer. The fans/HDD
spin up, the POST appears on-screen, but immediately after (or maybe even
during the POST) the screen goes blank (black), the fans slow down *but do
keep spinning* and nothing else happens.

Having got my nice, shiny, new PSU tester, I thought a good place to start
was to test the PSU - especially as it's only a 250W one. However, it's a
24-pin connector and when I test it the +12V2 reading is still "LL"

Does that mean that the PSU is faulty as there should be something on the
extra 4 pins and showing a real value on the +12V2 display, or am I
completely wrong and the +12V2 display on the tester has nothing whatsoever
to do with the extra 4 pins of the 24-pin connector? The info that came with
the tester is very minimal and does not explain what should be seen with a
20-pin or a 24-pin PSU.



Re: ATX PSU - how do I test a 24-pin connector?

John wrote:
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12V2 is on the 2x2 ATX12V connector. You are probably supposed to connect
the 2x2 connector, to the position shown as "4p" in the diagram. "4p" is
one half of the 2x4 section. Once the 2x2 is connected, your "LL" will
disappear, to be replaced by the CPU 12V supply voltage reading. \specs\ATX12V_PSDG_2_2_public_br2.pdf

The extra four pins on the 24 pin, are redundant, and don't add any additional
information to your LCD ATX tester. They add current-carrying capacity to
the interface.

And the problem with your tester, is the interface doesn't allow watching the
voltages, while the cables are connected to the computer. It would be
real interesting to know what the voltages are, when the fans in your
friend's computer slow down. The 12V rail is being loaded by something, and
perhaps removing hardware a bit at a time will uncover it. Or the power
supply is simply being asked to supply too much power.

You could probably build some custom "Y" cables, to make the voltages
accessible, but the manufacturer of the tester should have thought of
that. Adding such cables, isn't the best, because it adds voltage drop.
The remote sense capability cannot compensate for the drop, when the sensor
wire is not right at the load. (In the above document, note that pin 13
has two wires connected to it. The brown wire is remote sense, and is
there to improve the precision of the delivered 3.3V. Some supplies have
three remote sense wires, and 3.3V, 5V, and 12V have improved precision.
That is a feature on an Antec supply.)


Re: ATX PSU - how do I test a 24-pin connector?

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Thanks very much for that Paul - very useful info and has given me a better
understanding of what's what.

Cheers mate,


Re: ATX PSU - how do I test a 24-pin connector?

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It's better to buy a simple tester or volmeter and test the various
It's easy and more realistic.
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