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Old February 21st 19, 09:07 AM posted to alt.comp.hardware
Paul[_28_]
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Default What is the potential value of a used 400W ATX power suppy.

Norm X wrote:

Thanks Paul. The link to article you gave was useful. A nice man at a repair
shop gave me a short Molex to six pin GPU power adapter cable. I gave him
RAM I could not sell. I was confused a bit by an 8 pin connector that could
be split into two identical four pin connectors on the new Amazon Coolmax
PSU. I left the old Dynex PSU with the shop for refurbishing. That may have
been premature. I used scotch tape to identify all the power connectors I
used. The wire colors tell what is correct. Either I get a call from the
shop to tell me all is A OK, or I can study the article you gave to satisfy
my concern. I don't want to power up unless everything is electrically
correct.


If you use the short Molex to six pin GPU, note that the Molex connector
has enough ampacity to do the job, but then there's nothing left over
for hard drives. For example, doing this would be marginal, from the
HDD-requirements point of view. The GPU on the video card will likely
tolerate the tolerances on the 12V rail if done like this.

PSU
|
+--- HDD 1.5A
|
+--- HDD 1.5A
|
|
+--- adapter --- PCIe 12V @ 6A for 75W video card

What can happen if you do that, is the HDD may "spin-down" in
mid-session, because the voltage on the cable drops to around 11V
instead of 12V. The HDD is doing an "emergency shutdown", as it thinks
the PSU is about to turn off the power.

If using the adapter, you might do this. Now, if the cable voltage
drops to 11V because of ohms law, the video card won't shut off.

PSU
|
+---
|
+---
|
|
+--- adapter --- PCIe 12V @ 6A for 75W video card

Some of those adapters, come with two Molex. And they're intended
to be used like this. You connect each molex to a different chain,
in an attempt to spread the load, so the wires from the PSU work in
parallel and the voltage drop is slightly less.

PSU ---------------+
| |
+--- ---+
| |
+--- ---+
| |
| |
+---X X---+
\ /
+--+--+
|
PCIe 2x3 12V @ 6A

On a modern desktop PSU, the two wire looms in question
are likely off the same output, so you're not putting
outputs in parallel.

*******

The 2x4 that splits into two 2x2 is for ATX12V by the processor socket.

Traditionally, a Pentium 4 motherboard back in the day, had a 2x2
with two yellow wires and two black wires. The connector was keyed
by the shape of the nylon shroud around each pin. The motherboard
manual, on the good ones, would even show which pins where
+12V (yellow) and which were ground (black).

Later, there were several standards for server processor connectors
using 2x4 pins instead. Those don't split. And those are not
compatible with desktops either. Fortunately, you don't run into
those too often when buying desktop power supplies.

Enthusiast motherboards need more than 144W. Or 12V @ 12A on the 2x2
ATX12V. To solve that need, they combined more yellow and black wires.
By using a 2x4 with four yellow and four black, they get room
for 12V @ 24A to flow, or 288W. There are still some overclocking
experiments that draw more power than that (D-805 at 4GHz), but then
you'd probably be melting the motherboard when doing that anyway :-)

There is a 2x4 that splits into a 2x3 and a 2x1. That's PCI Express
and solving both 2x4 and 2x3 input requirements. One of the pins
involved there is a "sense" pin rather than a "current flow" pin,
and tells the video card a proper 2x4 PCIe power was plugged in.

You should have been able to see those on Playtool.

This is the ATX12V that splits in two pieces for usage with
legacy motherboards (near the CPU socket).

http://www.playtool.com/pages/psucon...plus4index.jpg

Whereas this is the PCI Express that does 2x3 or 2x4 PCIe power.

http://www.playtool.com/pages/psucon...plus2index.jpg

The shroud on the connector, is intended to prevent inappropriate
combinations. On the ATX12V that splits in half, one half has
the "right" shape for the 2x2 on an old motherboard. The second
half is the extension portion, and is the "wrong" shape to fit
a 2x2 on its own. This is purely arbitrary - what matters is
that no yellow wires on the PSU end, touch pins intended for ground,
and that causes a short and causes the PSU to shut off (if you're lucky).

This is a picture I drew previously. The drawing shows the shapes
in the plastic ends of the connectors involved for ATX12V.

ATX12V 4/8

TTTTTT TTTTTT End view of connector pair
___ ___ ___ ___ showing shapes which help
| | | | | | | | control mating only one way.
| | | | | | | |
\_/ --- \_/ \_/ You will be using the *left* connector in pic.
___ ___ ___ ___ The right connector is used if
| | | | | | | | the motherboard had a 2x4 and room
| | | | | | | | for all eight pins.
--- \_/ \_/ \_/
TTTTTT is the tab which latches to
|| a feature on the side of the connector
\||/ on the motherboard and provides a
\/ visual cue on how to do it.
___ ___
| | | | Normal motherboards have just 2x2 [144 watt]
| | | | and have a tab for the latch on the
\_/ --- PSU end to attach to. The right-hand
___ ___ one above would not insert unless you
| | | | had an enthusiast 2x4 on the motherboard
| | | | with it's (approximate) 288W rating
--- \_/

HTH,
Paul