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  #11  
Old 19-06-13, 05:00 PM
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alpenwasser alpenwasser is offline
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Quote:
Originally Posted by sushisky View Post
why do the connections have double wires? (really confused about this)
I'm not an absolute PSU expert, so if anyone spots an error feel free to correct me.

Every PSU needs to do voltage control. This means that the PSU needs to check that its 12 V
line actually puts out 12 V, the 5 V delivers 5 V and so on. The important part is that those
voltages need to be supplied at the connector to the components, and not where the PSU
connects to its cables.

Now, there's a nice little equation linking resistance, current and voltage drop over an electrical
element:
Code:
U=R*I
U: voltage drop over element
R: element resistance
I: current through element
Every wire that goes from the PSU to each respective component (say, the 24 pin connector
on the M/B ) has a resistance and a current which flows through it (usually). That current can
of course change depending on how much load the system is under.

So the voltage drop between the PSU and the 24 pin connector on the mainboard will not always
be the same and will change with system load. Voltage regulation needs to take this into account.

In order to be able to do that, the PSU needs to measure the voltage at the M/B end of the
connection. In order to do that, you use a second wire which you connect to the M/B crimp
terminal on one end and to a voltage sensor input on the PSU side. That wire will have almost
not current
flowing through it, meaning that the voltage drop over that wire from the 24 pin back
to the PSU will be practically nothing (or, to use an engineer's favourite word: negligible ).

Example:
Let's assume you have an 18 AWG wire going from the PSU to the 24 pin, delivering a current
of 5 amperes. The voltage drop you will have from the PSU to the M/B in that wire will then
roughly be:
Code:
U=R*I=rho*L*I=0.00002096 ohm/mm * 500 mm * 5 A = 0.05 V = 50 mV
U = voltage drop over wire
rho: resistance per length of wire, 0.00639 ohm/foot
L: length of wire, 0.5 m in this example
I: current, 5 A here
Source for wire resistance value (I've metrified it for this calculation): link.

In this example you would have a voltage drop of 50 mV between the PSU and the 24 pin
on the M/B, which is not insignificant.

The secondary wire has no current flowing through it (or almost none, not 100 % sure on that,
somebody who's an expert on PSU design might be able to say), which means that the voltage
drop over the secondary wire from the 24 pin back to the PSU will of course be:
Code:
U=R*I=R*0=0
And this allows you to accurately read the actual voltage at the 24 pin connector. Of course I
have not taken into account the actual resistance in the crimp connector and all that stuff, but
I hope this makes things a bit clearer.

This is also why I'm not sure if there might be problems when you connect the voltage sense
wire to the current carrying wire almost back at the PSU. The voltage you will read with the
sense wire will be incorrect in that case. Whether or not the error will be large enough to cause
any problems I'm not sure about.

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  #12  
Old 20-06-13, 08:09 AM
stevebarnes15 stevebarnes15 is offline
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They have double wires sometimes because the connectors sometimes have less pins. eg, power supply cale for graphics has 6 pin connector for gpu end and 8 pin at the psu end.

Power Supply cables are coloured for a reason and that so we can identify the voltage of the cable as each colour has it's own voltage.
It's just the industry standard which can be confusing but with a little reading becomes apparent.

To braid a split cable the best way is to use 3 pieces of braid, 1 for each straight line of the Y section with a heatshrinked joint which will be hidden behind the motherboard tray after if that makes sense.

What i found easier when starting out was to sticky label each wire numbering them 1-24 and then draw a diagram of the connector so you know which wire goes where when reassembling.
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  #13  
Old 20-06-13, 05:05 PM
sushisky sushisky is offline
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so since the second wire is for voltage reading and regulation, it would be bad to cut it off or recrimp it?
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  #14  
Old 20-06-13, 05:32 PM
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alpenwasser alpenwasser is offline
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Quote:
Originally Posted by sushisky View Post
so since the second wire is for voltage reading and regulation, it would be bad to cut it off or recrimp it?
I'd say cutting it off completely is probably a pretty bad idea since you would
take away the PSU's capability to control and regulate the voltage coupled to
that wire. I would not be surprised if you'd actually break something with that
(example: cutting off the + 12 V sense wire will lead to the PSU getting a
measurement of 0 V on the + 12 V line, which might lead to some very bad
voltage regulation reaction).

Lutro0's version of simply branching the wire out somewhere at the back I'm
not completely sure about. Voltage measurement will be somewhat compromised
but since you can adjust your voltages in the BIOS (usually) you could actually
compensate for that to some degree. The voltages measured by the PSU would
be slightly too high (the actual voltages at the connector would be somewhat
lower than the PSU believes them to be), so you're not going to accidentally fry
your components or anything, it's just not exactly what I'd call an optimal solution
from an electrical point of view, and if your setup is sensitive to bad voltage
regulation (say, if you're running some significant overclocks) then this might
lead to failure sooner than a proper two wire cable.

The most elegant solution I'd say is to run two separate wires for the full length of
the cable inside one sleeve and then branching out of the sleeve somewhere
close to the PSU. If that's not possible, then I'd personally run two separate wires
with separate sleeves, but if you want you could try Lutro0's version, I have not
yet heard about anything horrible happen due to that, it's just not something
I'd personally do to one of my own rigs.
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  #15  
Old 20-06-13, 05:51 PM
stevebarnes15 stevebarnes15 is offline
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For 24pin motherboard connector google power supply unit computer and hit up the wiki page, you can find a full pin out with voltages and colour guides there.
Very very useful.
So long as you remember where each cable goes the braiding itself is very easy but also very time consuming.
If you can get away without cutting wires all the better but if you have to, just take your time and think about how you want your end result to look, draw diagrams if nessecary, and label the cables as suggested before.

Another thing to keep in mind is that psu's store energy so be careful when opening your psu up, some can hold charge for days after being unplugged and i for one wouldn't like a zap off one.
Nice to see someone having a go though so kudos for that.
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  #16  
Old 21-06-13, 06:20 PM
sushisky sushisky is offline
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wow. is it possible to have a psu with cables that are all single wire? or will there always be some mixed in :/
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  #17  
Old 21-06-13, 07:28 PM
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alpenwasser alpenwasser is offline
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Quote:
Originally Posted by sushisky View Post
wow. is it possible to have a psu with cables that are all single wire? or will there always be some mixed in :/
I don't see how you could do a (good) PSU with only single wires since then you'd
have to implement another control mechanism for voltage regulation, and I can't
think of a way to do that without the sense wires.

Best you can hope for is that the pinout on the PSU side is reasonable enough so
that you don't have a lot of tangled and crossing wires, but I don't know of any
PSU that's really perfect in that regard at the moment.
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  #18  
Old 22-06-13, 12:24 AM
stevebarnes15 stevebarnes15 is offline
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Is this what you are trying to achieve?

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  #19  
Old 26-06-13, 12:42 PM
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alpenwasser alpenwasser is offline
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Just a quick annotation: I've found the Wiki article describing what I was talking about
above with regards to the sense wires: Four-terminal Sensing

I knew there was a name for this but had forgotten what it was.
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  #20  
Old 30-06-13, 06:15 PM
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Reloaded2 Reloaded2 is offline
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Trst Me without heatshrink.
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