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HELP ? ATX PSU LEADS


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Guest MARSMAN

I was wondering if it would be possible to modify the output leads (low voltage side) of a ATX/12V-PSU.
Basically i want to make an extra long set of leads (harness/loom),to go from the PSU to the MOTHERBOARD via a PANEL on the rear of the tower case.
I would like to site the PSU as far from the computer as possible

My questions are: What IF any is the MAXIMUM length possible (would 5-6 Mtrs be pushing it)?
And, if crosstalk is a cause of concern could it be reduced by using screened wire/cable for each seperate wire/cable making up the (harness/loom)?

Is there something that i am missing.?

Any idea's or relevant links welcome.
Hoping you can help.

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Hi, MARSMAN

Mars is obviously connected to the World Wide Web these days. Welcome to this forum MARSMAN! Seriously; what you should watch out for is the voltage drop. A modern CPU consumes a lot of current and you will get serious voltage drop especially if you use standard wire. I think 5-6 Mtrs is not possible. The CPU harness is an untested issue as far as I know, I can

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Marsman,
You could separate the CPU and PSU only if you use very heavy wire (or copper pipes), which is very impractical.
However, some PSUs have "remote sense" wires that allow the PSU to adjust its voltage to make-up for the voltage drop of long wires that you desire. Maybe your PSU already has these wires but they are jumpered to its output. If so, then cut the jumpers and try it with each output wire (fairly heavy) and each sense wire (not heavy) joined at the CPU.

Ante and Marsman,
Have you seen this link? I have tuned-in since December:
http://marsrovers.jpl.nasa.gov/home/index.html

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marsman, I have done what you are talking about, but I have not run the wires to such length. You will not have cross talk, but you will require more current than what you needed before. How many watts is this power supply rated? If it is an older computer with a small power supply (230 to 300W), you could purchase a newer PSU on the internet for around US$15 in the 400Watt category. You can figure the resistance of your wire by the gauge. I have a chart somewhere and I will look for it. At the worst, you might have to increase the size of the wire, making a special cable.
A lot will also depend upon how many devices are powered. Floppy drive, CD, hard drive, etc. To streamline, get rid of anything that is not necessary. This will lighten the load.

MP

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A more powerful PSU will not increase the current because its voltage will be the same. You will need more VOLTAGE to overcome the voltage-drop of the long wires. Then it will require only a little more power.
You may need to add capacitors at the CPU to smooth the digital hash.

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audioguru, the regulation of the PSU will match the voltage. You need more current to allow this due to the added resistance of the wires. This is why you now see computer power supplies going up to 500W when they used to be only 230W.
Voltage is only potential. It is the current that causes the circuit to work. If you have 1,000 volts and no current, a circuit will not do you much good.

Now, getting away from the theory part of this subject, I have experimented with this myself. One of my older computers gives me a bench power supply from the plug through the back panel.

I liked your copper pipes idea, but thought that it might be a little extreme in this case. ;D

MP

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MP,
I am sorry to continue to disagree with you.
The current will NOT increase due to the added resistance of the wires, if the supply voltage remains the same. The resistance of the wires is in series with the load, not in parallel. Therefore the load will receive less voltage due to the voltage-drop across the wire resistance. And with the load receiving less voltage, the current will also be less.

Example #1 (5.0V supply, 0.5 ohm load):
Original With 0.1 ohm wire resistance
Current: 10A 8.3A
Voltage at the load: 5.0V 4.2V
Power from supply: 50W 41.7W
Power at load: 50W 34.9W

Example #2 (6.0V supply, 0.5 ohm load):
With 0.1 ohm wire resistance
Current: 10A
Voltage at load: 5.0V
Power from supply: 60W
Power at load: 50W

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It is ok to disagree. But that is not what I said. As I look at my last post, I agree it is not real clear, nor was it worded in a way to get the meaning across.
What I said was that the circuit would "require" more current. Thus he would want a PSU rated at higher watts. Added was my suggestion of a larger gauge of wire. This is what actually reduces the resistance. For example 24 AWG wire has a resistance of 84.2 ohms per 1000 meters whereas (larger)16 AWG wire is only 13.2 ohms per 1000 meters (as an example, not that this length is even considered). Point is, as you increase the gauge, you have considerable less resistance. However, you also need more current. Sorry, I do not have a measurement for copper pipe or I would also post it as per your earlier suggestion.
But as funny as that might sound, if you had enough current to power it, yes, you could use the pipe. ;D

MP

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MP,
In this series circuit that will have extra wire resistance, where is your "more" current going to go? See my example #1 to see that the current is LESS when wire resistance is added to the loop.
Why is a more powerful PSU needed? With more wire resistance, the current and power will be LESS. If you use a larger wire guage or copper pipe, then the current and power from the PSU will NOT be more than it was with original short wires.
My example #2 shows that IF you could raise (maybe not possible) the voltage of the PSU, then the voltage and current at the CPU will be normal, but the PSU supplies a little more power.

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audioguru, this is not rocket science. This is the same method used by even the car industry since Henry Ford was building cars. For example, if you are building a hot rod and you have to move the battery from the front of the car to the trunk, you use a larger cable. You do not go out and buy a 24 volt battery. ::)

MP

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