Possible PSU externally bu lengthening cables?

[mikeguava]

Hi,

I am trying to custum build a mini PC which has a very high power consumption ( ~400 - 450 Watts)
I cannot find any standard PSU that meet my size requirments and I am considering custumizing a Power Supply to have it sit externally - by lengthening all the cables ( 24 Pin etc. ). This would help the size of the unit as well as in terms of heat.

Question: Would the extending the cable length cause any problems - I am talking about 3m / 9feet in total length.

Thanks in advance for any input and suggestions.

Mike

[Ice Czar]

not if you also increased the diameter of the wire
the longer the wires the greater the resistance (leading to voltage drop among other thngs)
you offset that by dropping the AWG (increasing the diameter) and you can also increase the baseline voltage (using the pots)
personally Id use 16AWG or 14 AWG instead of the normal 18AWG, and then do a test to see what kind of voltage drop there is (with a multimeter)
its also to a certain extent dependent on temperature
if the voltage drop is too great to compensate with the pots youll need to make it shorter
review > Extention Cord Basics (http://www.taunton.com/finehomebuilding/pages/h00010.asp)

[clocker2]

I ran my SFF Ideq for several months with an external 350w PSU and had no issues, but then again, I did not have to lengthen any cables.

3M/9ft.????
Is the power supply even going to be in the same room as the case?
That seems excessively long for an external supply.

[mikeguava]

Hi

Thanks for the replies

you offset that by dropping the AWG (increasing the diameter) and you can also increase the baseline voltage (using the pots)
personally Id use 16AWG or 14 AWG instead of the normal 18AWG,

Thanks for the tip - I think I will try some 16AWG first - this is going to be a very thick cable to start with...
I assume having small bit of 18AWG in the beginning and end of the cable will not make any difference.

Do you have any recommendation for a good PSU with easy accessible pots?

I am trying to power a mini itx mobo ( 6.9" x6.9" 915G) with a 3.8GHZ p4 2 WD Raptor Sata drives and an ATI X850 PE.

Maybe I should to go higher than 400watts...

[QUOTE=clocker2]
I ran my SFF Ideq for several months with an external 350w PSU and had no issues, but then again, I did not have to lengthen any cables.

3M/9ft.????
Is the power supply even going to be in the same room as the case?
That seems excessively long for an external supply.[QUOTE=clocker2]

yep it does seem a bit excessive - maybe I'll go 7feet - the thing is I want to have the PSU on the floor - away for the computer to avoid excessive noise and heat - apart from wanting to build a miniature system. Mini system with some punch...


CHeers

Mike

[sangram]

Why not go dual supply route? Use one for the main mobo power, use the second one for the P4 +12 volt connector and the PE's Molex connector, all the drives, etc.? May be a better route than taking the whole thing external, you will avoid the hassle of losing voltage in a long wire.

At times the 12 Volt rails are pushing 20 amps, maybe more. Even if your wire + connector resistance is .01 ohms, you're losing what - 0.2 volts? And we get scared when voltages drop more that .02 volts... And imagine the 5V line, that gets scarier....

[mikeguava]

Part of the problem in me wanting to go external is size and heat - the case is going to be 8x8x8 inches. I could put 2 dc2dc boards in there which are 200Watts - conected to two external "laptop type" Ac to dc power supplies. I am not too sure about those boards - in terms of how stable the current from the dc2dc boards will be. Having two SFX poawer supplies will we huge - they still are 5x3.5x2.5 inches and would take up most of the space -well actually all the space.

I was initally hoping to put a server type 1U Power supply which are around 7.75x4x1.5 - nice and flat - but then comes the heating issue - and they still take up a lot of space.

Mike

[sangram]

If so I would recommend you figure out how thick the wire needs to be. not advisable to just add wire and splice it into existing, would recommend you make up a completely fresh connector. Soldering wires of different guages together will create more joints = more voltage drops = unstable systems.

8x8x8 still gives you space to put in two SFX with their butts stuck together. You'll lose a lot of space though, as the resulting size will almost be as big as one ATX supply.

I like your idea of the two boards, with an outboard voltage source. If the boards are for lappys, they'll be stable, more so than PC supplies, the regulation will be tighter, etc. I don't know of their load carrying ability but since laptops switch power modes ever so often I believe that the laptop boards will be more stable than the desktop supplies, not less.

[mikeguava]

Thanks for your input

Do you really think that soldering joints create voltage drops? These joints are actually being thicker than the standard cable - why would there be a drop? Are you speaking with experience or is this an assumption? If so I really need to make sure how to go about this

The problem is to connect heavier gauge wire to the connecting pins - they won't fit. There might not be a way around it.

ANy PSU in a 8x8x8 cube would create a lot of space problems ( it actually is precisely 7.5x7.5x7.5 - just said 8 for the ease)
- if you were to look at it in person you'd realized that it really wouldn't work to put in 2 sfx PSUs - not if your are planning to have anything besides the PSUs inside the box.

Even a single SFX would create massive space problems. A standard CD-ROM drive would not fit in the cube that I designed. I had to get a laptop slot drive in order to have a rom drive. A cooler is a must for a P4 ....space I have not...

Yes the external DC2DC would be sweet - again looking at the boards ( http://www.mini-itx.com/store/?c=10#p1808 ) I have not the confidence that they can really power my system. T
hey do not come out of a laptop. Laptops don't need the massive power that my set-up will draw.I will be running a P4 3.8 GHZ, 2 WD raptors and a Pioneer DL DVD burner as well as a ATI Radeon 850 PE Garphics card.
I don not want to mess with unstable current. I haven't looked into Laptop powersupplies yet - maybe some of the high-end Dell XPS laptops have some interesting parts - If I could find them.

But then if I am having to run power off two Ac/DC external power supplies - I might as well have a single regular PSU. WOuld be a lot cheaper and straight forward. Just have to build a nice case around that.

Cheers

Mike

[sangram]

Hang the PSU outside the case!

I just saw someone dremel his PS opening so it was just large enough for the PS to slide through, then added a plate behind the ps to stop it from falling off, and bingo - you get the full space of the inside of the cabinet, and yet have a mini PC.

Unfortunately, it'll look like the thing from that movie - "The basket case".

OK solder joints have a resistance, but that's not all. The resistance changes with mechanical and thermal loading, and there can be resistance spike of over 300 ohms in some cases. There is a definite relationship between the mechanical stress (which would be more in lengths of flexing cable than on fixed PCBs, which is why every high-stress connection uses crimping, not solder - ATX cables, Molex, Automotive) applied to the joint and its resistance.

As a starter, try this file

perneg.home.cern.ch/perneg/cable_rev/baseline_voltagedrop_02Feb21.xls

username password are "anonymous" as far as I could tell.

Seems like resistance is a real problem in the real world, which you may or may not have a problem with in your application.

I am aware that the larger cable may not crimp well on the ATX connector. Then you can try what we in the audio world refer to as 'umbilicals', which are huge diameter wire (2-guage or better) to carry the main bulk of the current and then redistribute using the smaller wires, river-tributary style. you'll have to mod the PS, and one umbilical per voltage should be enough.

Only way to tell is to figure it out empirically. Solder first, and then figure out if all's well. btw that is one hell of a config for a mini-PC

[mikeguava]

The basket case - :D funny!

I love the "umbilicals" Idea very much. This sounds like something I'll try to accomplish. And have the distribution inside the computer case.

Thanks a lot for your input

[Ice Czar]

I think the umbilicals idea is the way to go
with a port you can disconnect as needed
would be rather tirsome to have a 7 > 9 foot cable attached to the box when you need to work on it or transport it

you overcome the length resistence through the larger guage to the box and then distribute it as pointed out, adding additional interfaces causes some resistance but not all that much
the figure generally accepted for a 4 pin molex is along the order of 0.01 ohm