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2016-04-20, 13:32   #12
Mark Rose

"/X\(‘-‘)/X\"
Jan 2013

B6B16 Posts

Quote:
 Originally Posted by bgbeuning Those use 18 gauge wire which I would recommend limiting to under 10 A current. (Wikipedia says 14 A heats AWG 18 wire to 90 C.) 300 W at 12 V is 25 A so the PSU wire needs AWG 12 or 14. And it would be having 12.5 A on the first Y cable. The above is the worst case. The load is probably spread over multiple wires. If none of the wires feel hot to the touch, it should be safe. A "loop" current meter will let you check each wire.
There are three 12 V wires, one in the ATX-24 connector, and two in the ATX-4 connector. Each circuit in the ATX-24 connector is supposed to be rated for 6 amps, and each in the ATX-4 connector 8 amps.

http://www.playtool.com/pages/psucon...onnectors.html

Four motherboards on a single power supply is probably very close to a limit on one or more of the circuits.

2016-04-22, 05:25   #13
Prime95
P90 years forever!

Aug 2002
Yeehaw, FL

22·3·641 Posts

Quote:
 Originally Posted by Mark Rose There are three 12 V wires, one in the ATX-24 connector, and two in the ATX-4 connector. Each circuit in the ATX-24 connector is supposed to be rated for 6 amps, and each in the ATX-4 connector 8 amps. http://www.playtool.com/pages/psucon...onnectors.html Four motherboards on a single power supply is probably very close to a limit on one or more of the circuits.

There are two 12V wires in ATX-24 connector and two wires in the ATX-4. Do we have any idea how well balanced most motherboards are in drawing from those 4 wires? I don't own a current meter.

My Skylakes are drawing 57W at the wall. PSU is 92% efficient so each board is consuming 53W. If all of that is 12V, that's 4.5A. My thought is to put one ATX-4 splitter on each plug of the EPS12V4+4 connector. This will halve the load on each of those lines. Since those are rated for 8A they will definitely not be a problem. The only question is the ATX-24 cable where the 12V lines are rated for just 6A. Do we think the motherboard will draw more than 1.5A from either of those 12V lines?

 2016-04-22, 10:42 #14 henryzz Just call me Henry     "David" Sep 2007 Cambridge (GMT/BST) 13·457 Posts My dad's old Athlon II x4 was supposed to need a ATX-8 connector in the motherboard but would run with just a ATX-4 connector plugged in. When he got to the point where he needed to replace the power supply he discovered that the plastic on the connector/motherboard had melted. This led to a new pc. I would be careful with this sort of thing. http://www.playtool.com/pages/psucon...onnectors.html was an interesting read. The Athlon II 620 x4 was a power hungry system though with a TDP of 95W while yours are lower. It looks to me that based upon the link above you should be fine with the 20+4 splitter as you are not using graphics cards which need 12V power. It sounds like the two lines are usually connected so the total is more concern than the individual. Will your pc boot with just 20 pins? If so you should be fine.
2016-04-22, 11:47   #15
bgbeuning

Dec 2014

FF16 Posts

Quote:
 Originally Posted by Prime95 Do we have any idea how well balanced most motherboards are in drawing from those 4 wires? I don't own a current meter. Do we think the motherboard will draw more than 1.5A from either of those 12V lines?
My loop current meter only seems to work on AC power.

2016-04-22, 13:35   #16
Xyzzy

Aug 2002

836410 Posts

Quote:
 Originally Posted by bgbeuning My loop current meter only seems to work on AC power.
https://en.wikipedia.org/wiki/Current_clamp

2016-04-22, 14:40   #17
bgbeuning

Dec 2014

FF16 Posts

Quote:
 Originally Posted by Mark Rose
The link for ATX-24 shows the +5 V (5 wires) and +3.3 V (4 wires) rails can have twice the current as the +12 V (2 wires) rail. But since the +5 and +3.3 have twice as many wires, that is OK.

If the 60 W is split fairly evenly over all 13 wires (11 in ATX-24 and 2 in ATX-4),
then the Y cables seem pretty safe. But like George said, the evenly part is the big question.

2016-04-22, 19:02   #18
chalsall
If I May

"Chris Halsall"
Sep 2002

274416 Posts

Quote:
 Originally Posted by bgbeuning My loop current meter only seems to work on AC power.
Further to Mike's link... Yes, loop current meters only work with AC.

This is a fundamental truth based on physics; the "flux" generated by alternating current allows the measurement.

It is possible to measure the voltage of both AC and DC circuits by placing probes appropriately, and can be "out of circuit" (read: "parallel").

It is, however, only with AC circuits that it is possible to measure the current externally.

To measure the current of a DC circuit one must be "in circuit" (read: "in series").

(There are actually more complicated ways of measuring DC current "out of circuit", but they're non-trivial, and not available retail.)

Last fiddled with by chalsall on 2016-04-22 at 19:10 Reason: English is such a subtle language....

2016-04-22, 19:58   #19
sdbardwick

Aug 2002
North San Diego County

10101101112 Posts

Quote:
 Originally Posted by chalsall Further to Mike's link... Yes, loop current meters only work with AC.
Some clamp current meters can work with DC. See from Mike's link https://en.wikipedia.org/wiki/Current_clamp#Hall_effect
and Fluke model 365 (around $250 USD) for example. 2016-04-22, 21:21 #20 chalsall If I May "Chris Halsall" Sep 2002 Barbados 274416 Posts Quote:  Originally Posted by sdbardwick Some clamp current meters can work with DC. See from Mike's link https://en.wikipedia.org/wiki/Current_clamp#Hall_effect and Fluke model 365 (around$250 USD) for example.
I am aware of that.

Are you aware that such measuring devices are highly inaccurate?

2016-04-22, 23:00   #21
sdbardwick

Aug 2002
North San Diego County

5·139 Posts

Quote:
 Originally Posted by chalsall I am aware of that. Are you aware that such measuring devices are highly inaccurate?
Depends on the context. Accurate enough for field work (diagnostics), but you wouldn't use one as an engineering tool. They are much better than they used to be - cheap DSP helps a great deal.
IIRC, non-specialized Fluke clamps (Hall or otherwise) are good for about 2% full range accuracy. Resolution is not great @ around 0.1A

2016-04-23, 21:41   #22
Serpentine Vermin Jar

Jul 2014

31·107 Posts

Quote:
 Originally Posted by chalsall To measure the current of a DC circuit one must be "in circuit" (read: "in series"). (There are actually more complicated ways of measuring DC current "out of circuit", but they're non-trivial, and not available retail.)
I'm going to agree with chalsall on measuring DC current. You really need to measure in series (directly for low amp or use a shunt for larger amperages).

Anything else is really no better than an educated guess. It might be close enough, so if that's good enough for whatever you need, then knock yourself out, but I wouldn't use it for anything where accuracy matters.

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