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How far can you tap the Mac Pro's power supply?

TheStrudel

TheStrudel

macrumors 65816
Original poster
Jan 5, 2008
1,134
1
It has no doubt come to the attention of many that the Radeon 4870 requires two PCIe six-pin power taps off of the motherboard, which is exactly how many  allots. Say we wanted to use a GeForce 8800GT and a 4870 at the same time, which would require a total of three PCIe six-pin power taps. What I've read on the forums here indicates that powering them would be possible, but what ways would work?

As far as I can see, I've got two options:

1) Split one of the 6-pin power taps with a Y-splitter, and use the resulting two taps for one card and the other tap for the other card.

2) Use one of the ODD 4-pin molex power taps, convert it to a 6-pin tap, run it down to the card. But I'm running two optical drives; would it be possible to split one of those power taps into two, then run one out converted to a 6-pin and into the card?

I don't know nearly enough about the motherboards and power sources to know which way would work - if any - and if there is any risk involved. I'm running an early '08 mac pro, but I'm curious if this would work in other models as well. Anybody know enough about the hardware to say?
 
Cool, thanks for testing it as far as you have - let me know when your 4870 comes and you get it revved up, I'm curious to see what that'll be like. I guess that means it'll be safe to tap out from optical and run it down. I'm curious about just splitting PCIe taps, though, just because someone else suggested it. Any clue whether or not the logic board or PSU can support that?
 
I wouldn't do it, personally...

The 4870 is rather power-hungry, which made me very reluctant to consider splitting the PCI Express power connectors to support another card at the same time, even the low-ish power 3870. Given the max draws of all of the devices in question, the safest place to borrow the power from really seemed like the optical bay. Plus it was an excuse to do a fairly unusual mod. :cool:

Note: in order to squeeze the connector through with the other wires from the optical bay area, I did have to partially unscrew the SATA 1 connector and pull it forward a bit to make enough room. There's plenty of room for the wires, but those fat connectors needed a little extra room.
 
I wouldn't recommend attempting to split the 6 pin PCIe power connectors to run a 4870 and an 8800. Almost certainly too much current draw for the traces on the logic board to handle (say 225 - 250W, or just over 20A). You could actually burn them out. :eek: :(
 
voyagerd said:
So I guess the only option is to get a second PSU.. fun..
Click to expand...

If the 8800 is a secondary card, the power draw would probably not exceed what could be provided from the optical bay split, especially if the 3D were not used concurrently with burning, which is probably an unlikely combination anyway.
 
KevinPlusPlus said:
If the 8800 is a secondary card, the power draw would probably not exceed what could be provided from the optical bay split, especially if the 3D were not used concurrently with burning, which is probably an unlikely combination anyway.
Click to expand...
It's quite possible, but I've never seen anything on the # of 12V rails or their current ratings. Nor how they're used inside the MP (any model). No way to know how the loads were divided to which rail.

The only thing I can think of, would be to get into it with a meter, and load test. :(

I do wonder, as it's possible both optical bays and all 4 HDD bays run off a single rail. (If so, figure ~5A, but without the details, it's possible to overload it and fry the PSU). :confused: Details Apple, details. :p
 
KevinPlusPlus said:
If the 8800 is a secondary card, the power draw would probably not exceed what could be provided from the optical bay split, especially if the 3D were not used concurrently with burning, which is probably an unlikely combination anyway.
Click to expand...

If I got a secondary card, it would be something faster than the 4870, which would probably draw equal or more power than it.
 
Your secondary card would be faster than your primary card? Odd choice. Care to elaborate? Is it for windows, perhaps?
 
Most modern PSUs can manage multiple GPUs, it would be almost insulting if the one in the mac pro cannot
 
Aegelward said:
Most modern PSUs can manage multiple GPUs, it would be almost insulting if the one in the mac pro cannot
Click to expand...

It's not just a matter of the total power draw, but the power draw on each individual rail. It's just like in your house - you maybe can put a laser printer on each room/circuit and print with all of them at the same time, but if you plugged all 8 into one outlet with a power strip you would surely trip the breaker in a hurry.

Likewise, if you split any one power rail inside the computer too much, you run the risk of over-drawing, which could cause instability at best and a meltdown of sorts at worst.

It sure would be nice of Apple to include more than 2 PCI Express power connectors in the future, although I'm not holding my breath.
 
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