MP 1,1-5,1 Pixlas Mod and (6+8) GPU connection.

[Noobraino]

Hey guys,

I did the Pixlas Mod on a cMP with an open end to 2 x (6+2) pins connectors. I know it is kind of the standard and the connection on the Power Supply should look like this

But if i want to do the Pixlas Mod again on a new Power Supply but using a open end to a (6+8) connectors since i’ll have only 3 grounds instead of 5 is there something very specific i should do in terms of the connection order or it is pretty much straight forward, 12v on any yellow and grounds on any black?

Thanks so much.

 

[sp1tf1r3]

What did you end up doing ?

From an electrical standpoint, this PSU has only one 12V and one GND. There are multiple cables, because doing so allows to spread the current, each cable not exceeding 240VAC (12V @ 20A). On the backplane connector's end, each pair of 12V and GND is merged into 1.

The large cables (12V and GND) are made of 12AWG, so what you want is not exceed the current they should normally carry, including what the backplane is pulling, and what the PSU can provide.

One 6-pins is 75W / 2 cables = 3.125A, one 8-pins is 150W / 3 cables, so a max of 50W / 12 V = 4.16A per cable if the gfx card plays by the rules.

If you already did 2x (6+2), tapping into 3 of the 4 12V, going (6+8) is not much different. To spread the current, you should have at most 2 (not 3!) of your open ends tapping into any one big 12V (same with GND), giving at least 11A (= 20A - 2x4.16A) of headspace per big 12V cable for whatever the backplane needs.

Are you using the internal PCIe aux power at the same time ?

(I went with the variant of soldering into the connector with one common 12V and GND, no need to think about what goes into which cable, only about the total load. Do you want more details about that ?)

 

[mikas]

Had some spare time, or let's just say I took some spare time for my hobby.
Most of you know what that is in the pic.

It's a dual Pixlas mod.
I have tried to google about AWG specs for what ever situation, temprature, wire and use cases. It's a mess I think. BUt here is a good starting point, reliably looking numbers by user sp1tf1r3, thank you.

There is no good escape from tapping three times a GND wire in a dual Pixlas, is it?

What do you people think of that. I think I have seen this done by some people already, but didn't catch the calculations or logic entirely. I'll have to search those threads/users again.

And yes, I thought about doing a "Trixlas" too for a good moment or two (Trixlas= 3x Pixlas). But did not put it through - at least not yet. I have to study and try Martin Lo´s Radeon VII downvolt strategies first. And I have to carefully think about safety, and if there is any point in doing it for me. I do own 3 pcs Radeon VIIs at the moment.

There is still room for 8 more taps easily. Ok not easily, but they will fit if needed to. No way trying to put the SuperDrive shield plate back in there, and maybe not the drive itself either. But I wouldn't mind about that, no need for optical discs anymore for me.

I am quite confident I am not gonna see smoke with this. But what about the Triple Pixlas, could I possibly be hitting the limits of power supply cables somehow. I guess that is possible. We don't know which of those cables feeds what, which components draw amperage from where in there.

I know the total power supply should be enough for two VIIs, but there can be, and probably always is, a weak link somewhere in the chain.

I'll get back when the next Mac Pro is up and ready for tests. I'd like to test those 3 VIIs after downvolting and limiting the powerdraw with help of powertable tunes (like by h9826790 has adviced).

Just squeezing the last out of these venerable tanks before retirement (or more sadly, before obsolesence).

 

[rx78]

Had some spare time, or let's just say I took some spare time for my hobby.
Most of you know what that is in the pic.
View attachment 1731020
It's a dual Pixlas mod.
I have tried to google about AWG specs for what ever situation, temprature, wire and use cases. It's a mess I think. BUt here is a good starting point, reliably looking numbers by user sp1tf1r3, thank you.

There is no good escape from tapping three times a GND wire in a dual Pixlas, is it?

What do you people think of that. I think I have seen this done by some people already, but didn't catch the calculations or logic entirely. I'll have to search those threads/users again.

And yes, I thought about doing a "Trixlas" too for a good moment or two (Trixlas= 3x Pixlas). But did not put it through - at least not yet. I have to study and try Martin Lo´s Radeon VII downvolt strategies first. And I have to carefully think about safety, and if there is any point in doing it for me. I do own 3 pcs Radeon VIIs at the moment.

View attachment 1731026

There is still room for 8 more taps easily. Ok not easily, but they will fit if needed to. No way trying to put the SuperDrive shield plate back in there, and maybe not the drive itself either. But I wouldn't mind about that, no need for optical discs anymore for me.

I am quite confident I am not gonna see smoke with this. But what about the Triple Pixlas, could I possibly be hitting the limits of power supply cables somehow. I guess that is possible. We don't know which of those cables feeds what, which components draw amperage from where in there.

I know the total power supply should be enough for two VIIs, but there can be, and probably always is, a weak link somewhere in the chain.

I'll get back when the next Mac Pro is up and ready for tests. I'd like to test those 3 VIIs after downvolting and limiting the powerdraw with help of powertable tunes (like by h9826790 has adviced).

Just squeezing the last out of these venerable tanks before retirement (or more sadly, before obsolesence).

Wow that looks so good. How will you mount the second pixlas and get it to power on with the first?

Or am I missing something?

 

[mikas]

There is no need to switch anything or to power on anything but the computer with Pixlas mod.

I'm going to install two Moddiy (2x8pin open ended, sleeved) cables. When it's all assembled and ready, to what ever the 8-pin connectors are attached to, the power will be drawn at demand. As long as the PSU can provide all the juice, that is. If the cables are de-attached, no power will be drawn from those connectors. The other one of the cables is 70 cm and the other is 80 cm of lenght, to better match the clip/tap locations - or spade terminals, what are these called again (please see pic, tools and equipment, the difference in cable lenght).

The reason I use two of those cables, and not just splitting the first one with a splitter adapter, is because I want to spread the amperage evenly to those thinner cables. If I did a split at the end of that one cable, there would be double the amperage at the first cable of the power draw chain. And it would mean more stress to that part. It might be ok for the cables, but I want to be a little bit more on the safer side of it.

The only tricky part in my opinion are the PSU screws or bolts. They are secured with bolt glue or something, and they are really tight. I broke one bolt head taking the PSU out, finally had to force it out and ruined the bolt head in the making. It was only this one PSU though, the other one was easier to work with.

Besides that, I feel that every other aspect of this mod is manageable for almost to everyone with at least some degree of hand skills AND right tools for the job, as always.

And Thanks again to everyone sharing their knowledge and tips over the years. I really appreaciate it.

 

[rx78]

I think i completely misunderstood what you were doing!

I thought you were going to have two PSUs in one Mac, but you're actually doing two mods on one PSU.

 

[mikas]

Exactly.

I do have a spare Seasonic 600W fanless power supply, and bought a couple of switch buttons and jumper cables too, to put it in one of my Mac Pros. I did not ever realize my project with that. I think it would have fitted in with some fidling and right cables. It just seemed too tight in there for the PSU to live there long enough, at least to my eyes. Thought about thermals, fan speeds, fittings and everything there is to it, and decided to let it go. Then grabbed the Pixlas method, it's more to my taste.

 

[rx78]

Exactly.

I do have a spare Seasonic 600W fanless power supply, and bought a couple of switch buttons and jumper cables too, to put it in one of my Mac Pros. I did not ever realize my project with that. I think it would have fitted in with some fidling and right cables. It just seemed too tight in there for the PSU to live there long enough, at least to my eyes. Thought about thermals, fan speeds, fittings and everything there is to it, and decided to let it go. Then grabbed the Pixlas method, it's more to my taste.

View attachment 1731682

That Seasonic looks right at home in that bay. I've been powering my Vega 56 using a Corsair PSU but I've got no where to put it so it stays sitting on top of the Mac.

 

[mikas]

You are right, looks like it was designed to fit in there. Or the other way around, don't know.
I used to power Vega 56 in low bios mode from 2x6-pins+2xsata power, from feb 2020 if I remember it right. So I still am actually, in one of my Mac Pros still. It's working just fine, no problems whatsoever. But Pixlas is coming this spring there too. And one of the VIIs accompanying it I guess.