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 Post subject: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Sat May 11, 2013 8:20 pm 
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I'm looking at doing a wiring project which would connect a SMPS in series to a PicoPSU and PCI-E connector in parallel. The idea is essentially to retain all the benefits of the PicoPSU (less wires, more efficient) while at the same time, allowing a single PCI-E connector for a 150W GPU.

In theory, the idea should work without being too taxing on the power system. The SMPS supplies 12V 25A at a single screw terminal output. My idea is to use copper lugs to run 5 18awg wires off the +12v terminal and 5 18awg wires off the ground terminal. This would be 10 wires total which would fill all the pins on both the PicoPSU connector and the PCI-E.

I guesstimated my total load to be about 225W and the SMPS is rated for 300W, or approximately 30% higher - which is exactly what the manufacturer recommends so as not to be too taxing.

Questions: Is there any current loss by running all these wires in parallel?

Remote sense- The SMPS has both DC output connectors and signal connectors. Remote sense ships with the "local sense" jumper, but if I use RS to increase efficiency, should I wire just one sense to the PicoPSU? Or should I also wire remote sense in parallel? I'm sure one RS would be good, but how about 2?

Finally, 5VSB: there is a 5VSB header pin on the SMPS. I'm wondering what I need to do with this, if anything. My thoughts were that the PicoPSU should supply +5VSB, but should I direct wire the 5VSB from the SMPS to the PicoPSU ATX connector pin? I just don't grasp how these things interface...


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Sun May 12, 2013 11:03 am 
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I'm guessing that it might make more sense to connect the remote sense wire to the PCIe connector because the motherboard probably does not draw as much 12v current as the video card and the picopsu is a dc-dc switcher, the 5v and 3.3v lines are going to be regulated the same unless your 12v line droops a whole lot.

Both points should be tied together through the motherboard so if there is more ir drop between the PSU and the video card than between the PSU and pico and you sense at the pico, your video card will have a slightly low 12v or maybe cause more current to flow through the motherboard than ideal.

Another option some people might have is a volt meter probe point. I have a motherboard with a special header for testing voltage. I still think the PCIe is probably the best sense point but its an option for some.

Running multiple sense lines is not the way things are supposed to work, the slightly different voltages at each point will cause current to flow through your sense line. That's a no-no.

For the 5v, probably just leave it alone or maybe use it to power an active USB hub.


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Sun May 12, 2013 3:45 pm 
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Hmm, I like your idea about the 5VSB, that would be pretty sweet to wire it to an active USB hub! I could definitely use it, because I've got at least 4, if not more USB devices. Thanks!

As far as remote sense, it sure is a useful function. However I'm not so sure it should be wired to the GPU since my GPU is 150W. In theory, 75W are loaded on the PCI-E side, and 75W loads on the PCI-E bus. That means with a 65W CPU, 75W on the PCI-E bus, and some 5-10W on peripherals there's a total 150W load on the PicoPSU whish is twice the PCI-E, so better to wire RS on the higher load no?

That's all I can think of. I suppose if I were to run a 225W GPU, then it would be better to RS the GPU. I really don't know in this specific circumstance and honestly there's zero information on the Internet about doing it in this way. My gut sense is to put RS on the larger load but I could be wrong. Hmm


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Sun May 12, 2013 9:53 pm 
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I would be really surprised if your GPU was broken up in to two separate circuits that each draw 75 watts.

My assumption is that the PCIe connector ground and 12v lines are electrically connected to the ground and 12v traces on the card edge connectors. If the PCIe cable has less resistance than going through the ATX12v connector and then through several inches of motherboard trace, then most of the current flowing to the video card is going to go through the PCIe cable which brings me back to sensing current there.

I didn't explicitly state that earlier but that was part of my reasoning.


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Sun May 12, 2013 11:17 pm 
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Xbitlabs used to have really good power consumption figures with distribution across connectors, but not any more. The latest I found was for a GTX 580. It draws up to 2 Amps from PCIe slot and the rest from the PCIe power connectors.

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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Mon May 13, 2013 11:46 am 
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QUIET! wrote:
I would be really surprised if your GPU was broken up in to two separate circuits that each draw 75 watts.

My assumption is that the PCIe connector ground and 12v lines are electrically connected to the ground and 12v traces on the card edge connectors. If the PCIe cable has less resistance than going through the ATX12v connector and then through several inches of motherboard trace, then most of the current flowing to the video card is going to go through the PCIe cable which brings me back to sensing current there.

I didn't explicitly state that earlier but that was part of my reasoning.



The PCI-E connector and edge connectors are connected to the same ground, thus, in theory current will flow in the path of least resistance - through the PCI-E connector. Good observation!

But some current *does* flow through the bus connectors. I was doing some research and I found this:

http://www.xbitlabs.com/articles/graphi ... html#sect0

Xbitlabs used a special setup to measure current draw on the PSU rails: 3.3v, 12v, and PCI-E 6 pin/8 pin. With a full load on Crucis Warhead, a 5750 draws only 1.4A on the 12v, and 3.3A on the PCI-E. But on a 5770 the 12V draws significantly more, 2.3A vs 3.9.

So if you calculate the CPU draw on the PicoPSU, in total there will be more current flowing to the PicoPSU then the 6 pin pci-e. Not by a great amount but certainly more.

What do you think about that? If it was your system and you wanted to prevent voltage drop, would you still put RS on the PCI-E side? Of course, I suppose there's also the additional Vdroop function on the motherboard itself, but still.


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Mon May 13, 2013 2:31 pm 
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Well mostly I think any GPU that draws more power through the slot than through the PCIe 12v plug has a very poorly designed circuit board (more resistance between the PCIe 12v connector and the load than through the card edge, motherboard traces and the ATX 24 pin connector).

It looked like the worst case card drew 4.5 amps through the slot, if yours was equally as bad, that still leaves 8 amps for the PCIe cable. If its like some of the other cards and draws maybe 1.5 amps from the slot, that leaves 11 amps for the cable.

If its 11 amps, that's more than half of your 225 watts and very likely to be the point where your 12 v rail sags the most which is why I think you want to sense there.

But that's just a slightly educated guess, it would probably be a good idea to test several 12v points under load to find where it sags the most and use that point.


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Tue Jun 11, 2013 11:38 am 
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I have actually done this, I'm powering an undervolted HD6870 va 2x PCIe power connectors & the motherboard & my monitor all from a single 200W 12V Brick type supply. (actual load when stressing everything was 15.5A - leaving only about 0.5A of headroom...)

Before you do this, measure the resistance between the +12V pins of the PCIe power connector(s) and the +12V pins on the ATX 24 pin connector. If there is a low resistance (<1Ohm), it will not work, unless you switch the graphics card power with a relay fed from one of the molex outputs. (When the PicoPSU is off, 12V will be fed to the mobo via the PCIe slot connector, so it never turns off properly!)

In my case, there is no connection, so the PC does turn off. if you like overclocking (and get graphics crashes etc,) remember to power off the card manually, as otherwise the memory holding the clockspeeds & voltages does not reset. Again, a relay to switch the graphics power would be useful here...

As for the sense, I would just connect it to the output of the PSU. a few mV (or 100mV) drop down the cables won't affect anything! I've had mine running as low as 11V and no problems...


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 Post subject: Re: Wiring a PicoPSU in parallel with PCI-E?
PostPosted: Wed Jan 22, 2014 10:20 pm 
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Joined: Wed Jan 15, 2014 2:14 am
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What capacity of SMPS will be required for 946 gzis, Pentium D 2.8 pro also which brand SMPS must I go for ? I have 450 watts SMPS in my present configuration is not able to run any more peripherals other than one hard disk. Whats the difference between the local SMPS which cost only 600 Rs, Which branded one will be best for future use. what wattage ?


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