Passive computer power supply questions - Page 1
Forum section: electronics / general

author: RichardF
author_rank: Contributor
timestamp: December 11, 2014, 06:10:36 pm
content: 1) Is it possible, with the current available equipment and without significant DIY (like soldering), to use two passive power supplies in a personal computer — without there being serious drawbacks? Don't worry about cost, unless we're talking about one of those super-expensive linear supplies marketed for pro audio.2) Is it possible to avoid any potential cross-contamination issues (problems related to chaining two power supplies together) by forcing a graphics card to draw all of its power from PCIe power connectors (none from the slot), in order to use one power supply just for powering graphics cards? If it's not possible, why isn't this something the industry has thought of?3) Will someone here make either a double-sized passive switching PSU for computer use (given that a single ATX passive PSU can be found in up to 520 watts), so enthusiasts can get 750-850 watts out of a passive model — along with a compatible case?This device,AddPSU, has good ratings on Amazon.Please don't tell me to just get a PSU with a fan on it. They all make too much noise when they're loaded enough. The closest thing to a quiet one is the Corsair RM 850 and it has questionable caps and cooling. It seems to me that the best way to enable a passive (or a PSU with an extremely slow-moving fan) high wattage power supply is to either use two of the fanless units or have a double-wide unit. Without so much surface area than ATX provides I don't see how it can be possible to extract more watts out of a passive or nearly passive unit.A person with a bad attitude responded with this. I am wondering if there is more to it than what he wrote which is why I'm asking here:Quote from: grouchCan someone kindly explain to me if it's truly unwise to use two passive power supplies, and why. Also, if it were possible (assuming it isn't) to power just graphics cards with one power supply, would that eliminate the main problems — since they're caused by connecting two power supplies together? Thanks!

author: akis
author_rank: Frequent Contributor
timestamp: December 13, 2014, 12:29:14 am
content: If I remember correctly modern PSUs have large openings with very large fans almost covering the whole bottom area of the PSU and they turn really slow not making any noise. For sure the GPU and the the CPU fans will make much more noise. The GPU will have two or three onboard fans 1/4 of the size of the PSU fan and probaly 10 times the loudness.Regarding putting two PSUs together I have never thought about it and I am not sure how you would do it. For certain it might be like paralleling batteries: unless the voltages are 100% the same down to mV, then one power source will "play" into the other, meaning you will have large/huge/catastrophic currents flowing from one PSU into the other. If you plug one PSU into the GPU there is also the PCI connector that carries power lines, that is how the GPU card gets power even if you do not use its dedicated power connectors. So again, by plugging the extra PSU into the graphics card , if the voltages are different even slighlty, it might create large currents through the common connection, the PCI connector, and again I imagine blow-up.Unless there are specific PSUs which are designed to be paralleled by the manufacturer, I would not try to put them together.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 13, 2014, 12:44:17 am
content: What is a "passive power supply"? A power supply is ACTIVE, by definition.I rather agree with Bad Attitude. The gear was not designed to do what you want to do, and you could buy a whole new computer for less $$$ and trouble than trying your proposed hack.Daughter boards are not designed to operate on "alternate" power schemes unless they specifically designed them to do that. And that is probably way to fiddly for 99.9999% of their customers.And you can say the same about mother boards and slots.

author: Fsck
author_rank: Super Contributor
timestamp: December 13, 2014, 12:54:53 am
content: I believe he means passively cooled when he says passive.there isn't a good solution without diy. the above are correct.

author: ajb
author_rank: Super Contributor
timestamp: December 13, 2014, 08:09:46 pm
content: If you really want to use multiple power supplies, the safest way to do it is probably to keep all of the core components (motherboard, add on cards) on one PSU and use a second to run your drives. By ensuring that both power supplies are never plugged into a single component you sidestep any concern about accidentally connecting PSU rails together through your video card or something.That said, I would think that the video cards that really need the extra current required by that PCIe Aux power connector probably wouldn't tie the PCIe Aux 12V directly to the 12V pins on the PCIe card slot, that seems like asking for trouble--Likewise for the motherboard Aux 12V connector and the main power connector. On the other hand, I've definitely seen mid-range motherboards and video cards that workwithoutthe aux power connectors plugged in, so I don't know if they do some fancy input switching or just don't bother trying to isolate the inputs from one another. High performance components that really need gobs of power probably have different power topologies from the mid-range components I typically use anyway.As a final note, how much power do youreallyneed from your power supply? "Enthusiasts" have been known to buy way more power supply than their components actually demand. A good sanity check would be to throw a Kill-A-Watt on your computer and see what it actually draws from the wall under load. The actual output of your power supply will be somewhere around 70-90% of that.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 08:39:44 pm
content: Quote from: ajb on December 13, 2014, 08:09:46 pmUnfortunately, the main reason to use two fanless power supplies is to use one to power the GPUs since fanless single PSUs max out at around 520 watts.Quote from: ajb on December 13, 2014, 08:09:46 pmI'm not sure about this, although some cards will function in 2D mode when one forgets to plug in the connectors, using the maximum of 75W from the slot. This page looks like a card will drawfrom both the slot and the connectorswhen gaming.Quote from: wikipediaQuote from: ajb on December 13, 2014, 08:09:46 pmEstimated power load is under 750 watts peak (Furmark + Prime) based on power draw figures from review sites such as Tom's Hardware. Overclocked 5930 and two overclocked 970s. Moreover, this is not just about my setup. Some people are overclocking the AMD FX which is abigpower hog. I was planning to use two external 1080mm radiators with four 180mm fans on each, or possibly eighteen 120mm fans, depending upon the cooling-noise ratio.The quietest fan PSU I could find is the Corsair RM 850 and it hits43 decibels of noise at 670 watts.Quote from: Richard CrowleyIt seems like there is an opening in the industry for GPUs that can be powered solely by PCIe connectors. There is a lot of specialization in enthusiast hardware, such as watercooling equipment. It seems trivial to design a card that just draws its power from the connectors.

author: extide
author_rank: Regular Contributor
timestamp: December 13, 2014, 08:44:45 pm
content: Why are you running a 5830 and Two 970's? You might be able to get away with two 970's and a LGA 1150/55/56 CPU on a single 520w fanless psu. Adding in an old radeon 5830 seems silly... Unless I am mistaken?

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2014, 08:44:58 pm
content: The best way to have a silent PC power supply is to use an external power brick of the kind laptops and docking stations use. My docking station at work has a giant power brick (can't remember the power output offhand, but it could be ~200 W or so).Since modern computer boards generally route high voltage (12 V+) power around and down convert it locally where needed, a power brick with a single high voltage DC output will work fine to power such systems.To make the whole thing work, however, requires the whole computer system to be designed from the ground up this way, with correct choice of main board, graphics system, etc. You can't just unplug a regular ATX power supply and replace it with a laptop brick.In short, silent computers have to be designed as silent computers from the ground up. To retro-fit an existing desktop system and make it quieter the options are more limited.

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2014, 08:46:15 pm
content: An example of a silent PC designed as such from the ground up is an iMac. No fans anywhere, completely noiseless.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 08:47:21 pm
content: Quote from: extide on December 13, 2014, 08:44:45 pmTypo. Sorry. 5930. It's the CPU. Intel has a 5820 so I assumed the next model up, the one with 40 PCIe lanes, would be a 5830. But, no... it's a "5930". lolQuote from: IanB on December 13, 2014, 08:46:15 pmThe goal here is to make a system with a high level of GPU performance that isn't noisy.

author: extide
author_rank: Regular Contributor
timestamp: December 13, 2014, 09:01:59 pm
content: Ah, right the 5930K -- yeah thats the LGA2011 platform. Might still be doable on a single 530 if you don't overclock anything. I never suggest using more than one PSU in a PC as the rails will never be exactly the same voltages and you could end up dissipating quite a bit of current through places that are not designed at all to do so. Maybe power the entire pc from one, except for the PCIe connectors on the video cards, and then the second psu powers those. You don't even need a PC PSU for the second one -- all you need is a hefty 12v power supply for the PCIe connectors. You can rig up a relay to turn the second one on/off at the same time as the first.

author: mariush
author_rank: Super Contributor
timestamp: December 13, 2014, 09:08:39 pm
content: Here's the problem dude.Probably 99% of the passive power supplies (the ones without fans) are designed to rely on the air around them moving through the power supply case to keep them cool. There has to be some difference in temperature between the outside air and the inside air for this passive cooling to work.You will find in the manual (or instructions) of these power supplies notes saying that if your system only has one processor fan (integrated graphics cards for example) you should install a case fan to move the air inside the case and basically "motivate" the air to move through the passively cooled power supply as well.The point is, you're not looking at the whole picture. You want to make your system silent by removing the fans of the power supplies, but the power produced by the power supply is consumed by some devices (video cards, processor) that will use fans to keep themselves cool and those fans will be quite noisy when you play games or something very gpu intensiveModern power supplies have fans that adjust their fan speeds with the power consumption. For example, my now years old Seasonic X-650 doesn't even spin the fan until the power consumption goes over 150-170w and then runs the fan at a very low comfortable speed up to about 300-350w where I can barely start to hear the fan.So when in idle (in windows, or when watching a movie), your computer should already be silent and the power supply shouldn't even spin its fan, if you get one that's of high quality. In games, you wouldn't care, because the psu fan will be overwhelmed by the gpu fans.There's no point using two passive psus.. If you want something really nice, you'd be better off buying a 850w-1000w power supply. Then remove the insides from the case and put them in a bigger case, enough to install a 200mm fan and use a temperature sensor connected directly on the heatsinks of the power supply to adjust the fan speed.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 09:08:53 pm
content: Quote from: extide on December 13, 2014, 09:01:59 pmI wonder where I can find one that's good quality. I'll do a Google search.Quote from: extide on December 13, 2014, 09:01:59 pmIsn't that what the AddPSU unit does?Quote from: extide on December 13, 2014, 09:01:59 pmPerhaps, as long as I avoid overclocked 3rd-party GPUs, which isn't easy. Tom's Hardware foundmuch higherpower consumption for a Gigabyte 970 than for a reference 980.Quote from: extide on December 13, 2014, 09:01:59 pmThat was my original idea, but since the GPUs also draw power from the slot I was concerned about problems.

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2014, 09:12:17 pm
content: Quote from: RichardF on December 13, 2014, 08:47:21 pmThat tends to define a high power demand from the system.Fans provide good cooling in small enclosures. Equal cooling can be provided without fans given a larger enclosure. What are your space constraints? Are you willing to repackage the system in a different enclosure with custom thermal engineering?One way to provide strong passive cooling for a component like a power supply is to remove it from its original enclosure and provide it with directed inlet vents, modified heat sinks and a tall "chimney" on top. The warm air rises up the chimney and draws cool air in at the bottom. With a tall enough chimney and good design you can get plenty of air flow without fans.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 09:14:26 pm
content: Quote from: mariush on December 13, 2014, 09:08:39 pmThe idea was to use case fans (although kept to a minimum for noise) to keep the PSUs cool as well as to cool the motherboard VRMs. The GPU and CPU fans aren't relevant to power supply heat dissipation since they are outside of the case via the two 1080mm radiators.Passive power supplies are sold and used successfully. Why it would be unthinkable to have two of them just because of cooling issues is beyond me. Certainly it would create more incentive to move the hot air out and it would take more space. A custom enclosure will most likely be necessary. In fact, I was thinking about having a custom enclosure built that would encompass the power supplies and the radiators, so I could have dust filtration on the latter.How slow fan speeds are at low wattages in fan-cooled PSUs isn't really central here, is it? The point is to have a quiet system when it's fully loaded.Quote from: mariush on December 13, 2014, 09:08:39 pmActually, I think you're not reading what I've written because you're talking about CPU and GPU fans inside cases and PSU loads at low wattages. None of those apply.

author: taylor12
author_rank: Contributor
timestamp: December 13, 2014, 09:16:44 pm
content: I have a Corsair AX860 and when i load my system down i cant hear it. i have a 4770k and a gtx780ti both overclocked. Just feeling he air rejected out of it it seems to not spin up much with the load.

author: Someone
author_rank: Super Contributor
timestamp: December 13, 2014, 09:18:03 pm
content: Quote from: IanB on December 13, 2014, 08:46:15 pmAll apple computers have fans in them still, but they only spin up when needed and are designed to be quiet. Many people believe their mac book air or pro or iMac is is fanless only because they never use intensive applications.Quote from: RichardF on December 11, 2014, 06:10:36 pmCheck the manuals for "fanless" power supplies, they almost universally require airflow from case fans as mariush mentions above. At low loads you can try and get away without the forced cooling but expect short lifespans for the computer. I have built several fanless computers over the years and the challenge is to reduce power consumption at every possible point.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 09:18:12 pm
content: Quote from: IanB on December 13, 2014, 09:12:17 pmThis is going to be a new build. I don't have space constraints, although I would like to have dust filtration. Spraying compressed air after disassembling fans from radiators is a pain to say the least. I am also thinking that a basic enclosure would also cut down (due to the dust filtering material) on high frequency noise some, such as coil whine.Quote from: IanB on December 13, 2014, 09:12:17 pmWouldn't the PSUs already be adequately cooled so that even a single 140mm case fan at low RPM would be enough to move their heat out as well as cool the motherboard VRMs?In fact, since having two PSUs doubles the cooling potential and cuts down on the load on each, it seems like they would both be less stressed.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 09:19:47 pm
content: Quote from: Someone on December 13, 2014, 09:18:03 pmI guess I wasn't clear, sorry. I was not expecting to build a completely fanless computer. I was instead trying to use two fanless power supplies to avoid the 40-50 decibel noise contribution from a fanned PSU.

author: ajb
author_rank: Super Contributor
timestamp: December 13, 2014, 09:21:32 pm
content: There are some pretty powerful single-output industrial power supplies that are passively cooled. Should be fairly straightforward to build a device with 3.3V, 5V, and 12V inputs that implements the required ATX power supply behaviors on the output. The whole setup wouldn't exactly fit in a standard PC case, though.

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2014, 09:22:34 pm
content: Quote from: RichardF on December 13, 2014, 09:18:12 pmI'm talking about removing the complication of trying to double up on two PSUs and just using one conventional PSU. The complication is moved from the electrical issues of doubling up to the thermal issues of case design and air flow.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 09:25:07 pm
content: Quote from: IanB on December 13, 2014, 09:22:34 pmI'm not going to do it if electrical issues will cause danger to the equipment. As for air flow, that doesn't concern me. I am not going to try to stuff passive PSUs into a form factor that prevents them from being adequately cooled. Whatever way I install them, air flow will be adequate.The main reason I posted this topic is to find out of it's feasible to use two power supplies without having dangerous to the equipment electrical incompatibilities. I don't have any background in electrical engineering so I figured I would ask people who do.

author: RichardF
author_rank: Contributor
timestamp: December 13, 2014, 09:35:51 pm
content: Quote from: taylor12 on December 13, 2014, 09:16:44 pm15 decibels at 380W. 42 at a bit over 400W. 50 dBA at 700W. 55 dBA at 800W.Louder than the RM 850 and not quiet unless you stick with low wattage. These "quiet PSUs" are exactly the motivation for this topic.http://tpucdn.com/reviews/Corsair/AX860/images/fan_noise.jpgHaving two fanless power supplies reduces the overall load on each and provides a lot more surface area for cooling. If it's not feasible, due to limitations in present GPU design, then that's unfortunate. However, the addition of surface area for cooling to reduce fan noise is a sound one, eh?

author: Vgkid
author_rank: Super Contributor
timestamp: December 13, 2014, 10:15:03 pm
content: here are a few passive choices, up to 550W.http://www.jonnyguru.com/modules.php?name=SearchThe JG forums would be a better place to ask.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 13, 2014, 11:25:54 pm
content: I have probed quite a few GPUs and the 12V inputs on the different connectors are not connected to each other. (Double check with a multimeter.) They will work just fine running from different PSUs as long as they all come up prior to the power good signal going high. It's done because some dual rail PSUs are in fact two PSUs in one, as opposed to just separate filtering in most decent multi rail units.Splitting the load between two PSUs will help a lot for reducing thermal density.

author: dannyf
author_rank: Super Contributor
timestamp: December 13, 2014, 11:30:06 pm
content: Quote"Passive power supply"? You probably have to build it using passive transistors and passive ICs?

author: gxti
author_rank: Frequent Contributor
timestamp: December 14, 2014, 12:30:19 am
content: Quote from: dannyf on December 13, 2014, 11:30:06 pmYour joke has already been sniped by Richard Crowley. Better luck next time. The grouch battle is over, but the grouch war is yet to be decided -- don't give up hope!As for the technical constraints, I don't see any reason why this wouldn't work if you balance the power consumption appropriately. I don't think the concerns about PCI-e vs auxilliary power are well-founded, because after all that is already two separate routes from which it draws power. It has to deal with differences in resistance etc. not to mention multi-rail power supplies. But I do tend to agree with those who are saying that concept overall is misguided. The screaming from my GPU and CPU coolers has always drowned out anything from the PSU.

author: akis
author_rank: Frequent Contributor
timestamp: December 14, 2014, 08:38:37 am
content: Quote from: gxti on December 14, 2014, 12:30:19 amI am not sure I agree. Whatever comes out of the same enclosure PSU , regardless what we call it, multi rail, single rail or whatever, HAS to account for shorts between its own rails of the same "nominal" voltage. So if there are 2 12V connectors coming out of the same PSU, I, as a user, have to be assured, that I can connect then together (in parallel) without blowing out the PSU. In other words I , as a user, do not care how the PSU is made internally, and whether the various 12V connectors feed into the same or completely separate circuits/subcircuits inside.But noone makes a guarantee that two 12V connectors coming out of separate PSUs will also behave like that. It would be like paralleling batteries or any other kind of PSU and the only way to do it reliably is via (Schottky) diodes and an assured negative current coefficient, so that as one PSU gives out more current, its voltage drops, and then the other PSU takes over so in the end they balance out sharing the load.And we cannot depend on the internals of a computer, with hard drives, optical drives, expansion cards and whatever else, that if we feed half and half from separate supplies, there will not be paths between the 12V rails. Because as far as the motherboard and busses are concerned there is one 12V rail, not multiple. How would a GPU separate the 12V supplies coming out of the back connectors and the PCI slot without wasting power? And why on earth would it go into the trouble of the additional components required.QuoteI thought exactly the same but then noticed that the OP has mentioned somewhere (he was not explicit at the start) that he uses passive cooling for his CPU and GPU. I presume that his PSU is the last bastion of sound pollution inside his case.

author: extide
author_rank: Regular Contributor
timestamp: December 14, 2014, 04:53:04 pm
content: Quote from: RichardF on December 13, 2014, 09:08:53 pmThat would require the second unit to be an ATX psu as well. If you went with some sort of industrial 12v only psu, then the AddPSU thing wont work, as they rely on the ATX turn on wire.Also, most high end video cards actually do NOT pull power from the slot, they are designed to pull it entirely from the PCIe connectors. There are some exceptions to this, but I am fairly sure the 970 and 980's are like this, at least the ref PCB ones.

author: RichardF
author_rank: Contributor
timestamp: December 15, 2014, 01:12:03 am
content: Its seems there isn't consensus about whether or not it is safe to try to use a separate PSU to just power the GPUs, via the PCIe connectors. These posts indicate that even high performance GPUs may draw power from both the slot and the connectors:Quote from: kitgurulinkQuote from: techpowerupQuote from: techpowerup

author: RichardF
author_rank: Contributor
timestamp: December 15, 2014, 02:08:58 am
content: Quote from: 3roomlab on December 15, 2014, 01:33:40 amThis can be seen in many ATX power supply noise/fan speed/wattage load graphs. Many newer units will not even run a fan until they're loaded above 250 watts or so (something around 400 for the Corsair RM 750 & 850). At a certain load rans will rapidly ramp up, greatly reducing the efficiency of the cooling in terms of noise generated. Some go above 50 decibels. Anandtech said the Seasonic SS 1050 could be heard "rooms away".My take on the charts is that the ATX form factor, even when lengthened a bit, is not designed well for handling such high wattages quietly. I think much bigger form factor would help, especially if waste heat would be exhausted out of the case. But, since nothing like that yet exists in the marketplace, I thought about the possibility of using two fanless power supplies rather than just a single power supply. The dual sourcing of power (PCIe slot and connectors) for graphics cards seems to be the biggest obstacle.While figuring out how to put them in an enclosure with adequate cooling is one issue, the one that concerns me more is danger to the equipment and possibly having "dirty" power that causes instability or stress on the equipment. I'm not nearly as concerned about cooling strategy because I don't have significant space limitations. I would like to have them in an enclosure to reduce EMI and dust exposure, but the passive cases I've seen are just cases with a lot of holes in them. That doesn't seem to be terribly complicated. Plus, I can always add a large very slow-moving fan to supplement the convection.Quote from: 3roomlab on December 15, 2014, 01:33:40 amThat was my reasoning behind a double-wide PSU or something similar. It would have twice the surface area for cooling. Since 520 watt fanless models are offered from reliable vendors, then why can't doubling the space given to cooling make it possible to increase a fanless PSU to, say, 750 watts? Or, if you want a higher wattage then put something like a 200mm fan on it and run it at a low rate.Quote from: 3roomlab on December 15, 2014, 01:33:40 amIs this because the paint provides more surface area? If so, then would matte paint with grit in it (silica or something) be even better? At what point, though, does the paint become a heat insulator?

author: ajb
author_rank: Super Contributor
timestamp: December 15, 2014, 03:28:36 am
content: Quote from: RichardF on December 15, 2014, 01:12:03 amThe concern isn't that a card may draw power from two or three sources, the concern is that the card maydirectly connect those sources together, which can cause currents to flow between those sources through the card. If the card described in your quote can independently regulate the the portion of it's operating power that comes from each source, that suggests that the different sources supply different phases of the main buck converter(s). They mention five phases, so probably two come from each PCIe aux connector and the fifth comes from the PCIe slot. Using a polyphase converter to balance power draw from multiple sources is interesting, I'd be curious to see that circuit.It's pretty trivial to check if any given card ties its power sources together, just need to measure continuity among the 12V inputs.Quote"Dirty power" is a pretty squishy term. Ripple on two supplies won't be any worse than on one supply. Under/overvoltage is not permitted by the ATX spec, so shouldn't be a problem on a decent supply. Both supplies will be required to meet the hazardous power limits of the ATX spec, so should shut down on overcurrent. I think as long as you use two high quality, ATX-compliant power supplies, suitably bond the DC grounds together, and ensure that all of the positive rails are mutually isolated, the only real concern I would have from looking at the ATX spec is that whatever mechanism you use to tie the power on/off of the two supplies together doesn't violate the risetime requirements, and ensures that if one power supply shuts down the other immediate follows suit. I don't think the little gizmo you linked on Amazon would do that. If I were to do this myself I'd probably build an adapter that ANDs the PWR_OK signals to the motherboard, and routes the PS_ON# signal to both PSUs via some logic that immediately deasserts PS_ON# if either PSU misbehaves.

author: RichardF
author_rank: Contributor
timestamp: December 15, 2014, 03:15:44 pm
content: Quote from: ajb on December 15, 2014, 03:28:36 amThat's what I was referring to. That's why the issue of whether or not a card is going to draw from both the slot and the connectors seems critical.Quote from: ajb on December 15, 2014, 03:28:36 amFrom the little I've read, it seems like the slot may also load the 3.3 rail.Quote from: ajb on December 15, 2014, 03:28:36 amHow would I do that?Quote from: ajb on December 15, 2014, 03:28:36 amDoes this require an additional modification?Quote from: ajb on December 15, 2014, 03:28:36 amThat's unfortunate. It seems to me that if someone is going to market a device for tying ATX supplies together they should cover all the important bases.Quote from: ajb on December 15, 2014, 03:28:36 amIt seems I'm out of luck, then. The one friend I have who has this background is back in Finland.Thanks for the info.

author: extide
author_rank: Regular Contributor
timestamp: December 15, 2014, 05:30:27 pm
content: Quote from: ajb on December 15, 2014, 03:28:36 amYes, this is exactly the issue right here.

author: ajb
author_rank: Super Contributor
timestamp: December 15, 2014, 06:10:34 pm
content: Quote from: RichardF on December 15, 2014, 03:15:44 pmExcept it's possible for a device to draw power from more than one source without tying those sources together. This is a necessary condition of being able to independently regulate the amount of power drawn from each source. From the description of the cards that you quoted, manufacturers of high end graphics cards are accomplishing this by splitting the sources among separate phases of a polyphase buck converter, which means the sources are isolated from one another by their own dedicated switches and inductors.QuoteAs long as the slot is the only source of 3.3V available to the card, then it's no concern.QuoteSince you'd need to interface to both PSUs' ATX connectors to get proper startup/shutdown behavior, probably build your interface gizmo with two 24 pin ATX headers and have all of the 24 pin connector's grounds tied together there. The supply rails from ONE PSU would head out to the motherboard from there, along with the PS_ON and PWR_GOOD lines from the interface logic.QuoteNope, just proper connections to your peripherals, as we've been discussing in this thread.

author: RichardF
author_rank: Contributor
timestamp: December 15, 2014, 11:19:08 pm
content: Quote from: ajb on December 15, 2014, 06:10:34 pmI seem to recall reading that the slot provides both 3.3V and 12V power. Unless that 12V power is handled by a separate phase it seems like there could be issues with a second PSU.Quote from: ajb on December 15, 2014, 03:28:36 amIs there a way to modify the AddPSU device to accomplish this since it already handles the startup/shutdown issue? I am looking for a solution that involves as little DIY soldering and the like as possible. It is not my area of expertise.Quote from: ajb on December 15, 2014, 03:28:36 amProper meaning what? The big potential problem appears to be the GPUs due to the power being provided by the slot. If GPUs are getting both 3.3V and 12V power from the slot, is it still OK to power them via another PSU?

author: ajb
author_rank: Super Contributor
timestamp: December 16, 2014, 03:48:16 pm
content: Proper meaning make sure none of your peripherals will directly tie a supply rail from one power supply to a supply rail on another.The AddPSU is nothing but a 4 pin drive connector, a 24 pin ATX connector, and a relay. It's a far cry from what you'd need to ensure proper startup/shutown behavior with any confidence of staying within the ATX spec's timing requirements. At a minimum you need a handful of transistors or a logic gate or two to implement proper startup/shutdown behavior. If you wanted to be defensive you'd add a window comparator for every supply rail.The fact that the PCIe slot provides 3V3 is no concern because there's no other source of 3.3V available to the card, so there's no chance that the card may connect one PSU's 3.3V rail to another PSU's 3.3V rail. The fact that the slot provides 12V IS a concern because the PCIe Aux power connectors also provide 12V, so if you have the motherboard (and hence PCIe slot) coming from one PSU and the Aux connectors coming from the other PSU, it would be possible for the card to connect all of those 12V inputs together, which could mean that you have the 12V rails from two different power supplies shorted together. HOWEVER, it seems (as shown in links you've provided) that high power video cards typically segregate the 12V supplies from the slot and the PCIe Aux connectors onto different phases, so as long as your card does that (you can check with a multimeter) there's no concern there.

author: RichardF
author_rank: Contributor
timestamp: December 16, 2014, 11:04:13 pm
content: I appreciate the information and clarifications.Now, all we need is for someone to make a better product than AddPSU and sell it on Amazon.