Passive computer power supply questions

[RichardF]

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:

It's a really bad idea. There have been attempts before on other parts of the Internet to run 2 PSUs and it is an extremely bad idea. As has been pointed out, you need both PSUs to start up together. What then happens if one PSU goes off and the other carries on supplying power? Or if one PSU has an earth fault and the other doesn't and a shock takes the path of least resistance to earth... through all of your components.

Efficiency is worse than with a single larger sized PSU. Conversion is always more efficient on a larger scale so doubling up a smaller sized PSU will waste more electricity, giving more heat.

Now that you bring up electrical noise... you do know that with those two PSUs you're going to have a separate ripple voltage on each along with all of the other kinds of power distortions. You're ruining your power quality by having 2 PSUs and a dirty common GND between them (earth looped of course) even if things do work correctly (that gadget you linked to only gets over startup issues, nothing else). Worse power quality means worse overclocking potential. 

A single decent fan cooled PSU can't possibly be as loud as all of the other components you're going to have. Why bother having something quieter when it will be drowned out anyway?

Can 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!

[akis]

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.

[Richard Crowley]

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.

[Fsck]

I believe he means passively cooled when he says passive.

there isn't a good solution without diy. the above are correct.

[ajb]

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 work without the 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 you really need 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. 

[RichardF]

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.

Unfortunately, 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.

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 work without the 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.

I'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 draw from both the slot and the connectors when gaming.

They can use up to 75 W (3.3 V × 3 A + 12 V × 5.5 A), though the specification demands that the higher-power configuration be used for graphics cards only.

As a final note, how much power do you really need 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.

Estimated 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 a big power 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 hits 43 decibels of noise at 670 watts.

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.

It 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.

[extide]

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?

[IanB]

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.

[IanB]

An example of a silent PC designed as such from the ground up is an iMac. No fans anywhere, completely noiseless.

[RichardF]

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?

Typo. 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". lol

An example of a silent PC designed as such from the ground up is an iMac. No fans anywhere, completely noiseless.

The goal here is to make a system with a high level of GPU performance that isn't noisy.

[extide]

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.

[mariush]

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 intensive

Modern 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. 

[RichardF]

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.

I wonder where I can find one that's good quality. I'll do a Google search.

You can rig up a relay to turn the second one on/off at the same time as the first.

Isn't that what the AddPSU unit does?

Might still be doable on a single 530 if you don't overclock anything.

Perhaps, as long as I avoid overclocked 3rd-party GPUs, which isn't easy. Tom's Hardware found much higher power consumption for a Gigabyte 970 than for a reference 980.

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.

That was my original idea, but since the GPUs also draw power from the slot I was concerned about problems.

[IanB]

The goal here is to make a system with a high level of GPU performance that isn't noisy.

That 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.

[RichardF]

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 intensive

Modern 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.

The 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.

The point is, you're not looking at the whole picture.

Actually, 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.

[taylor12]

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.

[Someone]

An example of a silent PC designed as such from the ground up is an iMac. No fans anywhere, completely noiseless.

All 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.

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? 
...
Can 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!

Check 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.

[RichardF]

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?

This 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.

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.

Wouldn'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.

[RichardF]

Check 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.

I 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.

[ajb]

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.

[IanB]

Wouldn'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?

I'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.

[RichardF]

Wouldn'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?

I'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.

I'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.

[RichardF]

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.

15 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.jpg

Having 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?

[Vgkid]

here are a few passive choices, up to 550W.
http://www.jonnyguru.com/modules.php?name=Search

The JG forums would be a better place to ask.

[NiHaoMike]

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.