FAQ: Dual Power Supplies - Overclock.net - An Overclocking Community

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FAQ: Dual Power Supplies

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post #1 of 62 (permalink) Old 12-01-2011, 09:09 PM - Thread Starter
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The short answer: Don't.


The vast majority of people who use two power supplies don't need to, and waste money or endanger their rigs doing so. It's only a valid approach in a very small handful of scenarios, and is otherwise either an unnecessary over-complication, or a dangerous jury rig solution.

The main thing people do wrong with dual power supplies is to think they even need it. They look at a GTX580, then look at power consumption charts and think "OMG 500W OMG OMG NEED MOAR POWER" when in fact that's the total system power consumption; the card itself only pulls ~300W. So then they decide to supplement their already more-than-sufficient 650-850W PSU with another PSU, wasting money and making life more difficult, what with having to stash another PSU somewhere inside their case and hiding a bunch more cables.

The next worse thing people do is do dual PSUs with cheap units. Most power supplies under $100, and all under $50, use a technology called "group regulation" to reduce costs. There's a bunch of technical stuff I could throw at you; but the gist of it is that the current on the +12V and +5V rails on a group regulated power supply must stay within a certain approximate ratio to one another (say 3A of +12V for every 1A of +5V, +/-20%) or else the voltage regulation will go out of whack. +12V load too low? +5V voltage drops and +12V voltage soars. +5V load too low? +5V soars and +12V drops.

The secondary PSU in a dual-PSU usually has no +5V load at all, or at most 1-2A, meaning that the +12V voltage will droop significantly; on high-end group regulated units it will fall to 11.6-11.7V, which is in-spec but poor. On low-end group regulated units +12V might drop to 11.3-11.5V, out of spec or almost. Out of spec voltage (>11.4V) can cause component malfunction; bluescreens and random shut downs, and will prevent many hard drives from functioning at all.. Very low, but in-spec voltage (11.4-11.6V) can cause poor overclocking results, occasional instability, excessive wear on component power circuitry, and can cause hard drive issues over time.



Dual PSUs should only be used when both power supplies are high quality and use independent regulation, or even better "DC-DC" regulation. It's only a useful approach when dealing with systems that pull >1000W; then two quality 650W+ indy/DC-DC regulated units may be used in place of a single 1000W+ unit. Even then you may run into issues of crosstalk leading to greater effective ripple on power regulated devices (motherboard, graphics card, RAID controllers), and units that rely on +12V v-sense may suffer poor voltage regulation if the cable with their v-sense wires is not in use.



So just don't.

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post #2 of 62 (permalink) Old 12-01-2011, 09:12 PM
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Well written, high quality content. Well done. I always look forward to content like this. smile.gif

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post #3 of 62 (permalink) Old 12-01-2011, 09:25 PM
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Thanks man! I can't believe I was thinking of doing this.

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post #4 of 62 (permalink) Old 12-01-2011, 09:26 PM
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I'm not intending to contradict what you are saying but what about using one of the (Add2Psu) adapter such as I posted in this thread? https://www.overclock.net/t/1084863/add2psu-dual-psu-adapter-21-99-shipped Then using the additional power for system fans, pumps, possibly additional 8 or 6 pin graphics card connectors. Other than that yes i agree you should not be mixing power supply for main-board, CPU, memory and such.

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post #5 of 62 (permalink) Old 12-01-2011, 09:39 PM - Thread Starter
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Originally Posted by drufause View Post

I'm not intending to contradict what you are saying but what about using one of the (Add2Psu) adapter such as I posted in this thread? https://www.overclock.net/t/1084863/add2psu-dual-psu-adapter-21-99-shipped Then using the additional power for system fans, pumps, possibly additional 8 or 6 pin graphics card connectors. Other than that yes i agree you should not be mixing power supply for main-board, CPU, memory and such.

Not as dangerous, sure, but still a pain in the ass and usually not even necessary or worthwhile.

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post #6 of 62 (permalink) Old 12-02-2011, 02:12 PM
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Phaedrus.

What do you suggest then for people using Quad Nvidia 580 and the new 3930k/3960x that can draw 500w when OCed?

Is there any other choice then using dual-PSU?

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post #7 of 62 (permalink) Old 12-02-2011, 02:17 PM - Thread Starter
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Phaedrus.
What do you suggest then for people using Quad Nvidia 580 and the new 3930k/3960x that can draw 500w when OCed?
Is there any other choice then using dual-PSU?

Antec HCP-1200, Corsair AX1200, Thermaltake Toughpower 1350M, Silverstone ST-1500, Enermax Platimax 1200W

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post #8 of 62 (permalink) Old 12-02-2011, 02:20 PM
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Good write up. My servers still need their redundant psu's though :p



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post #9 of 62 (permalink) Old 12-02-2011, 02:46 PM - Thread Starter
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Good write up. My servers still need their redundant psu's though tongue.gif

Oh yeah, redundant PSUs are totally different. tongue.gif

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post #10 of 62 (permalink) Old 12-02-2011, 04:31 PM
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Quote:
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Antec HCP-1200, Corsair AX1200, Thermaltake Toughpower 1350M, Silverstone ST-1500, Enermax Platimax 1200W

So you think a single AX1200 could do: 4X 580 OC (or 4X Lightning 6970 heavealy OCed), 1X 3930k OC (taking around 500w), 32 Gentle Typhoons fans (6X 3000rpm in those 32), 4 pumps MCP655, 6x 2TB HDD, 3 SSD and 1X Blu-ray writer?

I'm asking this, because I'm trying to use 2 PSUs for this system, and the results are a dead AX1200, and a dead Antech HCP-1200, in the last week. I was trying to pair those with an AX850 with the Add2PSU thingy. But there is 2 dead PSUs, a dead Rampage IV and a dead 3930k in the last week...

I don't know if using 2 PSUs killed the AX1200 and the HCP-1200, but the AX850 is still alive and kicking!

What would you use for a system like that? I have a brand new AX1200, a brand new HCP-1200 and a 2 weeks old AX850 with me right now that I can use. biggrin.gif

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