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Author Topic: Multiple PSUs and Grounding  (Read 9424 times)
DBordello
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January 11, 2012, 06:27:35 PM
 #1

How does everybody feel about utilizing two (or three) PSUs?  Do you feel that connecting their ground is a good idea?

I traditionally thought that grounding them together was the way to go.  I built a molex adapter that simply connected ground to ground.  However, I went to disassemble my rig that had been running for months without issue, and I hadn't even hooked up my ground adapter. 

I recently tried expanding to 3x500W.  Something bad happened and I shorted the mobo.  I was grounding all three PSUs together. 

Discussion?

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bulanula
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January 11, 2012, 06:29:41 PM
 #2

I think grounding them together is the best way to prevent power fluctuations and other such stuff. I'm no expert but that's what I heard.

When you shorted the mobo, did it die afterwards ?
DBordello
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January 11, 2012, 06:31:55 PM
 #3

It did.  I am not entirely confident I know what happened.  I think I was getting cocky about 3 PSUs = 1.

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January 11, 2012, 07:25:50 PM
 #4

Grounding will be necessary... it's keeping the +ve rails apart that I would worry about.
You absolutely do not want to connect them up in parallel.
P4man
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January 11, 2012, 07:43:13 PM
 #5

I thought I read the exact opposite; something about avoiding a ground loop?

FWIW, Ive run a rig on 2 PSUs for a few months, sharing nothing. Each PSU fed different cards, and it worked fine, but its not something I can guarantee or even recommend. Ive since upgraded the PSU to be safe.

I will point out there are VGA power supplies that provide extra 6/8 pin PCIE connectors, and afaik they are not connected to the main PSU - other than for powering up automatically.

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Gerald Davis


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January 11, 2012, 08:07:07 PM
 #6

I don't think internally sharing a ground is good idea. 

Each PSU is going to share the same external ground anyways.  You likely don't want to connect multiple PSU to different outlets where they could experience different resistance to ground but if you got 3 PSU connected to a power strip and it is plugged into the wall outlet I don't see what splicing ground wires together achieves beyond that.

As an example N+1 redundant power supplies for servers don't have any grounds tied together and there grounding can be more of a challenge because each supply should be on a different branch circuit connected to different UPS otherwise you compromise redundancy. 
Swishercutter
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January 11, 2012, 08:24:16 PM
 #7

Depends on what type of "grounding" you are talking about.  If you mean connecting to a common "earth ground" on the AC then they already share the same ground point when they are plugged into the wall (your ground wire in the house is connected to a common stake that is driven into the ground outside, or connected to a cold water pipe).  If you mean a common negative on the PSU's DC voltage then that is a different thing, I connected the PSU's together via DC negative on my setup.  You need to keep the connection between the 2 short to prevent ground loops though. 

Do a quick search as this was discussed in a much greater detail back around June/July...couldn't find a link to a writeup but I know I linked them in the past.
jake262144
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January 11, 2012, 09:57:04 PM
 #8

I think grounding them together is the best way to prevent power fluctuations and other such stuff. I'm no expert but that's what I heard.

I don't mean to sound offensive here but I remember clearly you claiming to be an electrician.

...I work for a local power company as an electrician. No stolen cables or altered meters etc. ...

If you do work as an electrician and are no expert at what you do, God save the Queen Smiley
bulanula
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January 11, 2012, 10:03:17 PM
 #9

I think grounding them together is the best way to prevent power fluctuations and other such stuff. I'm no expert but that's what I heard.

I don't mean to sound offensive here but I remember clearly you claiming to be an electrician.

...I work for a local power company as an electrician. No stolen cables or altered meters etc. ...

If you do work as an electrician and are no expert at what you do, God save the Queen Smiley


Well there's always the disclaimer : if your GPUs / stuff blows up don't blame me Wink
jake262144
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January 11, 2012, 10:13:09 PM
 #10

That disclaimer should be on by default when trusting any information from the net  Grin

Does the answer to OP's question depend on the type of wiring at their premises?
Does it depend on whether their installation uses three wires (building has its own grounding) or just two (in which case the grounding is supposed done at the power station)?
I do remember reading an article about being supposed to connect ground to neutral if the building isn't grounded and the electrical installation is done with two wires.
...or am I just prattling nonsense?
DBordello
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January 11, 2012, 11:03:53 PM
 #11

I have just finished debugging my failed setup.  It turns out that the CPU and Motherboard were taken out in the fiasco. 

There are numerous good arguments here for not connecting the DC ground.  1) VGA PSUs and 2) Server PSUs (although, in this case the components are probably always powered by one or the other?) 

I think I am going to move away from my setup of connecting the DC ground together.  In the end, they are all connected to the same power strip (with ground). 

Thank you for everybody's advice.  I think I am still going to consider moving away from 3x500 to something simpler.  Maybe 2x750.  I am trying to power 6x5850.  Thoughts? 

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DeathAndTaxes
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Gerald Davis


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January 11, 2012, 11:06:19 PM
 #12

I have just finished debugging my failed setup.  It turns out that the CPU and Motherboard were taken out in the fiasco. 

There are numerous good arguments here for not connecting the DC ground.  1) VGA PSUs and 2) Server PSUs (although, in this case the components are probably always powered by one or the other?) 

I think I am going to move away from my setup of connecting the DC ground together.  In the end, they are all connected to the same power strip (with ground). 

Thank you for everybody's advice.  I think I am still going to consider moving away from 3x500 to something simpler.  Maybe 2x750.  I am trying to power 6x5850.  Thoughts? 

I doubt you need 1500W to drive 6x 5850s.  I drive 3x5970 (6 total GPU) which are more powerful and AC wattage at the wall is 870W.  DC is probably closer to 800W.
ArtForz
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January 11, 2012, 11:20:08 PM
 #13

The "internal" grounds are already pretty damn well connected.
1. On ATX PSUs, secondary GND is directly tied to safety ground, otherwise it'd be illegal to sell in most of the civilized world. Don't believe me? Just measure it.
2. Graphics cards have all PCIe power connector and card edge connector grounds tied into a massive ground plane. Again, if in doubt, measure it... use a 4-wire mOhm meter or you'll only be seeing 0.0 Wink

So, where does the "you have to tie grounds together" myth come from?
Simple.
Basically you need the following scenario:
1. Building with improper/unusual wiring and several V difference between safety ground potential between "close" outlets.
2. 2 PSUs that are *not* electrically connected via a common metal chassis (if both are screwed to the same steel chassis, that's usually a low enough resistance path to reduce the potential difference to safe levels of a few dozen mV).
3. said PSUs being plugged into said outlets. Now we have 2 PSUs where #1s GND is a few V different from #2s GND.
4. One connected to mainboard, another connected only to drives or similar. Now we have mainboard GND and drive GND differ by a few V...
5. Data cable from mainboard to drive has ground/shield wires (duh). Now you have grounds differing by several V connected with a few strands of AWG26, smell the PVC? Worse... if data lines make connection first, (relatively) large currents flow through the I/O pin protection diodes, usually resulting in dead controllers and/or drives.
In *that* scenario, connecting the grounds via a few AWG18 jumpers or similar reduces the potential difference between grounds to a safe level.

So short version... if it makes you feel good, tie secondary grounds together all you want, but it's flat out pointless if you're powering GPUs.

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jake262144
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January 11, 2012, 11:34:28 PM
 #14

Good info, makes perfect sense. Thanks Art.
DBordello
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January 12, 2012, 12:43:29 AM
 #15

The "internal" grounds are already pretty damn well connected.
1. On ATX PSUs, secondary GND is directly tied to safety ground, otherwise it'd be illegal to sell in most of the civilized world. Don't believe me? Just measure it.
2. Graphics cards have all PCIe power connector and card edge connector grounds tied into a massive ground plane. Again, if in doubt, measure it... use a 4-wire mOhm meter or you'll only be seeing 0.0 Wink

So, where does the "you have to tie grounds together" myth come from?
Simple.
Basically you need the following scenario:
1. Building with improper/unusual wiring and several V difference between safety ground potential between "close" outlets.
2. 2 PSUs that are *not* electrically connected via a common metal chassis (if both are screwed to the same steel chassis, that's usually a low enough resistance path to reduce the potential difference to safe levels of a few dozen mV).
3. said PSUs being plugged into said outlets. Now we have 2 PSUs where #1s GND is a few V different from #2s GND.
4. One connected to mainboard, another connected only to drives or similar. Now we have mainboard GND and drive GND differ by a few V...
5. Data cable from mainboard to drive has ground/shield wires (duh). Now you have grounds differing by several V connected with a few strands of AWG26, smell the PVC? Worse... if data lines make connection first, (relatively) large currents flow through the I/O pin protection diodes, usually resulting in dead controllers and/or drives.
In *that* scenario, connecting the grounds via a few AWG18 jumpers or similar reduces the potential difference between grounds to a safe level.

So short version... if it makes you feel good, tie secondary grounds together all you want, but it's flat out pointless if you're powering GPUs.

Excellent analysis.  What if the two PSU are *not* electrically connected?  Ie, an open rig without a chassis. 

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jake262144
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January 12, 2012, 01:04:31 AM
 #16

Excellent analysis.  What if the two PSU are *not* electrically connected?  Ie, an open rig without a chassis.  

The "common ground" between them might be slightly less common than expected.
As Art already pointed out, the voltage at "ground" might differ between these not interconnected PSUs.
If they power different components of the same PC, current will run where it's not supposed to (from one "ground" to another).
You can fry your components that way. That's not a very likely scenario though.

Mind you, there's nothing to worry about if you're running those PSUs from the same power extender - the power strip will provide common ground anyhow.
DeathAndTaxes
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Gerald Davis


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January 12, 2012, 01:44:18 AM
 #17

What if the two PSU are *not* electrically connected?  Ie, an open rig without a chassis. 

If each PSU is properly grounded then they are electrically connected via ground wire.  If you for some insane reason broke off the ground pin on the power cable then you likely would have an issue.
DBordello
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January 12, 2012, 03:51:57 AM
 #18

What if the two PSU are *not* electrically connected?  Ie, an open rig without a chassis. 

If each PSU is properly grounded then they are electrically connected via ground wire.  If you for some insane reason broke off the ground pin on the power cable then you likely would have an issue.

Awesome, sounds great!

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January 12, 2012, 04:25:59 AM
 #19

The big concern I have with running two power supplies into one rig is that even if the grounds are externally linked, the output voltages can be different between the supplies. Lets say one supply puts out +12.1V and another puts out +12.4V, then there is a 0.3V potential between them, and current will flow between the two power supplies. The amount of current is only limited by the design of the power supply regulation and the resistance in the path. If one power supply is connected to your motherboard and supplies a video card +12v through PCIe slot (up to 75W), and the other is connected to the video card power connectors, then you seem to have made a path through the card and motherboard where this current can flow with little limitation until a trace goes up in smoke. While I have run such a system with two power supplies and it worked, frankly I am surprised it did.

ArtForz
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January 12, 2012, 05:02:45 AM
 #20

The big concern I have with running two power supplies into one rig is that even if the grounds are externally linked, the output voltages can be different between the supplies. Lets say one supply puts out +12.1V and another puts out +12.4V, then there is a 0.3V potential between them, and current will flow between the two power supplies. The amount of current is only limited by the design of the power supply regulation and the resistance in the path. If one power supply is connected to your motherboard and supplies a video card +12v through PCIe slot (up to 75W), and the other is connected to the video card power connectors, then you seem to have made a path through the card and motherboard where this current can flow with little limitation until a trace goes up in smoke. While I have run such a system with two power supplies and it worked, frankly I am surprised it did.
No graphics card I know of does that, it's also a massive no-no according to PCIe spec.
Simple reason, defeats per-rail overcurrent protection -> fire hazard.
Not to mention the havoc that could cause with PSUs that have 12V rails coming from 2 independently regulated 12V supplies (not too uncommon in >1kW units nowadays).
Too lazy to dig up the exact text, but basically you're forbidden from creating a galvanic connection between each 12V aux connector and 12V from slot.
Also cards have to accept any power sequencing of 12V for any amount of time without sustaining damage.
They're not required to function if time from first to last 12V input coming up is > 100ms or if any input drops out after the initial 100ms.
But so far all cards I've seen seem perfectly happy with aux 12V coming up long before slot and staying on while slot 12V goes away and comes back (= aux supplied from 2nd always-on PSU).

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