Preventing ground loops when using multiple ATX PSU - Page 1
Forum section: electronics / projects

author: TheHippySeal
author_rank: Contributor
timestamp: August 27, 2012, 04:22:03 am
content: Hello Guys. Someone suggested this would be a problem when I posted before. Lets say I'm using one to power the graphics cards and fans, and one for the rest. Is there a way to prevent this from occuring? I found that you can just remove the ground for audio amplifiers when you connect them. Could I do that with these power supplies?

author: Rerouter
author_rank: Super Contributor
timestamp: August 27, 2012, 07:48:14 am
content: seeing as you did not fully specify 2 atx supplies, providing they are in the same casing you can only prevent it if one of them allows you to disconnect the ground from both case and negative,if not then its not the biggest issue in the world, as i had something else in mind until you mentioned your graphics card,

author: amyk
author_rank: Super Contributor
timestamp: August 27, 2012, 09:40:06 am
content: You need to worry about current balancing and voltage regulation when having multiple PSUs in parallel.

author: TheHippySeal
author_rank: Contributor
timestamp: August 27, 2012, 09:49:52 am
content: Ok, so the ATX PSU casing is isolated from the computer case, and the second PSU casing. Then I snip the common pins, or ground, off the mini fit jr plugs on the isolated psu and terminate ground from the non isolated PSU in the plugs instead.That reduces the chance of any problems?

author: AndyC_772
author_rank: Super Contributor
timestamp: August 27, 2012, 10:01:47 am
content: The problem won't be ground loops, it'll be load sharing.There's absolutely no need to separate the grounds. You won't even be able to to; they'll be connected to the PC chassis, the case of the PSU, and mains earth.What you do need to be concerned about is what happens if you connect both supplies' outputs together, either intentionally or through one of the things you're powering. Chances are they're not designed for it, and won't like it.The danger is that, suppose one of them has a "12V" output set at +12.07V, and the other is set at +11.92V. At what voltage does the combined output sit, and how much current comes from each supply? I'll give you a clue, it won't average out to +11.99V with 50% of the current coming from each... they'll fight each other, could go unstable, and might even smoke.Get one power supply that has a high enough rating to run your whole PC.

author: TheHippySeal
author_rank: Contributor
timestamp: August 27, 2012, 10:46:13 am
content: Yer. I don't think I'm going to be doing this any longer. I don't need to at the moment either. I measure the load peaking at about 600W and my PSU is 700W. It was interesting to think about doing though. Anything to save a couple of pennys and recycle this old junk.I'm interested in this ground looping topic. Something left out in my oh so thorough University education. I'm supposed to know all about sources of noise.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 27, 2012, 10:55:30 am
content: Ground loops are an issue when two devices each a potential which they call "ground", but which aren't actually the same potential for the two devices.Connect them together at a single point, and some current may flow through the connection to equalise the ground potentials - and that's OK provided the current isn't so large that it causes other problems. But connect them together at more than one point and there's no way to know the precise route the current is taking, or the potential with respect to ground (any ground!) at any particular point in the system.Do be aware that PC power supplies are rated in "Marketing Watts", which are quite a bit smaller than real Watts (as defined in the system of SI units). A "700W" power supply is highly unlikely to be able to sustain a continuous 600W (SI) load. What's the average load, and how are you measuring it?

author: TheHippySeal
author_rank: Contributor
timestamp: August 27, 2012, 11:24:10 am
content: Ok, that was more of a guestimate from the components. It actually doesn't use more than 400W really. That's with the processor on the standard clock and 2xGTX470 @ 450MHz. Fans on full power because, the one I got which is supposed to be overclocked to 700MHz, actually has worse heat disappation! It doesn't have much exhaust flow at all, compared to my original one, which seems intuitively counter productive.

author: TheHippySeal
author_rank: Contributor
timestamp: August 27, 2012, 11:25:30 am
content: brennenstuhl pm 230 is the device. Precision equipment right there. Can't be that far off.

author: T4P
author_rank: Super Contributor
timestamp: August 27, 2012, 12:08:22 pm
content: @AndyC_772Well, that is when we speaking of shithouse power suppliesGood PSU's can sustain full draw at rated efficiencies (Best efficiency is at half of the full power)@TheHippySealYou judge man, GTX470 is about 225W at max but seeing that you underclocked your cards by 150MHz ... yeah 400W should be possible.Which CPU are you using? Did you OC it?

author: madires
author_rank: Super Contributor
timestamp: August 27, 2012, 12:50:05 pm
content: Quote from: TheHippySeal on August 27, 2012, 09:49:52 amIt's not feasable to isolate ground in a standard PC since there are a lot of connections between ground and earth (= case). Ground loops are more of a problem for connecting a PC and a stereo/amp if the latter has a connection between signal ground and earth.

author: gxti
author_rank: Frequent Contributor
timestamp: August 27, 2012, 03:28:49 pm
content: Load sharing in a typical PC shouldn't be a problem, even if you're connecting it to graphics cards. Many power supplies already have multiple 12V rails internally so graphics cards are designed to accept power from possibly unrelated sources. And a ground lift is definitely not necessary and likely to cause more problems than it solves.

author: T4P
author_rank: Super Contributor
timestamp: August 27, 2012, 04:56:01 pm
content: Usually i see those really high-end PC modders use two exact power supplies in the same case ...

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 28, 2012, 04:01:46 am
content: Quote from: AndyC_772 on August 27, 2012, 10:01:47 amMy new PC actually does have two power supplies supplying the same 12V rail, but the intent was to efficiently integrate battery backup and alternative energy into the system. If you have one set to a higher voltage than the other and the difference is too high to let resistance balance it out, the power supply with the higher voltage takes the load. Not a problem if both power supplies are capable of supplying the load alone and if they play nice with another supply pushing the voltage slightly above its reference. (Some synchronous rectified converters can have problems with that.)In order to allow load distribution control, I then have a microcontroller vary the output voltage of the supplies slightly. It can then command 100% AC power, 100% DC power, or any value in between, with hardware based failover. A large capacitor smoothes out transitions and DC/DC converters derive the other rails in the ATX standard.BTW, most multi 12V rail power supplies cheat and don't truly have multiple 12V rails, some have filters separating the rails, and only very few really have completely separate converters. What would make sense is to have a main 12V rail for all the VRMs and a filter for the drives, but it's overkill.