Computer power supply conversion - Page 1
Forum section: electronics / general

author: cubemike99
author_rank: Contributor
timestamp: March 25, 2013, 04:37:47 am
content: I've seen a bunch of tutorials all over the internet for converting an ATX computer power supply to a benchtop supply by breaking out all the connections with binding posts, adding some LED's and switches, adding an LM317, etc. How do they compare in an electronics perspective with proper supplies sold specifically as benchtop supplies? Is it worth converting one? Thanks.

author: BravoV
author_rank: Super Contributor
timestamp: March 25, 2013, 04:53:20 am
content: Quote from: cubemike99 on March 25, 2013, 04:37:47 amLike heaven and hell, seriously.(you might check my sig below, there is a thread where people show their bench psus)Worth it or not, it depends on how and where are you going to use it for, for sure the generated power from pc psu is very noisy full with switching spikes & ripples that is not trivial to suppress.

author: Rerouter
author_rank: Super Contributor
timestamp: March 25, 2013, 06:53:19 am
content: if your tolerances are low, and switching noise isn't much of an issue (of for testing sometimes a benefit) then an ATX is fine, if your after under say 200mV accuracy to what you set and want low noise then you can either add a regulator like the 317 on the outputs of the atx of look into building or buying a decent one,

author: cubemike99
author_rank: Contributor
timestamp: March 25, 2013, 12:54:13 pm
content: And here I thought computers need rock steady supplies that were right on target.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 25, 2013, 01:08:08 pm
content: Quote from: cubemike99 on March 25, 2013, 12:54:13 pmYour mobo takes care of that part.

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2013, 07:27:38 pm
content: For a computer rock steady means under 500mV of ripple and within 5% of nominal, except for some rails where 1V is acceptable and 10% tolerance is fine. The CPU has a varying voltage due to load, and at 100A plus it is anything but steady.

author: Monkeh
author_rank: Super Contributor
timestamp: March 25, 2013, 08:41:24 pm
content: Quote from: SeanB on March 25, 2013, 07:27:38 pmYou're off by an order of magnitude on the ripple, and that's worst-case, and 100A is way more than most CPUs use.

author: mariush
author_rank: Super Contributor
timestamp: March 25, 2013, 09:13:31 pm
content: It's 120mV for 12v, about half of that for 5v and 3.3v.Most new power supplies manage under 30-40mV.

author: FenderBender
author_rank: Super Contributor
timestamp: March 25, 2013, 09:53:19 pm
content: Computer power supplies are mainly concerned with getting a lot of power out a small little box with reasonable efficiency and performance.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: March 25, 2013, 10:00:06 pm
content: Check this out:http://dangerousprototypes.com/docs/ATX_Breakout_BoardIt has some protection and it is a quick and dirty conversion.One main issue is that it does not have variable voltage.

author: ConKbot
author_rank: Super Contributor
timestamp: March 26, 2013, 11:28:17 pm
content: Dont forget the usual stick in the mud, ground referenced. no luck safely putting supplies in series, and without a constant current mode, paralleling them is a gamble at best.

author: Rerouter
author_rank: Super Contributor
timestamp: March 27, 2013, 01:21:56 am
content: to be fair most of the cheaper units can have the earth pin disconnected, for parallel, not that i would recommend it to anyone who does not fully appreciate just what the risks are from doing this,

author: ConKbot
author_rank: Super Contributor
timestamp: March 27, 2013, 02:08:31 pm
content: Quote from: Rerouter on March 27, 2013, 01:21:56 amYes I meant in general, its doable to lift the ground, but just a simple lifting of the ground could leave the case (and thus the negative output) floating at 1/2 the mains potential from the filter caps on the AC input. (110 VAC @ a few mA in 220V land should get someones attention) Ive seen someone stick 2 power supplies in an enclosure, leave the base one ground referenced, and feed the board of the 2nd one, on isolated posts, with the post-filter AC, to get a rather beefy 24V supply. The enclosure was grounded, so it all should have been safe, but its not a trivial task.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 29, 2013, 02:47:33 am
content: Quote from: Monkeh on March 25, 2013, 08:41:24 pmThe 6 core i7 is speced for 135A typical full load and 165A peak. But that's the draw from the voltage regulator on the motherboard. The draw from the power supply is more like 10-15A.Any good PC power supply should be very good with noise, certainly much better than most wall warts.

author: Rerouter
author_rank: Super Contributor
timestamp: March 29, 2013, 09:29:43 am
content: Quote from: NiHaoMike on March 29, 2013, 02:47:33 amIts fair to note the power supplies people salvage for these tasks are generally the absolute budget bin models, and the noise will either exceed the standard slightly (cheap clone with faked label) or be right on the borderline of the specs it was created to meet (1995 ATX supplies would not bee to uncommon in this practice)

author: Monkeh
author_rank: Super Contributor
timestamp: March 29, 2013, 04:26:07 pm
content: Quote from: NiHaoMike on March 29, 2013, 02:47:33 amWould you try that again, this time without filtering out the word 'most'?

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 29, 2013, 08:29:57 pm
content: Quote from: Monkeh on March 29, 2013, 04:26:07 pmThe desktop 4 core Ivy Bridge i7 is speced at 85A typical full load and 112A peak. That includes the 3770, a common desktop CPU. (Mobile CPUs, of course, very rarely exceed 100A, but mobile CPUs generally do not run from ATX power supplies.)

author: Monkeh
author_rank: Super Contributor
timestamp: March 29, 2013, 08:37:46 pm
content: Your idea of a common CPU is most amusing.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 29, 2013, 11:09:24 pm
content: This sort of discussion is what beer, trees or jogging was invented for, pick your poison, in fifty years CPU specs will be totally different anyway.

author: americanhacker1
author_rank: Newbie
timestamp: March 30, 2013, 04:02:31 am
content: I am working on one right now its my latest project but I just repurposed the molex connectors to just sit in my desk labeled 12v 5v and 3.3v and another connector was used to ground it has 3 inputs running back to one cable and the basic Idea I had was to mount it on he backside of my desk so I doesn't take up an needed room there is just one cable that runs from the wall into the unit it took some modifying and wondering what to do but in the end it all seemed to work out.