Computer power supply as bench supply OK?? - Page 1
Forum section: electronics / projects

author: Merlysys
author_rank: Regular Contributor
timestamp: January 05, 2014, 03:52:15 pm
content: Is there a problem using a PC psu for regular electronics design/troubleshooting work? ... for low DC voltages, under 15.They are literally free as many people upgrade PCs giving away the whole box. They provide several amps at 3.3, 5, and 12 each. Only thing they lack is a variable output but connecting its 12V to basic DC-DC board works for most projects.I think most hobbyists and many designers can do without a lab type power supply but they don't realize that an old PC psu will do, or am missing something?Is there a problem with ripple or overload or something else I haven't thought about?

author: don.r
author_rank: Frequent Contributor
timestamp: January 05, 2014, 03:59:52 pm
content: Electrically noisy and could be a disaster if a short was present in your project (no current limit). If you must, use one of these:http://www.seeedstudio.com/depot/atx-breakout-board-bench-power-supply-p-1222.html

author: orion242
author_rank: Unknown
timestamp: January 05, 2014, 04:08:16 pm
content: I use a couple on the bench and like them for what they are. Cheap, quick, have the voltages I need, and usually the outputs are well protected against overload.On the flip side, they can supply 20-30A on the some of the outputs and can fry things up pretty well. You may want to fuse them down to something more reasonable unless you need that kind of power.

author: homebrew
author_rank: Frequent Contributor
timestamp: January 05, 2014, 04:36:35 pm
content: If it is just used to permanently power a 12V device it is certainly ok but I would never use a PSU like this as lab equipment for several reasons:(1) Not designed for that use: As don.r said, no current limit (well yes, some 20A but you can't consider that as a limit ...). Also most PSU need a minimum load to be able to control the output voltages at all. They are not very accurate on regulating all channels, if a load is only connected to one rail (PC-SMPS only regulate the total power entering the transformer core ...)(2) Output is mains earth referenced. The common ground is directly connected to the metal case. That might introduce a whole range of ground loop problems ...(3) Not safe enough in my opinion: The primary and secondary windings of such SMPS are only separated by some few layers of mylar tape (if you are lucky and got a proper one ... otherwise there might only be a single layer.) - nothing compared to a proper mains transformer with separated windings, each in its own chamber on a mains earthed core.If you fiddle around with the supply turned on and you have inductive loads like coils or motors you might easily inject a couple of kV back into the supply ...Remember, you have to trust your life on such a lab PSU!I'ld rather build myself a supply from scratch before using a PC one ...

author: calexanian
author_rank: Super Contributor
timestamp: January 05, 2014, 07:33:08 pm
content: If its all you have i would at the very least use a couple of fuses for short protection and propably build a filter for the output. But for just power it up and see type of thing, if its all you got then its all you got. I would recommend even one of those $80 18 volt 3 amp variable ones off of amazon before a compy psu. They are good for driving motors though where noise is not an issue.

author: shims506
author_rank: Contributor
timestamp: January 05, 2014, 07:45:08 pm
content: Starting out its not bad of a project to convert your pc psu into a desktop power supply. But, I would recommend later on either building your own from scratch or finding some on auction because the pc psu has a lot of noise and some dont hold a stable voltage sometimes (at least mine kept jumping up and down).

author: calzap
author_rank: Frequent Contributor
timestamp: January 05, 2014, 07:51:18 pm
content: Here's a link to a JumperOne video on how to do it:I haven't watched it in a while, but seem to recall it was pretty good.Mike in California

author: pickle9000
author_rank: Super Contributor
timestamp: January 05, 2014, 08:01:45 pm
content: Current limiting is a must as far as I'm concerned.

author: homebrew
author_rank: Frequent Contributor
timestamp: January 05, 2014, 10:23:23 pm
content: Quote from: calzap on January 05, 2014, 07:51:18 pmOh man,he even drilled through the case with the PCB still installed. Fine metal parts inside an SMPS ... but hey, fireworks are ok around new year :-)The heat shrinking is good and absolutely required as the wires reach into the HV region of the supply. The heat sinks are often not grounded or even at mains voltage potential ...The safest thing would definitely be to bond the wires to posts outside the PSU and leave it untouched.Just be careful ...

author: Skaface
author_rank: Contributor
timestamp: January 05, 2014, 10:30:02 pm
content: incase you are unaware, theres a resistor on the controller IC which can be swapped out for a pot which will allow you to set the output voltage too. I had a setup like this and it worked well for my purposes

author: homebrew
author_rank: Frequent Contributor
timestamp: January 05, 2014, 10:36:15 pm
content: Quote from: Skaface on January 05, 2014, 10:30:02 pmYes certainly but it won't give you control over a single rail. You can just adjust for all rails together. Thus decreasing the 12V rail will also decrease everything else, as the voltage differences are determined by the secondary turn ratios on the transformer.You won't be able to go down to 0 Volts as the secondary control circuit would stop to operate and the upper voltage would also be limited by the component's (like max voltages on caps and diodes...)

author: richard.cs
author_rank: Super Contributor
timestamp: January 05, 2014, 10:56:44 pm
content: Quote from: Skaface on January 05, 2014, 10:30:02 pmIn addition to the things homebrew has pointed out most of these PSUs have an overvoltage shutdown that operates at 10-20% above nominal output voltage. It can be defeated of course but it's another thing to consider. This one usually hits people who are modifying them to shift the 12V rail towards 14V for lead/acid battery charging.

author: calzap
author_rank: Frequent Contributor
timestamp: January 06, 2014, 01:00:19 am
content: Probably the safest way to do it is hang a small box with binding posts on the side so it doesn't block vents; then bring the output wires into the small box and connect them there. Now I remember wincing when he did the drilling in the video. I would have made liberal use of a plastic bag or two and masking tape to keep the metal chips out of the circuitry. Years ago I was working on a computer (it was switched off) and dropped a screw on the motherboard. Of course, it instantly disappeared. Did not dare to to switch on the computer until it was found, but finding it involved removing the daughter boards, power supply and drive stack and doing a lot of shaking and swearing.Mike in California

author: mdszy
author_rank: Unknown
timestamp: January 06, 2014, 01:07:04 am
content: My very first electronics project was actually to convert an old PC PSU into a bench supply. It's honestly the only supply I have at the moment and it works beautifully well for me and what I've been doing with it (which is not much, really). After shorting it out a few times, I've found out that my particular PSU has really good overcurrent protection and will just turn itself off.For super basic uses, I'd say it's alright to use a computer PSU as a bench one, but I'm sure a real PSU would top it in every aspect.

author: G0HZU
author_rank: Super Contributor
timestamp: January 06, 2014, 01:17:55 am
content: I would not want to use one for a bench supply but I know plenty of people do use them.Partly why I would never entertain this is because I have seen what can happen when they fail. i.e. you sometimes get flames from them.So I would suggest you keep it away from anything flammable (and put it all inside a bigger metal box) or maybe try googling for:PC Power Supply FlamesIt might make you change your mind?

author: orion242
author_rank: Unknown
timestamp: January 06, 2014, 01:27:51 am
content: Rather than toss one out, why not reuse them as a bench supply? For most uses they work great and with a few added parts will serve you well.I mounted mine under the bench, and whacked all the connectors off. In standard electrical boxes I wired all the rails together and to a set of speaker banana jacks. Something likehttp://www.homedepot.com/p/GE-Single-Brush-Speaker-Wire-Wall-Plate-White-87693/202829295.This is mounted in the surface of my bench. I have a switch to toggle it on along with glass fuse holders fusing down the outputs a bit, and a power led. Its served me well enough for the few bucks I have stuck in it and seems to regulate all the rails just fine.I also have a switch on the surface of the bench that kills all the power on it. So when I leave, everything is powered off in one spot. No leaving on soldering irons, sketchy high power circuits, etc.I trust them enough to power my PC....why not my small circuits for testing?

author: mdszy
author_rank: Unknown
timestamp: January 06, 2014, 01:37:43 am
content: Quote from: orion242 on January 06, 2014, 01:27:51 amExactly!

author: Stonent
author_rank: Super Contributor
timestamp: January 06, 2014, 01:42:24 am
content: I wonder why someone just doesn't come up with an ATX box that you mount the PSU inside, plug in the ATX cable and have a front panel with on/off switches.Maybe throw some current limiting in.The 750W PSU I was dealing with the other day at work had a 60A rating on the +12 rail.

author: Dave
author_rank: Super Contributor
timestamp: January 06, 2014, 01:44:57 am
content: Quote from: orion242 on January 06, 2014, 01:27:51 amBecause they are capable of spitting out 30A+ on a 12V supply rail and accidentally shorting it would probably result in smoldering components.

author: orion242
author_rank: Unknown
timestamp: January 06, 2014, 02:11:19 am
content: Some sure will. The one on my bench will shutdown before blowing my fuses (15A) on a good short. Its pretty uneventful, not even a big spark. Others will go up in smoke on a short.None the less I pull the supplies out of every PC before death and sort the good from crap and use them.

author: calzap
author_rank: Frequent Contributor
timestamp: January 06, 2014, 02:30:58 am
content: I can see repurposing a computer PSU for appropriate, dedicated uses, but one would have to be hard-up for cash to use one as a general purpose bench supply. Really nice, used, 2-3 channel, digital display, current and voltage limiting PSUs from companies like HP, BK, GW Instek, etc. appear with regularity on eBay, and can be had for US$100 or less. I have a three channel GW Instek that sits in a box under my bench as a reserve. Cost $100 and looks virtually new, like it set in the closet of a tech firm that went bust. One of my bench supplies is an HP that also cost $100 and looked like it had never been switched on when it arrived. Takes some patience and skill at searching, but it's worth it.Mike in California

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 06, 2014, 02:41:02 am
content: Quote from: Merlysys on January 05, 2014, 03:52:15 pmPC psu is a good power source in term of power/weight, its main drawback is high noise leakage. forget about any other -ve stated by others, you can build linear front end for it and solve all the problems stated, except the high noise leakage, its the nature of SMPS i guess if someone can give a better solution i would like to hear, i cant think of any except maybe to put it in a thick Pb casing or any metal with good noise blocking. but if you dont care about the noise and risking blowing your stuffs, PC psu is 100% perfect. short answer, it depends on what you want to do...

author: mariush
author_rank: Super Contributor
timestamp: January 06, 2014, 02:43:09 am
content: Yeah, for you guys it's easy in US, because of surplus stuff and cheap shipping. Here in Europe there's few such power supplies and as they're heavy it costs a lot to ship them from country to country (in the $50+ range to ship them).On the other hand, you can buy a really crap chinese 450-500w atx power supply that can do an honest 150-200w for around 20$.I bought the power supply in the pictures below inside a computer case for around 35$... no way it's doing 450w but it can do 200w spread on the 3.3v, 5v and 12v:(now this aside, i know the problems a smps power supply has and I have a proper adjustable power supply on the workbench)

author: kolonelkadat
author_rank: Regular Contributor
timestamp: January 06, 2014, 05:04:31 am
content: I really wanted to do this. I had a box of old pc psus just begging to be repurposed. It seemed like a no brainer.Then I started price checking my BOM. binding posts, poly fuses, load resistors, etc. All told, it wound up something retardedly expensive, like 40 or 50 bucks. Then I still had to spend an hour or two assembling it...Its really not worth doing if youre going to do it right. With the time and money you invest into it, youd be better off mowing a couple of lawns or changing some peoples oil to get a few extra bucks to buy a proper lab supply.

author: don.r
author_rank: Frequent Contributor
timestamp: January 06, 2014, 06:38:41 am
content: Quote from: kolonelkadat on January 06, 2014, 05:04:31 amSee my reply to the OP. $14.

author: kolonelkadat
author_rank: Regular Contributor
timestamp: January 06, 2014, 06:50:43 am
content: nope. thats not even close to what i consider "done right "

author: don.r
author_rank: Frequent Contributor
timestamp: January 06, 2014, 07:03:34 am
content: Quote from: kolonelkadat on January 06, 2014, 06:50:43 amInteresting because it has all the features you mentioned. Personally, I still think the money would be better spent on a proper current limited supply.

author: BravoV
author_rank: Super Contributor
timestamp: January 06, 2014, 07:20:13 am
content: Simple summary :- I know EGGGGXactly what I'm doing, I understand the risks, I understand it's limitations, and the upmost important I "FULLY" understand how the load behaves.Highly recommended.- I'm not very sure, but Googled or watched Youtubes there are tons of these similar setups and they're very convincing, hence I'm going to use the PC PSU to power my (non PC) project/circuit/load and etc.Not recommended.

author: Icarus
author_rank: Regular Contributor
timestamp: January 06, 2014, 12:46:23 pm
content: Guys, why don't we as "people of the forum" design a hybrid lab power supply. Switching supply followed by linear reg.It'll be very challenging but fun too.Let's say1kW Hybrid2 Floating Output.CC and CVExtremly fast transient repsponse (Overshot 10uS max. 100mV max)Digital Display ? OptionalMoverover we should use jellybean parts as much as we can. So single IC solutions are no good.What you say ?

author: don.r
author_rank: Frequent Contributor
timestamp: January 06, 2014, 03:17:43 pm
content: Quote from: Icarus on January 06, 2014, 12:46:23 pm1kW is about 10 times more power than 99.9% of people need on their bench. The whole point of using an ATX PSU is that they are cheap (sometimes free).

author: orion242
author_rank: Unknown
timestamp: January 06, 2014, 05:15:46 pm
content: Quote from: Icarus on January 06, 2014, 12:46:23 pmWould be pretty slick if it could just use all the standard ATX connectors plop in the PSU and added CV/CC, linear regulation and the digital meters. I like the idea if the costs don't hit the moon.1KW is alot of power to deal with. I assume your not thinking about CV/CC on the full power output?

author: Jon86
author_rank: Frequent Contributor
timestamp: January 06, 2014, 05:40:04 pm
content: Quote from: Icarus on January 06, 2014, 12:46:23 pmLol good luck, When would you even need that kind of power at low voltages anyway? There's no way you're going to be able to properly regulate 1KW of power with CC/CV, at least not cheaply.

author: calzap
author_rank: Frequent Contributor
timestamp: January 06, 2014, 09:15:20 pm
content: When not being used as a bench supply, it could be used for arc welding.Mike in California

author: Icarus
author_rank: Regular Contributor
timestamp: January 07, 2014, 12:36:23 am
content: Once again I spoke before I thought.Well I've been crunching some numbers and It'll be expensive500W, 2-50V, 0.1-50A, CC CV Approximate cost $250 USD (if we buy components with retail prices)Block diagram will be something like this.It's too much expensive for hobbyist market

author: mariush
author_rank: Super Contributor
timestamp: January 07, 2014, 02:08:05 am
content: If you want something cheap, you can go with lots of small transformers like these:http://uk.farnell.com/pro-power/ctfc75-9/transformer-75va-2-x-9v/dp/1780861about 16$ each, two independent 9v ac secondary windings, each should be good for about 3a, so you get about 10-11v DC with each winding.Get 2 of these and use some relays to get 40v 3A, or 10v 12A, or 20v 6 amps etcCould be done for under 50$, the price of "value" switching power supply.

author: philpem
author_rank: Frequent Contributor
timestamp: January 07, 2014, 08:08:39 am
content: The big problem as I see it, is that most bench power supplies are single-output. When you've got a design which wants +5V (digital logic) and +/-12V (analog) you tend to lose a lot of bench space to power supplies.

author: Jibby
author_rank: Contributor
timestamp: January 08, 2014, 10:01:08 am
content: Using a computer PSU is good but for variable output use LM 317 at the 12 volt output.