Are ATX power supplies based bench power supplies "good"? - Page 1
Forum section: electronics / beginners

author: edude03
author_rank: Newbie
timestamp: November 28, 2010, 12:40:22 am
content: I'm considering building one mostly for use for experimenting and testing. I doubt I need a high end bench power supply but I wonder if a ATX based power supply is good enough?So question is should I build a variable ATX based power supply or should I shop around for a "real" bench power supply?

author: allanw
author_rank: Frequent Contributor
timestamp: November 28, 2010, 02:53:27 am
content: They work fine (considering they need to power your motherboard), but you probably want to add an current limit to it. The amount of current they can put out in a short circuit is pretty ridiculous and will easily fuse whatever shorted them out.

author: migsantiago
author_rank: Frequent Contributor
timestamp: November 28, 2010, 03:12:02 am
content: I'm using an ATX for my tests. I must say that its outputs are noisy, not clean enough. I have to build a second power supply in my tests in order to clean the atx supply.I also added a 15A fuse just in case of accidents.

author: Hypernova
author_rank: Unknown
timestamp: November 28, 2010, 04:00:12 am
content: They have the distinct advantage of a powerful 12V rail so if you need lots of power cheaply they are great, also makes a decent foundation to build on to step down the 12V to others. However they are switchers so yeah noisy.

author: the_raptor
author_rank: Regular Contributor
timestamp: November 28, 2010, 10:41:16 am
content: Quote from: migsantiago on November 28, 2010, 03:12:02 amThere is a difference between "MagicWonder 400 W ATX PSU" and a proper name brand PSU*. When I was a screwdriver monkey I had lots of fun fixing intermittent faults caused by crappy low end noisy PSU's. As a repair tech though generic PSU's are your greatest friend, they tend to die to the mildest power surge. Bad PSU's and viruses from porn sights used to be most of my work.*My current PSU has kept my computer going through brown outs that turn the lights off. Generic PSU's will power cycle when there is a brown out in the next state.

author: Bored@Work
author_rank: Super Contributor
timestamp: November 28, 2010, 11:06:26 am
content: PSUs as a bench power supply are a fire starter risk. Do you haven an idea what 15A, 20A or more can do on your bench? Expect red glowing lab patch cables and vaporizing hookup wire, while the PSU still and happily pumps out its juice.

author: saturation
author_rank: Super Contributor
timestamp: November 28, 2010, 01:27:53 pm
content: As mig says, the outputs are very noisy, not bench quality. Between a SMPS and a linear of bench quality, the linear is often cleaner and cheaper up ~< 3A, and the price drops quite a bit for 1A or under. For higher amps, SMPS can deliver bang for buck ~> 3A.For high voltages, look for bio-medical power supplies, keyword 'gel electrophoresis power supply'. They are often discarded in large amounts after a project, and they are capable of over 500-1000Vdc, and often cheap, delivering 500mA to 3A. You have to be careful around them, because these are truly lethal voltages, and you'd have to take CAT III precautions around them.The reason also they are being dumped is that most labs are going for programmable ones [ second hand ones are often manual] with more automatic high voltage safety as they are using higher voltages [ 3-6kV], for improved protein or DNA separation.I got one on eBay that delivers 3000Vdc, 3A, and 1000W for $10. I've never tested it above 900Vdc, the limits of my Fluke. The output is stable up to 10mV, sometimes to 1mV but that's it.My el-cheapo $70 Mastech bench linear PSU is stable down to 10uV-100uV, which I find very impressive. I use this and my original LPSU that delivers up to 1A, but so clean, you can eat on it. For most electronics work, 15-30Vdc to 1A is all you need, but clean is necessary.Quote from: migsantiago on November 28, 2010, 03:12:02 am

author: migsantiago
author_rank: Frequent Contributor
timestamp: November 28, 2010, 03:39:29 pm
content: Quote from: BoredAtWork on November 28, 2010, 11:06:26 amYou are right. I should get a smaller fuse.

author: Unknown
author_rank: Unknown
timestamp: November 28, 2010, 04:02:53 pm
content: Agreed with the previous posts. Keep in mind that most power supplies need a minimum load at 5V and 12V to keep them stable, which requires large power resistors. I would only consider a computer power supply if I'd need large amounts of current and 12V, with no concern for noise, which is what they were designed for. I would use a high-quality one in that case, not a cheap one with most protection circuits omitted and components that are too weak to carry the max current (see for example the JonnyGuru reviews of some cheap PSU's that blow up below the rated load current).If you want to use it as regular lab supply, you have to add voltage regulation, current limiting (not sure if a fuse is fast enough, the instantaneous current will still be tons of amps, plus I consider an adjustable limit mandatory), filtering and load resistors. By the time you're done, you might as well build or buy a linear supply, since your circuit is unlikely to be able to deliver more than a few amps (unless you use an adjustable switching regulator or have tons of cooling). The 12V upper limit (minus drop over your regulator) is a bit low. Don't think about using the -12V, it can only deliver a tiny amount of current for RS-232. Below 3A or so, a linear supply will perform better, it's not like an ATX supply is even smaller or lighter than a 12V/3A transformer.

author: Chris_R
author_rank: Contributor
timestamp: November 28, 2010, 04:22:56 pm
content: I'm just really repeating what everyone else has said but unless you have a good reason for requiring more than 1A then I wouldn't recommend you use an ATX but look for a proper bench power supply. You'll get a much cleaner output on it, higher precision and most importantly it's much safer.You can probably find a bench supply quite cheap, but you are likely to get stung on the postage as they tend to be pretty heavy things!In the off chance that you do need more than 1A (which if you're doing anything that doesn't physically move is unlikely) then make sure if you use an ATX that you have some extra equipment about to clean up the signal, make sure you add some protection into your design, make sure your equipment can handle the power output from a big supply and make sure you buy name brand!Oh and also keep a fire extinguisher handy (from experience, not my design I'd like to point out though!)

author: migsantiago
author_rank: Frequent Contributor
timestamp: November 28, 2010, 06:35:31 pm
content: I'm attaching some noisy pics of a 3.3V ATX power supply. The first two one are with an LED load. And the last 2 ones are with a 2.9ohms load.Vpp noise can almost reach 150mV.

author: Unknown
author_rank: Unknown
timestamp: November 28, 2010, 07:14:29 pm
content: Power for anything important like CPU or memory goes through another DC-DC converter anyway, so why spend time and money feeding this converter really clean power? This is also the reason why modern systems mainly use the 12V rail, since it's more efficient in terms of IR loss.

author: Simon
author_rank: Unknown
timestamp: November 28, 2010, 07:24:00 pm
content: true, due to the large currents required at low voltages they switched from directly supplying 3.3 or 5 V to converting 12V to 1.25 for the cpu and 1.8/2.5 for the ram, I would guess that the 3.3 and % volts are used directly (think disk drives) so hopefully these are decent in psu's but yea the 12V either runs a disk motor or is converted down onboard. find an old psu and you'll find that the amperages for 5/3.3 and 12V are reversed

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 28, 2010, 08:24:30 pm
content: I don't know why you are all crapping on PC PSU's, for the money they are pretty dam awesome. The ones that I use generate only about 20mV of noise on the 5V and 12V rails, most of that being 60Hz. If you're worried about the 20mV a few decoupling caps or an LC filter will take most of that noise out no problem. When you consider the price of PC power supply ($0-$20) compared to a benchtop power supply ($100-$400), the PC power supply is a pretty sweet deal.

author: Bored@Work
author_rank: Super Contributor
timestamp: November 28, 2010, 08:45:59 pm
content: Quote from: TopherTheME on November 28, 2010, 08:24:30 pmThey are they wrong tool for the job. It doesn't matter if they are cheap.An 860W PSU (a small one by todays standard), can pump out > 60 A continuously at 12 V. If something goes wrong when you connect it, everything gives in. Everything, but your PSU. Your board, your wires, your meter. Wana see you cables glow, your parts catch fire and your board becoming a piece of coal? No problem, these PSUs can make it happen.

author: Simon
author_rank: Unknown
timestamp: November 28, 2010, 08:53:23 pm
content: well I still get away with 400W although 500W may be a good upgrade, most psu's are full of marketing anyhow it's getting about as bad as PMPO watts for pc speakers

author: Unknown
author_rank: Unknown
timestamp: November 28, 2010, 08:57:46 pm
content: Quote from: TopherTheME on November 28, 2010, 08:24:30 pmPower per buck, sure, if you need the 20+A.Quote from: TopherTheME on November 28, 2010, 08:24:30 pmGood bench supplies are <1mV or so. Most ATX supplies generate fairly broad-spectrum noise, from the switching frequency and up. I'm surprised if 60Hz is the most significant frequency, sure there's not some kind of low-pass effect in your test setup? 60Hz would be easy to filter, broad-band noise not so much. Large capacitors (you need large caps to smooth large amounts of current) at the output also increase instantaneous current in case of short circuit, since the cap will discharge into your circuit. But since ATX supplies are designed as arc welders anyway, this may not be an issue for you.Quote from: TopherTheME on November 28, 2010, 08:24:30 pmThere's something wrong with your pricing. A high-quality ATX power supply (with acceptable noise and protection) is much more than $20, probably at least $60. A basic lab supply (eg. Mastech), something like 15V/1A, would be about the same. A lab supply has lower noise, adjustable voltage, adjustable current (i.e. less chance of your circuit blowing up in your face if you make a mistake), and has nice connectors, displays, knobs and a case. Yes, you can get used ATX supplies for less, but I also see plenty of cheap used lab supplies. Adapting an ATX supply for bench use also adds cost and complexity.

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 28, 2010, 09:50:50 pm
content: Quote from: BoredAtWork on November 28, 2010, 08:45:59 pm860W is small for a typical PC? No, thats huge. I've got an 850W in my PC and thats way overkill, even with my high end graphics card. Obviously, if your looking to use a PC PSU for your bench you're not going to get a 500W+ unit. I use two 250W units and have never had a single problem with shorts. The moment I short them the over-current protection kicks in the things turn themselves off. All this talk about arc welding and glowing wires is absolutely bananas.QuoteNope, thats strait from the 12V banana plugs with out any filtering. The only load was a battery charger (not charging) and two 10K 1 watt resistors on the 12V and 5V rails.QuoteNot really. You can easily find old PC supplies for free that people just toss or you can get them for under $10 off ebay.http://cgi.ebay.com/SPI-Sparkle-FSP300-60GT-Computer-Power-Supply-300W-/250710915171?pt=PCA_UPS&hash=item3a5f88f863Short the green and gray wires and add a few banana jacks and you're done.I've been using PC supplies and benchtop supplies for probably more than a decade and never had a single issue. Sure, benchtop supplies are a lot nicer but if you're a hobbyist on a budget I really can't see a better option than a modified computer PSU.

author: Simon
author_rank: Unknown
timestamp: November 28, 2010, 10:28:48 pm
content: I think the sort of use has tro be defined, if your powering a large power load you'll get more bang for buck from a computer psu, if your doing bench experiments get a bench power supply

author: migsantiago
author_rank: Frequent Contributor
timestamp: November 28, 2010, 11:02:47 pm
content: Quote from: TopherTheME on November 28, 2010, 08:24:30 pmYou have a nice ATX supply. My supply is so noisy compared to yours (150mV vs 20mV, and high freq noise vs 60Hz noise).I guess it depends on the ATX supply manufacturer. I'm 100% sure that my supply is a chinese one since it costs 20USD more or less. The computer that used to work with it is now dead.

author: Simon
author_rank: Unknown
timestamp: November 29, 2010, 06:39:09 am
content: they are all chinese, some just happen to carry a "name" and cost a lot more

author: Hypernova
author_rank: Unknown
timestamp: November 29, 2010, 02:57:16 pm
content: Quote from: Simon on November 29, 2010, 06:39:09 amForgot the name but there are a still some more premium brands that make them in Taiwan.

author: Simon
author_rank: Unknown
timestamp: November 29, 2010, 05:44:57 pm
content: well granted you'll get a decent one from china if you really want it and are willing to pay, frankly you might as well buy a UK (or your country) made unit if one were to exist.

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 29, 2010, 08:57:45 pm
content: One thing to note about ATX supplies that hasn't been addressed is the accuracy and stability of the rails. For example, the PSU that I measured for the pic in original post has a voltage output of about 11.8V at OCV. This voltage will change depending on the load put on the supply. If its a small load the voltage might go up to about 12V but a heavier load will drop it down to about 10 or so. I rarely require large loads and 99% of my projects use voltage regulators so for me this is not an issue but it could be for some.

author: Unknown
author_rank: Unknown
timestamp: November 29, 2010, 10:04:26 pm
content: Quote from: TopherTheME on November 29, 2010, 08:57:45 pm10V is definitely out of spec. I think it's something like +/- 5% or +/- 10%, depending on which rail. Cross loads (eg. max load on 5V, min load on 12V) may cause the 12V to go up, but a properly designed power supply should stay in spec as long as your load is.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 30, 2010, 08:53:19 am
content: Quote from: alm on November 29, 2010, 10:04:26 pmLatest spec is +/- 5% for the + rails and +/- 10% for the negative.

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 30, 2010, 03:36:52 pm
content: Quote from: alm on November 29, 2010, 10:04:26 pmYes a "properly designed" power supply will, but a cheap chinese supply I pulled out of the recycle bin probably won't. I think the supply I have on my desk in front of me was made in the late 90's and couldn't have cost more than $30 back then too.

author: edude03
author_rank: Newbie
timestamp: January 09, 2011, 11:41:10 pm
content: Sorry I haven't had a chance to weight in myself, (being OP) but it seems the discussion has really taken off.So basically it boils down to: No, a PSU can work in some cases but it isn't the right tool for the job.What I mostly need is a way to power anything quickly, for example a phone that I have no battery for, or a board I just built. If bench power supplies are only $100-$400 as one poster mentioned, then I'd rather pick one up than risk having problems with my home made PSU.Thanks to everyone who commented.

author: sonicj
author_rank: Frequent Contributor
timestamp: January 10, 2011, 02:19:35 am
content: Quote from: edude03 on January 09, 2011, 11:41:10 pmif you have a old psu, convert it! add reasonable fuses on the outputs and you have a safe, stable, cheap source for 3.3, 5, 12, -12 & -5v with plenty available current."What I mostly need is a way to power anything quickly"this is the ATX power supply's strong point; you can power anything quickly! with a bench supply, you can powersomethings quickly. if you are into designing and tuning your own circuits, get a bench supply. if you are into building circuits(kits), arduino, charging batteries, repairing broken electronics, and other general enthusiasts stuff, then the ATX should easily meet your demands.-sj

author: vtl
author_rank: Regular Contributor
timestamp: January 12, 2011, 02:40:32 pm
content: Quote from: BoredAtWork on November 28, 2010, 08:45:59 pmThats a lot of sweeping generalizations there. 860w small? You're crazy 860w is a huge overkill PSU. Around 500w is typical. Hardly any PSUs will pump out 60a on the 12v rail, theyll typically seperate them into multiple 17-20a rails. Besides, who would buy a brand new ATX psu to power their hobby stuff? Usually you grab one from an old comp which will be typically 300-400w (typically 12-17A 12v max)If I wanna see my cable glow I better go grab some LEDs because if I short anything out, the PSU will automatically shut off because of the short circuit protection. I've poked around and triggered the short circuit protection a few times by accident (computer case is earthed), completely harmless.Who cares if its the wrong tool for the job? If it means I don't have to shell out 500$ to power a crappy hobby circuit then so be it.

author: silicon_cockroach
author_rank: Newbie
timestamp: January 13, 2011, 03:16:29 am
content: Quote from: edude03 on November 28, 2010, 12:40:22 amThis is interesting.Correct me guys if i am wrong.Why not build one regulated power supply from scratch ? It is not hard to do,almost anyone who needs one,can actually build one with some help,most of the guys/girlswith no help at all.Just find some old transformer,and suitable box,everything else is so cheep.As mentioned before,AT or ATX supply's are sometimes dangerous,unless you want to power some car amplifier or similar beast.edude03 if it would help,i can send you pictures and schematic for one simple variable power supply which i built some time ago.It serves me well.If you want , just let me know

author: CodeDog
author_rank: Contributor
timestamp: June 25, 2011, 05:44:12 am
content: i built mine into an old ammo case from an army disposal store ... (cheap and rugged)... but I also use a real bench supply as wellcheers,

author: Psi
author_rank: Super Contributor
timestamp: June 25, 2011, 08:48:44 am
content: Quote from: BoredAtWork on November 28, 2010, 11:06:26 amhehe yeah, that's exactly right.They will totally destroy anything that shorts them out without a 2nd thought

author: Zero999
author_rank: Super Contributor
timestamp: June 25, 2011, 10:09:47 am
content: Yes, you need overcurrent protection for thin cable.It's probably a good idea to have a power distribution PCB going to a few sockets, each protected with a 10A polyfuse, with a 100nF capacitor for decoupling and an LED indicator so you know which has blown. There could also be a couple lower power sockets with a 1A polyfuse for breadboards which won't take much current to melt.

author: saturation
author_rank: Super Contributor
timestamp: June 25, 2011, 11:57:35 am
content: Nice idea, I presume you ground the PSU to the metal casing?Quote from: CodeDog on June 25, 2011, 05:44:12 am

author: PeterG
author_rank: Frequent Contributor
timestamp: June 26, 2011, 10:52:41 am
content: There is an upside to using an ATX SMPS, they are noisy so you can make sure your project can withstand a little noise on its power rail, not a bad thing in my book.My 2 cents worth.Regards

author: Wartex
author_rank: Frequent Contributor
timestamp: June 27, 2011, 04:05:49 am
content: Quote from: Simon on November 29, 2010, 06:39:09 amYou are talking out of your ass. This is my computer PSUhttp://www.hardwarecanucks.com/forum/hardware-canucks-reviews/5438-pc-power-cooling-silencer-750w-quad-power-supply-review-8.html16 mV ripple under high load, 30 mV at 60A load. Show me a bench PSU for under $200 that can do 60A at 12V.It's designed in USA with USA parts, only soldered and painted in China, that doesn't make it chinese. It's a premium PSU will beat some cheap bench PSU in terms of output clarity.

author: Unknown
author_rank: Unknown
timestamp: June 27, 2011, 08:42:22 pm
content: Quote from: Wartex on June 27, 2011, 04:05:49 amYes, if you need lots of power at 12V (grounded) cheaply, it's hard to beat a ATX PSU. Most applications don't need bench supplies that can deliver 60A, however, especially in these days of low power design (how long is your iPhone battery going to last if the phone would draw 60A?). And 60A is a lot of current to kill your circuit if you make a mistake, it will easily vaporize bond wires in most ICs.Quote from: Wartex on June 27, 2011, 04:05:49 amDo they still make components in the USA? Sure, some expensive/exotic parts, but what about standard passives or jellybean analog ICs? The kind found in ATX PSUs?You have to screw up pretty bad to get a horrible output from a linear regulator. Even using a 78xx without any caps should perform OK.

author: ndictu
author_rank: Regular Contributor
timestamp: June 27, 2011, 09:41:29 pm
content: I don't like the term "Chinese". It's a huge country, with diverse fabrication houses. Sometimes the expensive, branded thing and the cheap knockoff are from the same production line.The way it works is some company hires chinese production house for say 8 hours a day to make 100pcs of stuff. This is all made according to their plans, tested and everything. But they probably make a few pieces that won't meet the QC but still could be sold under price. Or, from what I've heard, the entire clothing industry works like this: they make a deal for 8 hours, make that and sell it,but really work for 12 hours, make the exact same stuff but without a brand logo and sell that too, with more profit margin for them.Made in China is stupid, it can be the best, most reliable product you own and it can be a teddy bear stuffed with asbestos.

author: saturation
author_rank: Super Contributor
timestamp: June 27, 2011, 11:17:47 pm
content: You are right since for the most part many good goods we buy are also probably made in China, but its changing as we speak. A problem has been that China has led the world in violating intellectual property, because those factories making products identical to those ordered by the contractor but sell it outside of the contractors supply chain or lack of an official label is still intellectual theft.A process known as the 'ghost shift'.http://money.cnn.com/magazines/fortune/fortune_archive/2006/05/01/8375455/index.htmChina is the #1 source of manufacturing of counterfeit goods in the US, for 10+ years in a row.http://www.cbp.gov/xp/cgov/trade/priority_trade/ipr/pubs/seizure/There is an entire chapter in Wikipedia regarding IP theft in China.http://en.wikipedia.org/wiki/Intellectual_property_violation_in_the_People%27s_Republic_of_ChinaBy comparison, when Japan and Taiwan were emerging in the electronics field, their enormous output in the 1970s through 1990s were not blemished by counterfeit products from the same factories, and moreso, not to the level of individual components, unfinished products or parts, compared to China.Hong Kong, when they were a protectorate of the UK prior to 2000, was a common source of counterfeit goods, but it was not to the scale that, now under China, they are #2 in the world,as a source for counterfeit.Quote from: ndictu on June 27, 2011, 09:41:29 pm

author: westfw
author_rank: Super Contributor
timestamp: September 13, 2011, 09:21:48 am
content: QuoteThat's about it. A "real" bench has variable voltage, variable current limits, and meters. A low-end supply will do up to 30V at 3A and cost less than $200, which is enough for most practical purposes, and a very useful piece of equipment to have (useful for charging most kinds of batteries, too.)A computer power supply has its uses as well, but they're different, and it's NOT a "bench power supply."

author: AlsInd
author_rank: Newbie
timestamp: October 03, 2011, 07:26:31 am
content: in my professional pc build & repair days i was always checking my builds with my scope. even the best pc psus are too noisy for precision electronic lab use but the majority of pc psus out there are cheap crap and really noisy. it is cheaper to buy a used lab psu from ebay than buy a high quality pc psu which still will not give you as clean a power. then you have to also add some voltage and current variable circuitry to it... it just does not make sense. you can get good enough lab psus used for under $50 on ebay... i just got another one, as if i needed it!cheers :-)al...