ATX Power Supplies - Page 1
Forum section: electronics / testgear

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 20, 2020, 10:11:21 pm
content: As I put the finishing touches on my ATX power supply conversion, I'm still trying to sort out what I sense to be a (very) strong bias against ATX power supplies here. I'm trying to figure out to what extent the apparent bias is, well, just personal bias, and to what extent it is justified and based in fact.Now one of the big concerns seems to be that the ATX supplies don't have current limiting, and therefore your expensive circuit can get damaged. But isn't that a bit like saying "Wrenches make crappy hammers". Well, yeah, it's presumed you know enough to use the right tool for the job, and AFAIK nobody ever claimed that an ATX supply includes current limiting. So don't use it where current limiting is important.On the other hand, a decent ATX supply has internal protection to shut off overcurrents over their set point. As well as overvoltage and other protective elements, depending on whether it's a piece of junk or name brand that follows (or exceeds) ATX specs. As I've shown before, I've done tests to show that my supply (and presumably other name-brand ATX-compliant supplies) has supervision circuitry to instantly shut off the supply at or below rated current. No spikes (unless they last for 10mSec or less and I missed them...). So we're back to knowing the specs of your power supply and applying it appropriately. Like you do for any other piece of equipment.So are people actually saying "never, ever, ever use an ATX power supply on your bench or else worlds will collide, and there's absolutely NO possible use case for you to ever justify using a converted ATX supply", or is it more like "yeah, they're fine when appropriate, as long as you consider their limitations, just like you do with every other piece of equipment".Is there something I'm missing? They're fine for many gazillions of computers around the planet, and assuming you buy one with all of the important certifications and protection (unlike some of the equipment many of us use on our benches), isn't it just a matter of making sure you don't do something dumb and apply it in the wrong situation (like on a $10,000 circuit where you need an appropriate power supply with current limiting in case you do something dumb?).Personally, I just used mine to test/calibrate a high current sensor, while my main power supply was busy. Worked fine. Was that a mistake?

author: tautech
author_rank: Super Contributor
timestamp: April 20, 2020, 10:14:11 pm
content: They have their uses sure but NOT as a precision bench PSU.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 20, 2020, 11:19:09 pm
content: Quote from: tautech on April 20, 2020, 10:14:11 pmI suppose it depends on how you define "precision", doesn't it? Mine was "precision" enough to help me calibrate a current sensor. I assume you're referring to more professional-oriented requirements for high precision to meet industry specifications, etc.?

author: Nusa
author_rank: Super Contributor
timestamp: April 20, 2020, 11:28:17 pm
content: Just don't call them bench or lab supplies in forums like this, as that triggers people that know what a real bench supply is, and how to use it.Sure, it's fine as a general purpose constant voltage supply for higher-current projects. But for many projects, it's like using a sledgehammer on a pushpin and then wondering why there's a hole in the wall.I wouldn't use it for breadboard projects...12 volts at 50+ amps for 10 ms (not "instantly"!) applied to the wrong pin by mistake is going to be fatal to a lot of components. And that's for a dead short. If there's some small resistance in the circuit, you've got a power supply that'll happily pump 40 amps until the smoke is released from something.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 20, 2020, 11:35:04 pm
content: Quote from: Nusa on April 20, 2020, 11:28:17 pmThanks. I'm curious of anyone has any references of actual test data for decent, name brand ATX supplies that shows what the actual transient overcurrent/overvoltage can be? Because my testing shows it can barely get up to its rating before the whole thing shuts down. No transient overcurrents at all. Again, my tests had 10mSec sample resolution and showed nothing. Hypothetically something could have happening in-between a couple samples, but that seems a big stretch. Something like 16A(?) rating for 12v rail, and it shut down around 12A.Has anyone actually done tests to show these high currents? Maybe I'm missing something?

author: masterx81
author_rank: Frequent Contributor
timestamp: April 20, 2020, 11:39:08 pm
content: I've used for years an atx psu ( 2003 ic) modded with voltage and current limiting (sense resistor, opamp and comparator). Worked really nice. Then i've bought the entry level bench psu korad kp3305... it's another thing. More precise, more stable on transients, almost no ripple... And for who use "true" bench supplies the korad is a toy. I trust it to power sensitive boards (as dual i can do both 3.3v and 5v), while with the atx i was not so confident and i was using always some linear regulation after the switching.The bigger thing that i miss is the 20a capability, but i'm planning to buy a dps3020 for that.Would be nice also an rd6006 but is almost redundant with my current setup.

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2020, 08:06:29 am
content: Quote from: engrguy42 on April 20, 2020, 11:19:09 pmNot at all.Without the ability to limit current an ATX supply can not only vaporise components but PCB traces too. Not only will there be risk of injury and that could to eyesight or burns to the person. Breadboards have been mentioned too and they could be damaged with high and uncontrolled currents also.Those with enough experience can mitigate some risk however the novice with limited knowledge and experience can be injured or permanently damage DUT's using uncontrolled current PSU's so as a responsible forum we are best to advise the use of user adjustable current controlled PSU's as we have absolutely no idea of the experience of our readers.It was not long ago the dangerous advice of how to float a scope was freely given as at one time it was common industry practice whereas today for just a couple of hundred dollars purchasing a differential probe rightly consigns that dangerous process to the foolish and to history....and yes, at one time I did it too !You are not the first here to extol the virtues of ATX PSU's and certainly not the first to be told forgettaboutit !The first half decent PSU I built used a LM338 5A regulator capable of 7A peaks but as it was otherwise uncontrolled current I rarely use it now instead opting for a SPD1305X precision PSU that more than suits my quite basic needs.For a while I used only a MCH K305D, a tiny 30V 5A SMPS however as these are somewhat electrically noisy it's been relegated to mostly charging duties.At one time I used the 3 channel SPD3303X-E hacked to the X model but customers keep buying them off me rather than wait weeks for more stock to arrive.

author: madires
author_rank: Super Contributor
timestamp: April 21, 2020, 09:15:55 am
content: An ATX PSU is fine for powering your PC or something else which needs lots of power. Imagine you are repairing a laptop with a power problem (doesn't turn on, no LEDs lit). A common cause for that is a shorted component, like a MLCC for example. Without current limiting the PSU will push as much current through the laptop's board as possible. Components in the power circuitry will heat up, might blow up, and PCB traces will get hot or even vaporize. With current limiting you'd limit the current to a reasonable value and look for the shorted MLCC without damaging the board.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 10:42:04 am
content: Thanks. So far it seems, again, the major issue people have with ATX power supplies is that they don't have current limit capabilities. Agreed. They don't.Just like a crescent wrench "sucks" because it doesn't drive wood screws.But other than that, does anyone have any actual data (not just opinion) to show that ATX power supplies are dangerous and worthy of mock and scorn and ridicule?Personally I don't care whatsoever about ATX power supplies. I don't like them or dislike them. They're just another tool, and I use them where appropriate. And I don't get "triggered" (as someone here mentioned) if someone dares to discuss them. Just looking for facts, to see if there's some limitation/danger I'm missing and need to protect for on my bench.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 21, 2020, 10:54:24 am
content: So is there a frugal way to get some limiting on ATX PSUs?lamps in series, ballast trannies,wet feetetc

author: BravoV
author_rank: Super Contributor
timestamp: April 21, 2020, 11:24:45 am
content: Quote from: engrguy42 on April 20, 2020, 10:11:21 pmIts fine if it fits in at your existing tasks, its just do NOT be too over confident ... or should I say be naive that a PC PSU, is all you need for more "serious" electronics tasks, thats all.As this is like reading a boy who can heat up a big nail using gas lighter, and did some simple soldering by melting some solder at PCB, and yelled proudly that he has the ultimate soldering tool, and denying the need of advanced and expensive soldering tools, as he is so confident that only using that nail only, can properly solder/de-solder SMD components.Hint, forget about the owners of the PSUs, just look at the bench PSUs collections in my sig at below right, and think why the hell all those manufacturers built them from the 1st place and reasons for that.My 2 cents.

author: Whales
author_rank: Super Contributor
timestamp: April 21, 2020, 11:34:14 am
content: Use whatever is appropriate to your tasks. I'm sure a good 50% or more of this forum wouldn't approve of my lab equipmentI'M HERE TO DONATE SOME BLOOD

author: Nusa
author_rank: Super Contributor
timestamp: April 21, 2020, 11:39:14 am
content: Quote from: Electro Detective on April 21, 2020, 10:54:24 amIf you mostly care about the oh-shit scenario, a suitably low value in-line fuse on the PSU output is cheap. Remember to have spare fuses, or you'll be tempted to eliminate it after the first oops.If you want real limiting, google "dc current limiter" and see what works for your situation.

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 21, 2020, 11:58:30 am
content: Quote from: engrguy42 on April 21, 2020, 10:42:04 amI don't understand why you are looking for data ?They are already there in the datasheet.In case of problems ( HS component, short circuit, handling error, tiredness), there is no current limiting to remain within reasonable orders of magnitude.Even if the current protection is very fast on ATX power supplies, there will probably be something along the way that will create resistance.So you will to stay below the max current of your ATX power supply and the OCP is not going to trigger. ( >40A on the +12V rail for mid range 500W ATX PSU ).You're just going to have a lot of smoke and/or an exploding component.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 12:26:18 pm
content: jemangedeslolos, are you saying that an ATX supply's OCP protection will only operate if the TOTAL of all the 12v rails exceeds the total current rating of all those rails?For example, my ATX supply has 3-12V rails, rated 8A, 16A, and 18A. As I've posted before, I actually tested this and applied current to one of those rails. The power supply shut down at 12 amps (see attached), not >40A.Maybe my test was in error? Or maybe it depends on the quality of the power supply and whether it fully complies with ATX?In any case, as I've said many times before, yes, an ATX power supply does NOT have current limiting, nor is it intended to.What I'm looking for is OTHER issues that cause people to dislike them so I can make a factual list of limitations.

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 12:42:37 pm
content: Use appropriately rated fuses on the outputs for basic circuit protection or get as elaborate as you want. As the supplies are normally mounted inside a case add some psu mesh to prevent screws and tools from finding their way into the air/fan vent. As always check the ground/earth is properly connected and use eyewear for soldering, cutting andblowing uptesting electronics.Saying you must have this or that to do electronics is a crock of shit. You can get by with very little to start off with. A proper adjustable bench supply is handy for obvious reasons but people make their own from parts or modify or use what they have, hobby electronics in a nutshell. In the US there is also a ton of surplus bench supplies on the market so I'd consider that first before dropping hundreds on a new supply.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 12:47:31 pm
content: Quote from: Shock on April 21, 2020, 12:42:37 pmCareful...when I suggested using fuses for ANY important circuits to allow for the fact that ANY power supply can fail, not just ATX, I got bombarded by judgemental finger-wagging. "Yeah, but what if your components can't handle the transient current before the fuse has a chance to operate and your entire city explodes??".

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2020, 12:51:54 pm
content: Quote from: engrguy42 on April 21, 2020, 10:42:04 amNope.The problem is people calling them "bench power supplies". They're not, they just "power supplies" (even if you place them on wooden benches).Avoid using the word "bench" and you'll be fine.

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 21, 2020, 12:52:06 pm
content: Quote from: engrguy42 on April 21, 2020, 12:26:18 pmAll ATX PSU have their own architecture.I have one Seasonic with just 1 x 12V rail (70A on the 12V) and one Be quiet with 4 x 12V rail.Even with only 10-12A on 1 rail, it is enough to blow up your logic board, cause cancer, kill seals and spread a global virus.PC power supplies are made for PCs ( crazy right ? )There is no need for 0.000000001% load and line regulation, ripple and noise is terrible and you have no UI.I don't know how it is possible to have precision result with that.It is not a solution to do serious work.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:00:08 pm
content: With all due respect guys, I'm not sure how many of you are actually engineers, but I'm trying to imagine an engineer telling a boss:Well boss, we shouldn't consider this piece of equipment, EVER, for these reasons:1. It "sucks"2. It's not "precision"3. It's not "bench" quality4. It's not for "serious" work5. It's analogous to a stupid kid doing stupid stuff6. It doesn't do stuff it's not designed to doIf I'm the boss, my response is "Oh really? You're fired". I'd then hire a guy who knows how to do a real engineering evaluation.Engineers look at facts, not emotional, judgemental finger wagging.

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 01:02:40 pm
content: Variable unregulated supply is a bench supply, that is the minimum qualifier. A regulated smps supply is just a power supply.I'd fire you for being silly. Was just seeing when you signed up, you are also starting to exhibit troll like behavior, calm down a little.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:07:02 pm
content: Quote from: Shock on April 21, 2020, 01:02:40 pmReally? So the difference between "bench" and something like an ATX is the "variable" part?Thanks. I learn something new every day.

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:11:18 pm
content: Quote from: engrguy42 on April 21, 2020, 01:00:08 pmIs it possible to have a live webcam during your next job interview ?

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:14:47 pm
content: Quote from: jemangedeslolos on April 21, 2020, 01:11:18 pmNah. I've been doing electrical engineering and managing (and hiring) electrical engineers for almost 50 years now. No more job interviews for me. Actually I was hoping to retire soon, but with the market the way it is that might have to wait.Though I've never been called a "troll".Wow.Not sure what it means other than "someone I hate cuz he disagrees with me" maybe?

author: masterx81
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:23:28 pm
content: You speak in that way because you have never tried a proper bench psu. Atx psu for lower the ripple have LOADS of output capacitance. And despite that, dynamically they can't mantain a precise voltage.On my atx i've also added the current control. But was not so precise, with dynamic load was HORRIBLE. I never trusted it.I've had your same opinion before using a bench psu, but now that i have one, the atx is on one side, i use it for something coarse but never anymore attached to a board. With a proper bench supply you can can set with mv/ma precision the voltage/current limits, dynamically they are way better. With a dynamic load i was also able to set the atx psu in some sort of resonance with incredible voltage spikes.On atx supply i wasn't able to have a precise current control, my supply had 2 ranges, 0-25v/0-10a, 0-10v/0-20a (from 5v rail and 12v rail, 2 ranges for have a bit more precision on the settings), i was able to let it goes to 0v (normally on the 2003 controller the minimum is 2.5v). I've lost a lot of time developing it, and was really far away from being usable on sensitive device. As stated before, i always had the need to use some other more reliable voltage and current control after it.Maybe with a good development can become more usable, but after i've already spent so much time for that result, i think that simply not worth.

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:32:03 pm
content: Quote from: engrguy42 on April 21, 2020, 01:14:47 pmIt is impossible that someone who has at least the slightest skill in electronics or who has worked at least once in his life on something other than a blink test with Arduino is not able to understand why an ATX power supply is not suitable for serious work.Are you one of those managers who never go wrong and who make decisions based on their excel spreadsheet ?I think that's why some people call you a troll and I join them.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 01:48:34 pm
content: As I posted before, the other day I used my ATX supply to calibrate a current sensor while my main "bench" supply was busy. It worked fine. Not sure if that qualifies as "serious", but anyway.Again, does anyone have actual data, or just name calling?

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 01:52:22 pm
content: Quote from: engrguy42 on April 21, 2020, 01:14:47 pmI got my first bench supply about 35 years ago. I'm surprised you're still on an atx smps after 50 years of doing electrical engineering.Quote from: engrguy42 on April 21, 2020, 01:14:47 pmI said "troll like", which would be someone with 50 years of experience "doing electrical engineering" who comes into a beginner forum and asks why they can't use an ATX smps in the same manner as a bench supply, that is a pretty accurate representation of trolling if you ask me.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 02:00:48 pm
content: If you actually read what I post, I never even implied the ATX is a substitute for a "serious" power supply. I said it's a tool that is useful in certain applications. Like any other tool.Unlike others, I'm not selling anything or proving a point. I'm trying to understand my tools and their limitations. I'm merely looking for facts. It's what engineers do. And I'm also not an electronics expert since my background is not directly in electronics. There are many other fields of electrical engineering in case you were unaware. Which is why I'm asking here, hoping there are people who have enough electronics knowledge to provide facts, not handwaving.

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2020, 02:04:13 pm
content: Quote from: engrguy42 on April 21, 2020, 01:48:34 pmThey puts out a voltage, sure. No arguments there.

author: nctnico
author_rank: Super Contributor
timestamp: April 21, 2020, 02:09:06 pm
content: Quote from: engrguy42 on April 21, 2020, 01:00:08 pmA good engineer forgot more than the average person learned so an engineer only remembers the do's and don'ts. It's up to you to take advantage of that or not. I have repurposed an ATX PSU a long time ago. The end result wasn't very satisfactory so I decided not to go that route ever again. One of the problems was that it wasn't always starting properly. It is too long ago to remember all the exact details.Another example: one of my customers didn't believe me when I said his circuit couldn't work because he was using an opamp as a comparator. It is one of those golden rules you learn early on. Forget the how and why. Just don't do it. It seems some people don't believe you'll experience pain by touching a hot kettle and go on touching to learn by themselves.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 02:15:34 pm
content: So with all the implied expertise here, has nobody ever actually tested a compliant ATX power supply to make a list of its actual limitations?

author: madires
author_rank: Super Contributor
timestamp: April 21, 2020, 02:26:16 pm
content: Switching noise is a problem when working on audio or T&M circuits. Another point is that the output's ground is tied to PE/earth.

author: madires
author_rank: Super Contributor
timestamp: April 21, 2020, 02:28:43 pm
content: Quote from: engrguy42 on April 21, 2020, 02:15:34 pmA modified ATX PSU isn't ATX compliant anymore.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 02:42:08 pm
content: Quote from: madires on April 21, 2020, 02:26:16 pmThanks. Those definitely belong on the "to be aware of" list for ATX supplies.

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2020, 03:04:30 pm
content: Quote from: madires on April 21, 2020, 02:26:16 pmYep. Anybody who's ever tried to power an audio circuit from USB can tell horror stories.

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 03:07:06 pm
content: Can't wait to see the results of the converted ATX supply. When you said you had 50 years of doing electrical engineering but had no direct electronics experience I readjusted my expectations of you a little.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 03:13:49 pm
content: Quote from: Shock on April 21, 2020, 03:07:06 pmAnd when you start providing actual facts and data rather than name calling and trying to discredit and act smart, I'll adjust my view of you too.

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 03:17:04 pm
content: I was agreeing with you, meaning at it's core functionality an smps is just like any other tool it does what it does. Like I said calm down.

author: masterx81
author_rank: Frequent Contributor
timestamp: April 21, 2020, 03:21:55 pm
content: My modified atx looked a bit more nice

author: Gyro
author_rank: Super Contributor
timestamp: April 21, 2020, 04:58:45 pm
content: Quote from: engrguy42 on April 21, 2020, 03:13:49 pmI don't understand what "actual facts and data" you are asking for. ATX supplies have specifications, both the generic ATX spec and individual manufacturer model specs. There is no particular reason to doubt these (aside from truly dodgy ones).Unfortunately the specs tell you that that ATX PSUs are not suitable as a replacement for Bench PSUs with adjustable current limits - where the limit can be set to what is appropriate to the circuit under debug / test.I don't understand what further facts and data you need.BTW, I think you will find that a lot of us here are engineers / engineering managers, many of us near or at the end of our careers. There are some very experienced folks on this forum. You are typically not arguing with hobbyists.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 05:28:18 pm
content: Gyro,As I have said before, multiple times...People have claimed they had ATX supplies that spiked to 100 amps. Others have said you can get >40 amps on a 12volt rail. Others make vague claims that ATX supplies will blow up your equipment. None of these claims are accompanied by data.I posted data from tests I did showing that my brand-name ATX supply shuts down below the rated amps of the 12v rail, in contradiction to what others are claiming.I don't know if this is due to an error in my tests, or others' supplies not meeting ATX specs, or a lack of clarity/uniformity in what the ATX spec requires. Does OCP measure the TOTAL of the 3 12v rails? Or, (as my test seems to show) each individual rail? Or is it not clear on exactly what's required? Without test data it's all just speculation and opinion.Also, are there other hidden problems with ATX supplies that I'm not aware of? Does the ATX spec allow some possibly damaging transient overvoltages in some cases? Does anyone have data on this?For example, I have a non-ATX bench supply that cost in the $100 range. After testing I found that if you leave the supply with the CV dial set to max while in CC mode, and you turn it off, at power on it will deliver an initial transient overvoltage up to the CV dial setting (30v max), then drop down to the voltage needed for CC. That's not good for those who worry about damaging, unexpected overvoltages.BTW, attached are scope traces from 3 different power on events of that supply, with 3 different dial positions for the CV dial.But that kind of stuff only comes from testing, right? Specs don't always tell you what you need to know. That's why people do tear-downs and tests, right?Why is this even something I have to explain?

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 06:03:29 pm
content: https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdfPower supply doesn't meet it's rating repair/redesign/rubbish.

author: newbrain
author_rank: Super Contributor
timestamp: April 21, 2020, 06:04:26 pm
content: Quote from: engrguy42 on April 21, 2020, 05:28:18 pmWhat's so strange? My ATX PSU (neatly tucked inside my PC, thank you very much) can provide > 60A on itssingle +12V rail. And probably more for a short time.750W are enough to vaporize most of the stuff on my bench.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 06:06:19 pm
content: Fine. Nobody has any data, only biased opinions and name calling. Great. I'll do my own tests. Geez.

author: bob91343
author_rank: Super Contributor
timestamp: April 21, 2020, 06:09:07 pm
content: What is with all this bickering and personal attacks? Definitely out of place here.

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 06:23:59 pm
content: He has problems with his power supplies and wants our data.

author: Fungus
author_rank: Super Contributor
timestamp: April 21, 2020, 06:26:49 pm
content: Quote from: engrguy42 on April 21, 2020, 05:28:18 pmElectronics 101: Anything with big output capacitors can produce big current spikes.

author: Gyro
author_rank: Super Contributor
timestamp: April 21, 2020, 06:28:12 pm
content: Quote from: engrguy42 on April 21, 2020, 05:28:18 pmI have no comment on this claim. All I can say is that the steady state current specifications - which will clearly be exceeded before the PSU goes into shutdown are sufficient to cause damage to the target circuit. That's all that is needed, different PSUs may have different peak currents but it's pretty academic when the lowest common denominator is too high anyway. The current shown in your plot is sufficient to cause damage to the average prototype.QuoteYes, the specification OCP does relate to the total of the 3 12V rails. If you look inside an ATX PSU, all of the 12V wires go to the same point. It is unnecessary and costly to generate multiple 12V rails. There is nothing in the ATX spec that requires it.QuoteRead the ATX PSU spec (it's actually called a Design Guide rather than a spec), it's publicly available. I have attached a copy for your easy reference though. Refer to Section 3.3.4 for transient response. Note also that the maximum ripple and noise specs in section 3.3.3 which indicate that it is probably not suitable for use with sensitive analogue circuitry.QuoteI can't really comment on your un-named (presumably single output) non-ATX PSU. Of course testing is necessary when integrating any purchased sub-assembly into a volume product....That isnotthe same however, as taking some ATX PSU and using it outside it's rated operating conditions asa bench supply - Importantly, the ATX spec includes normal conditions for load balancing across the various output voltage rails. Deviate significantly from those, by using just a single rail, as you almost certainly will if using as a bench supply, and you're on your own. You're in unknown territory and different manufacturers' supplies will behave in different ways, from going into overvoltage shutdown, triggered by the unloaded rails, to excessive current on the used rail before OCP cuts in. Your mileage may - and will, vary!I'm surprised that this is something Ineedto explain to someone of your apparent experience!Happy now?P.S. The version of the PSU design guide that Shock linked is far more recent and comprehensive than the one I attached, please refer to that one for specifics.

author: bd139
author_rank: Super Contributor
timestamp: April 21, 2020, 06:29:52 pm
content: Oh no not this again.

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 07:03:53 pm
content: Here is a list of tests to determine ATX power supply performance characteristics.+5VDC/+3.3VDC Power Sequencing+5VSB at AC Power-down+5VSB Short CircuitAcousticsAltitudeCapacitive LoadingCatastrophic Failure ProtectionClosed Loop StabilityCross RegulationDynamic LoadEfficiencyEMCHumidityImmunityInput UndervoltageInrush Current LimitingLine RegulationLoad RegulationMagnetic Leakage FieldsMechanical ShockMTBFNo-Load OperationOutput BypassOutput transient ResponseOvercurrent ProtectionOvershoot at Turn-on / Turn-offOver-temperature ProtectionOvervoltage ProtectionPower LimitPower Rail transient NoisePower-on TimeProper Labeling/MarkingPWR_OK TimingRandom VibrationRemote SensingReset after ShutdownRisetimeSafetyShort-circuit ProtectionThermal ShockVoltage AccuracyVoltage Hold-up Time

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 21, 2020, 07:06:14 pm
content: QuoteYou need to knock off the ad hominem attacks. People can make their own decisions about where to buy equipment, they don’t need your enlightenment.Contrary to what you say, you’re selling something alright. Your opinions. Like a kid picking at a scab, you don’t appear to be able to control the urge to repeatedly come back to a topic and flog it until you get the response you want to see.For those of us that have been on message boards for years, we’ve seen the behavior. It’s very predictable. You don’t want to participate, you want to dominate. You think you have something valuable to add to the conversation but when things don’t go your way, you challenge with pointless demands to drive the conversation in the direct you want to take it. It’s your way or the highway.You start a thread and then take control of it in an attempt to drive it to your desired outcome. That’s trolling, not having a discussion. So what are you really trying to achieve here? It sure doesn’t appear to have anything to do with benchtop power supplies. This thread is proof.

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 21, 2020, 07:20:30 pm
content: Yes.But where are your data?

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2020, 07:26:34 pm
content: Quote from: Shock on April 21, 2020, 07:03:53 pmYou forgot channel isolation !Equals NONE anyways.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 21, 2020, 09:23:07 pm
content: A bit late to the party, but here it is.ATX power supplies have a bad name because many of them, well, they're junk. Built down to a price, fake ratings, cheapest capacitors, no EMI filtering or PFC, thin wires, cheap connectors etc. They can get away with it because computers have local step-down regulators for anyting that's remotely important, so supply voltage accuracy, stability or ripple aren't critical. Keep in mind that a +/-10% voltage variation and 1% ripple is perfectly acceptable under the ATX standard and a cheap generic "ATX" PSU often doesn't meet even that.To give you an idea how loose the standards are at the ATX PSU low end, about 15 years ago I was negotiating with a supplier the purchase of an entire container of these. They only had one ATX PSU design (I think I evaluated that at 120-130W, enough for an office type PC back then) and they bluntly said they'll charge US$0.5/pc. for a 330W label instead of the 230W one; he even suggested that Delta is a popular brand, if we want to have that on the label. This was standard practice and people in the IT industry can probably confirm. Across more than a decade, tens or hundreds of millions of these bottom-of-the-barrel ATX PSUs were manufactured and plenty of them are still out there. Many of us have had to deal with them at least once and the only conclusion you're left with is "those ATX PSUs are an absolute dumpster fire". Things have improved in the meantime, but probably not enough.Today, every "DIY lab PSU for US$5" Youtube video starts with "find a scrap ATX PSU" and well, most of them look like the kind of stuff my supplier was pumping out. Intuition says to stay away way before the engineering aspects come into play.On the other side, there are good ATX power supplies out there as well; they have PFC, excellent EMI filtering, the output is stable and pretty clean. They have overvoltage, overcurrent and overheating protections and they use good quality capacitors. With some minor internal modifications, you have esentially a box that takes some AC from the socket and provides a stable DC output ... like a Meanwell, for roughly the same cost.Is that a lab PSU ? Well, it's not but it could be, if you add adjustable voltage and a current limiter, some reasonably decent meters and a few binding posts. Will it be cheaper than a comparable lab PSU ? Probably not and if you add your time, definitely not. Is it fun to do ? Of course, with every project you learn something, even if sometimes it's how NOT to do something.As a matter of fact, I've been using a modified ATX PSU (of good quality) for more than 10 years to power my ham radio station and a server PSU for the high-power amplifier/s. They're very suitable for fixed-voltage high-current applications and good enough quality to trust them power my equipment. The overcurrent protections saved a few expensive RF power transistors so far.Quote from: Gyro on April 21, 2020, 06:28:12 pmATX12V 2.2 and earlier had a safety requirement of "maximum 240VA per rail". As graphics cards started to use more and more power the PSU manufacturers figured how to avoid the restriction: by creating multiple +12V rails. The way that virtually everyone did it is to have a single +12V point inside the PSU and then split that with current sensing shunts that would trip OCP whenever one "rail" would exceed the 20A limit. As all +12V high-power cables had multiple wires for +12V and ground, sometimes in a cable one wire would belong to a rail and another wire to another rail, so you had 40A available while also technically meeting the 20A/rail limit. ATX12V 2.3 (circa 2007-2008) tried to put an end to the nonsense by removing the restriction, but by then having n+1 rails was a marketing feature and manufacturers kept doing it.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 10:08:07 pm
content: Wow, MOHZH, thanks much for the info. My sense has always been that folks get it in their head that the ATX supplies suck because of a bad experience with an old piece of junk. I see tons of those myths in the computer world too. A lot of people don't even realize that the decent ones have supervisors and the required ATX protections,etc.In any case, the ATX I'm converting is a Dell/Delta 460 watt supply which is 5-10 years old, and I think it's fairly reasonable quality. It has all that PFC stuff, and big 16 pin supervisor IC, and I'm *assuming* since it's Dell it's meeting the necessary standards.I've done a little testing, including some overloading the 12v to see how the supervisor OCP responded, and it shut down the supply instantly when the load got up to around 12A (the three 12v rails rated 8, 16, and 18 amps on the nameplate sticker). Does that seem reasonable, or maybe I made an error in testing? I'd think if the rails have those ratings on the sticker they wouldn't wait until >40 amps to shut down.Also, I've done some simple power-on transient voltage tests (with a few amps on each of the outputs) and didn't see any overshoot on any of the 12, 5, and 3.3 supplies (see attached). I'm trying to find out if there are any other "gotcha's" I need to worry about when using it.Thanks for any help.

author: Someone
author_rank: Super Contributor
timestamp: April 21, 2020, 10:31:24 pm
content: Quote from: M0HZH on April 21, 2020, 09:23:07 pmThis is probably the point people keep circling around but not getting to, an ATX supply is targeted at powering a computer with all its unique requirements. A similarly powered and quality DC supply without all the extras is around the same price, even if recycling/scrounging, broadening the view to enclosed power supplies in general will find better suited options.......bet demand for adopting random DC supplies into something variable spawned an entire product line:https://www.eevblog.com/forum/testgear/ruideng-riden-rd6006-dc-power-supply/So there is no point trying to compare one extreme of a bells and whistles ATX supply with 12 rails and low power standby, to a variable CV/CC lab supply, because there is so much ground in-between them littered with other options.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 21, 2020, 10:55:58 pm
content: Quote from: Someone on April 21, 2020, 10:31:24 pmHonestly I'm not sure why people make this into such a big deal. I have 3 desktop computers. Last year I upgraded two of them to add some powerful GPU's for doing graphics software and development (C++). Since they're so power hungry I bought some new power supplies, and had a couple of ATX supplies left over.So what do you do? Keep 'em or toss 'em? Well, it's free power, so keep them and tweak them and use them. Recognize their limitations, but use them as best you can. What's to discuss? Figure out their benefits and limitations, and use them. Tossing them isn't going to benefit you, so put them on your bench. What's there not to like?

author: Electro Detective
author_rank: Super Contributor
timestamp: April 21, 2020, 11:51:00 pm
content: Go easy on the OP folksfwiw the only thing I've used ATX units is for PC box psu backup replacement/spares and test/verifyingand sometimes powering older IDE hard drives externallyI have a few kicking around in older AT? and ATX and all fit nicely in a small box till neededTwo quick (?!!) questions:1. If the outputs of a poor mans psu ATX are fused, and one of the fuses blows due to an issue and or overload whilst all outputs are under load,will there be an imbalance of power and cause drama to the other working outputs, or they won't care ? anyone been there?2. Can identical ATX units be hardwird in parallel and or series or will they say 'NO'and release the Genie?3. ran for the phone to answer a 'wrong number' and forgot what the question was

author: maginnovision
author_rank: Super Contributor
timestamp: April 22, 2020, 12:07:42 am
content: Quote from: Electro Detective on April 21, 2020, 11:51:00 pmAssuming it goes overcurrent or the load drops below min(if there is one for the rail) I'd expect it to reset/shutdown. If it's your fuse which is rated under the OCP limit and it blows quickly enough without bringing rail below min it should hum along just fine.QuoteSince they're ground referenced I'd think no on the series, parallel shouldn't matter.

author: Someone
author_rank: Super Contributor
timestamp: April 22, 2020, 12:20:26 am
content: Quote from: engrguy42 on April 21, 2020, 10:55:58 pmBecause for all the use cases you are suggesting for an ATX power supply, there are vastly better alternatives available. The only advantage you have been able to point to is that someone may have an existing ATX supply to hand or available for "free". Great, you found a nice use for some garbage and did a little project. This is neither advancing other peoples knowledge, or introducing something new and unavailable, so most people really dont see any advantages and just the many many compromises/downsides.When I want a non adjustable DC supply, I pick one that matches the application rather than shoe-horning in whatever junk is around to save some money. When there is a need for an adjustable power supply, again I pick one that is designed for that rather than reverse engineering and then modifying a fixed supply. Surely if your ATX supplies are such great quality/performance, you'd probably be better off selling them for their purpose and buying what you actually need.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 22, 2020, 08:51:46 am
content: Quote from: engrguy42 on April 21, 2020, 10:08:07 pmDelta, Zippy, HP, Etasis and the like are more focused on server PSUs; their desktop / ATX stuff will generally be top-shelf. There are also plenty of desktop PC focused makers that put out good stuff, Seasonic, PCPC, Topower being some of them but there are too many to mention (or remember). With them, good specs like voltage stability, accuracy, low ripple are generally byproducts of good design and engineering and not necessarily something they aim for. One metric that is however important in a PC is transient reponse, as high-power CPUs / GPUs can have insane demands from the power cicuits. Generally plenty of very-low-ESR capacitors are used to swallow spikes, but the feedback loops should be pretty tip-top as well. If you see high transient spikes on ATX PSUs, throw them away.If your PSU tripped at 12A, you were probably using the 8A rail. Usually there is also a primary overcurrent trip (for total power) that may trigger even if the summed up power of all rails is over a certain limit.Generally the arrangement on ATX PSU's is to have separate SMPS transformer windings for 3.3V, 5V and 12V and a single common feedback loop derived from a resistor divider network that averages all 3 voltages; serves the purpose.

author: bd139
author_rank: Super Contributor
timestamp: April 22, 2020, 09:17:37 am
content: HP deskops no. I inboxed a whole office full of HP desktops once and two actually exploded on power up. We actually switched the rest on with a broom handle

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 22, 2020, 09:26:56 am
content: Quote from: bd139 on April 22, 2020, 09:17:37 amMaybe you can recover the ATX PSU to make high end bench power supplies ?

author: tautech
author_rank: Super Contributor
timestamp: April 22, 2020, 09:27:26 am
content: Quote from: bd139 on April 22, 2020, 09:17:37 amYep, their micro PSU's were a POS especially if they sat for a while and while cold the horrible porous gunt inside them soaked up atmospheric moisture and went BANG when you powered them up.Wouldn't touch another with a40'100' barge pole !

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 10:43:31 am
content: Thanks much MOHZH. It makes sense what you're saying about the OCP. If I was on the 8A rail and it tripped at 12A that's a very reasonable 150% trip point.So I guess it's reasonable to assume that for a decent supply with a decent supervisor you'd expect no more than 150% overcurrent before the whole thing shuts down. And adding a fuse might only give about the same level of protection (although slower near the trip point of course, due to inverse curve), in case you're worried about the supervisor dying or something. I also noticed three adjustable pots on the supervisor board (locked in glue) that I assume allow you to adjust the protection reference levels if you're not happy with how they're set.Once I get this thing buttoned up I might run it thru some more tests just to see. I'm reasonably confident now that I don't have to worry about power-on voltage transients, like I do with my "real" bench supply.Anyway, thanks again for the info.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 11:41:56 am
content: MOHZH, as I mentioned my ATX supply has what looks like a decent and fairly comprehensive 18 pin supervisor IC. I was never able to find the exact spec sheet for it, but I found another spec sheet for an 18 pin supervisor a while back, which was the reason I've always assumed that each rail has its own OCP and other protections.Attached is a snip of the application and pinout pages, and as you can see from the pinout pins 5, 7, and 10 are the separate input pins for the OCP for each 12v rail.Do you think this is typical for the name-brand supplies? I'm assuming that since it's a Dell branded supply they probably want to have some good protection?Thanks again.

author: bd139
author_rank: Super Contributor
timestamp: April 22, 2020, 11:53:17 am
content: I suggest you have a look at AoE X Chapters 1x2.6:https://x.artofelectronics.net/wp-content/uploads/2019/11/1xp2_resistors.pdf. Particularly the Larkin blaster which is a nice illustration of how bad dumping an SMPS secondary caps can be. If you have a short to ground the supervisor doesn't have a chance in hell of shutting the supply down before the DUT has turned into ash.Thus you might want to grab a DC current probe and scope and measure not the voltage transient but the current pulse at the output. You can get a rough estimate with a fat thick film resistor and differential scope probes across it but even a 0.05 ohm resistor can be interesting. Also consider wires, traces etc in your calculation.Also worth referring to Troubleshooting Analog Circuits by Bob Pease who describes a power supply design without any capacitance on the output as an ideal case.In this case the ATX power supply supervisor is designed not to protect the DUT which will explode instantly (MOSFET = fastest fuse ever) but to prevent current being dumped into the target continuously and cause it to burn the office it's in down. It is NOT suitable for protecting stuff in development so can we please stop flogging this dead horse zombified horse skeleton any further

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 04:49:12 pm
content: bd139, I'm trying to understand your specific concern.It sounds like you are defining a specific scenario in which you are applying an ATX supply to a specific type of circuit in which it is possible to short circuit the power supply thru a very low impedance component/circuit, thereby frying the component due to capacitive discharge.In your example you mentioned a MOSFET and the concern you could fry the MOSFET. While I'm sure that's true, I also assume that for many of us, frying a 20 cent MOSFET is a fairly minor concern, especially if it would only occur in the specific type of scenario and circuit you defined.As I mentioned before, the other day I mistakenly applied about 12 volts from my main bench supply to a 10 ohm, 2 watt resistor and it started to smoke. Dumb mistake, but I merely dropped the resistor in the trash and reached in the box for another. And no, my "proper" bench supply didn't protect it, and would have happily sat there until a fire started. Stuff happens.So while your concern and scenario is technically correct in some cases, I'm not sure how relevant it is to many of us. As I said previously, maybe this is more of a "yeah, don't use ATX supplies on important circuits" rather than a reason to never ever use them for anything ever.Personally, my concerns about electrical equipment are more centered on "can it hurt me?", and "can it destroy some of my expensive equipment?".

author: bd139
author_rank: Super Contributor
timestamp: April 22, 2020, 05:29:54 pm
content: Anything can be a low impedance circuit by accident.PC has billions of MOSFET fuses in them. If one goes you replace it. That’s the point.10 ohm 2 watt = smoke <— you are here4.7 ohm 0.25 watt = fire0.003 ohms unknown watts = metal vapour. In your eyes. <— you don’t want to be hereIt’s shit advice that needs to go in the trash because yes you can hurt yourself and there’s very little to stop you or anyone copying the shit advice and hurting themselves.In your other thread where you failed to take advice there. Look at the video I posted

author: M0HZH
author_rank: Regular Contributor
timestamp: April 22, 2020, 05:43:43 pm
content: "The concern to fry a MOSFET" might be serious depending on the situation. If it's a high-power RF mosfet (gates are still sensitive as hell on those), that's the price of a good lab PSU right there. If it's a tiny 20cent MOSFET but powers some fancy ADC/FPGA stuff, wallet comes out again and you also need to resolder QFP packages as a punishment. And for my own personal amusement and to completely confuse the Google search bots, I'll just mention Virtex Ultrascale in an ATX lab PSU thread.Quote from: engrguy42 on April 22, 2020, 11:41:56 amYes, that looks like something you would find on many decent quality ATX PSUs. But as bd139 said, these are meant to shut down in case of shortcircuits, not to limit pulse currents in case of an accident. They couldn't do that, there's no series regulating element to stop somthing like a 4700uF capacitor with sub-0.1ohm ESR to discharge directly in the load.

author: Gyro
author_rank: Super Contributor
timestamp: April 22, 2020, 05:48:35 pm
content: Quote from: engrguy42 on April 22, 2020, 04:49:12 pmI think this is the disconnect that has caused much of the 'heat' in this thread. For many people, burning up of components is much more of an issue than it seems to be for you. Aside from the collateral damage (damage to PCB traces, failure of related parts etc.), there is also the potential difficulty in rework, it's much easier to fit an smd part than replace one, they may damage their board in the process, they may not have ordered sufficient spares etc. This can cause much frustration, particularly for less experienced members.I'm not saying that your debug methods arewrongif they seem to work for you, but that probably isn't the case for most of us. I certainly don't subscribe to that methodology personally.Hopefully this clarifies what seems to be a fundamental misunderstanding and cause of argument.On a technical point, whilst the protection IC may be comprehensive, you may have to go to the extent of removing and changing current sense resistors to higher values in order to achieve more 'useable' current limits. You should also study the 'cross loading curves' in section 3.3 of the ATX spec that Shock linked in Reply #42 . These are upstream of the protection IC and are vital to the stable startup and operation of the ATX PSU, particularly if you plan to only load one rail at a time.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 06:07:51 pm
content: Quote from: bd139 on April 22, 2020, 05:29:54 pmI'm not sure what you're referring to as "shit advice". My sole purpose has been to define some of the important operating/response characteristics of decent ATX supplies so I can determine when it is or isn't appropriate to use them. I don't care what anyone else uses, nor am I advising anyone to do anything. I'm just trying to gather data for my own use.OTOH, if you're trying to push a position that nobody should ever, ever use them for anything solely because you can think of a very narrow set of special circumstances in which theymightcause a problem I think you'll have a tough time.

author: bd139
author_rank: Super Contributor
timestamp: April 22, 2020, 06:19:17 pm
content: I explained earlier how you want to attempt to analyse that and the perils involved. You completely ignored that bit.As for where and when: inside a computer, begrudgingly.

author: Fungus
author_rank: Super Contributor
timestamp: April 22, 2020, 06:37:14 pm
content: Quote from: bd139 on April 22, 2020, 05:29:54 pmCan he add an ordinary fuse to the output of his ATX power supply?Problem solved.(more or less)

author: maginnovision
author_rank: Super Contributor
timestamp: April 22, 2020, 06:50:01 pm
content: Except fuses take time to blow. Slow blow fuses are useless for anything other than preventing fire. I've had to buy some fast(and precise! safety critical) fuses before and they're very expensive if you're a hobbyist(if you're using an ATX supply you're probably this). The fuse holders are also fairly expensive for these best fuses. Mostly your fuse is going to prevent outright failure in case the ATX supply fails catastrophically, not protect anything you build if you consider you're tring to protect more sensitive devices.10ns peaks can blow a LOT of devices.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 06:54:36 pm
content: Quote from: Fungus on April 22, 2020, 06:37:14 pmI mentioned that long ago and got some resistance. But if you look at the attached Littlefuse fast acting fuse time characteristic, it seems like if you have, say, a 10 amp fuse (which is what I was planning on) it should blow in less than 100mSec at only 20 amps, and less than 10mSec at greater than 40 amps.Of course you can always come up with a "yeah but..." scenario where this would be disastrous, but anyway. Although I think the supervisor OC might also match that type of performance.

author: bd139
author_rank: Super Contributor
timestamp: April 22, 2020, 06:57:17 pm
content: However it'll also blow when it's filling up the electrolytic capacitors in your circuit (consider one empty electrolytic capacitor connected in parallel with a charged one and the charge equalisation). Which is why we have slow blows. Which tend not to blow up before the circuit does. Which is why fuses are mostly relegated to "stop the house burning down" applications now and we have hard current limits and crowbars etc.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 07:05:42 pm
content: Quote from: bd139 on April 22, 2020, 06:57:17 pmGot it. So now the criteria for not using an ATX is:1. Important circuit, and2. Circuit with any possible super low impedance short circuit path thru important device(s), and3. Circuit with lots of electrolytics that might blow a fast acting fuse while charging, so fuses won't help, and4. so on...Now that's determination

author: maginnovision
author_rank: Super Contributor
timestamp: April 22, 2020, 07:07:38 pm
content: No, these are things you NEED to be aware of. Even a free supply can cost you money if you don't know the limits. You said you wanted data why not and provided a bunch of why you should. Don't you think less experienced people deserve to get as much data as possible? I don't care what people do but I think they need info to make a decision.For instance, I don't typically test anything with large amounts of capacitance and add safety at the inputs too so a good fast fuse would likely save my circuits but most people want to test parts of circuits so they're not going to build a bunch of protection in since it'd take as long or longer than the circuit you're testing.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 07:14:21 pm
content: Quote from: maginnovision on April 22, 2020, 07:07:38 pmYou're absolutely right, this stuff is good to know. But it's also important to put it in perspective and see what's reasonable and relevant. Because at some point you're worrying about a zombie apocalypse. Anyone can add "what if's" to the list of scenarios, but we also need to put them in perspective, don't you think?

author: maginnovision
author_rank: Super Contributor
timestamp: April 22, 2020, 07:21:57 pm
content: Sure and I think they are. Fuses are good to prevent fire but not much else since most fuses allow significantly more current to flow. UNLESS your circuit is hardy enough to withstand these overages for a long enough time that the fuse is able to blow first(although at this point you need to think about breadboard/pcb traces and supply wires(probably ok) as well). You probably don't want to test a stereo amplifier, for instance, with an ATX supply. Too much capacitance and too many sensitive components. However, once you know the circuit is ok I'd be perfectly fine using an ATX supply to run it.Just the other day I destroyed the circuit protection on a board by accidentally setting the voltage too high. Didn't even look before applying roughly 3x the rated voltage. Maximum input voltage was roughly 16.8V protection kicks in at 17V. Only had to replace a couple protection components and then the board fired right up. This could have been prevented entirely if I'd bothered to set the current under 20A though too. A fuse would have done nothing here other than add one more thing to replace.

author: Nusa
author_rank: Super Contributor
timestamp: April 22, 2020, 08:18:45 pm
content: Remember that sarcasm does NOT come across well in print. So if you have trouble getting people to take you seriously after using it, you have only yourself to blame.

author: Fungus
author_rank: Super Contributor
timestamp: April 22, 2020, 08:38:05 pm
content: Quote from: maginnovision on April 22, 2020, 06:50:01 pmQuote from: bd139 on April 22, 2020, 06:57:17 pmSo... fuses take too long to blowandthey can blow too quickly.Got it.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 08:40:33 pm
content: Quote from: Fungus on April 22, 2020, 08:38:05 pmYes. At currents close to their rating they can take forever to blow, and at currents far above their rating they can blow very fast. Look that the example time/current characteristic I posted above.

author: maginnovision
author_rank: Super Contributor
timestamp: April 22, 2020, 08:54:16 pm
content: Quote from: Fungus on April 22, 2020, 08:38:05 pmIt all depends. If you have a lot of capacitance you MAY blow your fuse trying to charge the caps if you're near operating limits for your fuse rating instead of peak limits. Fuses also ALWAYS take time to blow, plenty long to damage things depending on the fuse. Sometimes you can use a differently rated fuse to offset. This is the same with some safety circuits and ramping. Sometimes you want a crowbar circuit to short at your limit, sometimes you want to account for inrushes so you begin clamping at some point possibly having a crowbar circuit at another point.IT ALL DEPENDS. It's not me being a fence sitter it's real. It's why we don't all use linear or switching supplies, it's why some people monitor voltage not current or vice versa. If you don't know what your circuit is going to do or how it will fail you'll possibly set a low current/voltage limit and ramp it as it seems to be ok. When you're confident you can try a normal startup.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 22, 2020, 11:27:26 pm
content: FWIW, I did some more tests on the 12v rail of my ATX to check the supervisor transient response to a near short circuit.I turned on the power supply until it stabilized, then switched a 0.5 ohm resistor onto the 12v rail. I measured with a diff probe the voltage across the 0.5 ohm resistor.As you can see from the attached chart, the voltage immediately went up to 10+ volts with no overshoot/transients, which equates to about 20+ amps thru the resistor. It lasted for 20 mSec and then the supervisor shut the PS down.One more data point I suppose. That's the smallest hi-power resistor I have, so it'll have to do for now.BTW, it also resets in 5 or 10 seconds and I can restart it no problem.

author: Cubdriver
author_rank: Unknown
timestamp: April 23, 2020, 04:24:34 am
content: Quote from: engrguy42 on April 21, 2020, 01:07:02 pmNo, the difference is that an ATX supply is designed to go in a computer case and power a motherboard and its associated hardware, and a bench supply is designed to be used on an electronics bench for things like circuit development and testing and experimentation. If you've been doing engineering and management for nearly 50 years and only just learned this today, I have to wonder what sort of engineer you are.Quote from: engrguy42 on April 21, 2020, 01:00:08 pmI'm glad you're not my boss. If mine tasked me with getting an engine for a 22" push lawn mower and I suggested an LS6 crate motor (a high HP Chevrolet 5.7l V-8 engine for those not in the US) he'd rightly smack me upside the head and ask WTF I was thinking. A sledge hammer and a jeweler's hammer are both hammers, but neither is a good substitute for the other. If I were speccing a power supply for a computer, I would not use a bench supply. When setting up a test bench, if I showed up with an ATX box and said "Well, we can add some circuitry to make it variable and (I hope) limit the current", I'd rightly be looked at as if I were an idiot. Part of engineering is knowing what is NOT an appropriate option for the application at hand and then not wasting time pursuing dead ends when there are obvious, established and more applicable solutions readily at hand.And for someone who's not selling anything or trying to prove a point, you sure seem to have a hard-on for using ATX power supplies for a purpose that is very different than the one for which they are designed, despite people telling you over and over that they are NOT a suitable substitute for a proper bench power supply.-Pat

author: miken
author_rank: Regular Contributor
timestamp: April 23, 2020, 05:05:25 am
content: With apologies to Tolstoy: Happy power supplies are all alike; every unhappy power supply is unhappy in its own way.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 10:26:17 am
content: I'm curious, does anyone happen to know about typical I2t (energy) withstands of electronic components? If you look at the trace I posted, the pulse was 20 amps for 20 mSec, which is an I2t of 8.I have no clue if that is a lot when it comes to electronic components, and might cause them to get fried, or not. Actually I'm kinda surprised that the supervisor responded so quickly, and off the top it seems like 20 amps for 20 mSec for a 0.5 ohm short circuit is pretty low in terms of energy injected.Thanks.

author: Gyro
author_rank: Super Contributor
timestamp: April 23, 2020, 11:32:38 am
content: For pretty much all 'non-power' semiconductors, 20A at 20ms iswaytoo high and long compared to the thermal time constant of their bond wires and dies by a large margin. They will die for sure.For passives (resistors) it depends on their value as to whether you will exceed their power rating at 12V. If you do, by a large margin then metal film and carbon film will blow first, followed by metal oxide and Carbon composition and wirewound.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 11:41:01 am
content: Quote from: Gyro on April 23, 2020, 11:32:38 amCool, thanks. So it sounds like the concern is components that are pretty much zero resistance having something like 20 amps thru them for 20 mSec. Which pretty much rules out resistors I suppose since their resistance will automatically drop the current.So I guess it's about non-power semiconductors operating at a VERY low resistance operating point, with no additional resistance in the path?

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 23, 2020, 11:43:49 am
content: Quote from: Cubdriver on April 23, 2020, 04:24:34 amIt is good to see that there are still a few with enough patience to explain things.But I think it's a waste of time.He will never see the light. ( well yes, but not the one i'm talking about).

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 04:01:15 pm
content: Okay, well I think I've settled on a decent list of considerations for ATX supplies. Thanks much to all who provided useful input. And for those who are familiar with ATX supplies in detail I'd appreciate any comments on my list:1. As with anything, don’t use junk. Stick with a newer, name brand power supply2. Use a power supply with a good supervisor circuit (eg., overcurrent, overvoltage, etc. protection on each rail), and preferably one that includes a 16 or 18 pin supervisor IC (for a 5 rail supply).3. Make sure you check the main motherboard connector to see if there is a voltage sense wire for voltage regulation on any rail. If so, make sure you include that connection in your final modification.4. Install a 5-10 ohm, 10+ watt resistive load on the 5 volt rail or the supply may appear to be okay, but may not operate correctly under all conditions.5. Since ATX supplies don’t provide a constant current source or variable source functionality, reconsider using an ATX supply if you need those things and are unwilling to retrofit.6. It may be possible in some limited cases for the power supply to discharge high transient currents thru your circuit due to capacitor discharge, and/or while waiting for the supervisor OCP to respond, under certain short circuit conditions. Therefore, reconsider using an ATX supply if your circuits under test are characterized by all of the following:- Important circuit, and- Circuit with any possible super low impedance short circuit path thru important device(s), and- Important devices that can’t withstand overcurrent pulses on the order of 20 A for 20 mSec (depending on supervisor OCP response), and- Circuit with lots of electrolytics that might blow a fast acting fuse while charging, in which case fuses might not be an option for providing additional protection

author: bd139
author_rank: Super Contributor
timestamp: April 23, 2020, 04:09:33 pm
content: Quote from: engrguy42 on April 23, 2020, 04:01:15 pmI've got a newer, name brand power supply. It's a Be Quiet! pure power 11. It wouldn't power up until it had been up for about 2-3 minutes. This was traced to a bad soldering joint. I had to repair itYou can buy a second hand bench supply for the price of a "decent" ATX supply which is still a volume product and made to a price point.I paid I 15 GBP for my 6286A (10A/20V).I paid 0 GBP for my PL330QMD (2x 3A 30V). (it was £100 but it arrived dinked and the seller just refunded the entire cash)Both of those are proper lienar bench supplies. Shop aroundALWAYS shop around.

author: newbrain
author_rank: Super Contributor
timestamp: April 23, 2020, 04:24:36 pm
content: QuoteMaybe, add some consideration about single rail vs. multi rail.As I previously stated, my ATX PSU can deliver more than 60 Acontinuouslyinto the 12 V rail - it only has got one.This is a very common (andoptimal) design for high power ATX PSU.That would correspond to 12/60 = 0.2, dangerously close to the sum of contact resistance + wires in case of a short: OCP would not have a chance to trigger.I would not like to be close or touching part of the circuit.Crappier PSU might be safer.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 04:30:32 pm
content: Quote from: newbrain on April 23, 2020, 04:24:36 pmThanks, excellent point. Maybe it's best to steer away from single rail, high power supplies altogether, and stick with multi-rail.

author: Cubdriver
author_rank: Unknown
timestamp: April 23, 2020, 04:43:29 pm
content: Quote from: engrguy42 on April 23, 2020, 04:01:15 pm7. Buy and use a decent variable bench power supply for experimentation on the bench.8. If you need scads of current at a given, fixed voltage, buy an appropriate fixed voltage power supply - they are readily available in either linear or switchmode varieties, and those of a decent pedigree come with documentation describing their intended operating ranges and the behavior they can be expected to exhibit within those ranges, along with hookup instructions. Voltage sense pins will be clearly called out.9. Read and follow (7) and (8 ) and you won't need to hack in load resistors on the 5V line to keep the power supply from maybe doing things you don't want or expect it to, nor will you need to open it to look and try to determine what kind of supervisory circuit it has.10. The best use for an ATX supply is to put it into a computer case and plug the connectors into the computer's motherboard and various drives and accessories as appropriate. The ATX supply will be very happy in this role, as this is the exact use for which it has been engineered.The only consideration I make when choosing an ATX supply is that it delivers enough power to supply the computer I'm putting it into, and has a bit of headroom so that it's not going to be running at its design limit. It is not something that I would consider for any kind of test setup at work (other than a computer), nor is it something those I work for would find acceptable.Horses for courses - there's a good reason that so manydifferentkinds of power supplies are manufactured.-Pat

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 04:58:31 pm
content: Again, the purpose of this list is to assist people like myself who might have some free ATX power supplies lying around and are considering if it's appropriate to use them, and what technical issues they might want to consider.For those who want to make a separate list providing recommendations on what to buy, feel free to start a thread.

author: 2N3055
author_rank: Super Contributor
timestamp: April 23, 2020, 05:08:59 pm
content: ATX PSU are so cheap that I tend not to reuse them.Usage that I found to be most useful is to throw out crap from inside and use them as small project boxes. It has power connector, power switch, good ventilation and fan. They are perfect for homemade loads, small specialist PSU, etc. They are also good source of various size heatsinks...I made a soldering fumes evacuation filter from one, just added frame on outside of FAN for filter (also cut out from ATX psu case and folded), and inside there was plenty space for small PSU...

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2020, 07:51:07 pm
content: Quote from: engrguy42 on April 23, 2020, 04:58:31 pmYes well that hinges on ones understanding of just what you're dealing with in an AT/ATX PSU.I have several tucked away in a drawer as the cheapskate in me struggles with throwing anything away might I have a use for it someday.I have repurposed the grand total of ONE AT supply in 25 years that I've had these things, as the base and PSU for a small PCB drill......since replaced with a Dremel setup for the same requirements.Of the others stashed away I've honestly considered using one on rare occasions when needing higher currents and in each case the voltage required was inadequate for the project.Knowing whatI requireand the pitfalls of ATX PSU's had me get a small 30V 5A SMPS mentioned early in this thread AND subsequently buying other 'proper' bench PSU's rather than even to attempt using a potential fire breathing ATX PSU.When I need 'grunt' SLA's or automotive LA's are brought to bear .....with the inherent risk they also present.I hold low opinions of ATX PSU's construction and internal clearances as a failure a year or two back alerted me to.After a couple of weeks travelling while mini tower PC powered down, repowering it on return had the PSU go BANG and investigation revealed a bug, yes a furking bug had ventured under the PCB and where a component leg had protruded sufficiently, had been electrocuted and remained there gently toasting away without affecting the PC in any way........until said bug was allowed to regain atmospheric moisture without a constant electrical toasting.....then cause the PSU to fail when power was reapplied......and all this with the PSU mounted on a mini tower case and not sitting on the bench with even higher chance of foreign body intrusion !Real experience counts for a lot .....not thinking you've discovered the lastest cost saving PSU that we can comfortably saywillbite you on the bum given the chance !The collective experience of other contributors here should also serve as a plain warning to readers that Murphy isalwayswatching !Readers, please be well aware of the folly of using ATX PSU's for anything but very carefully selected tasks !

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: April 23, 2020, 08:10:44 pm
content: Everyone has been telling him the same thing for 5 pages, but he doesn't understand.He has blinders the size of his egoHe will answer something like:78) Ok ATX PSUs are fine unless you live near some bugs.

author: bd139
author_rank: Super Contributor
timestamp: April 23, 2020, 08:22:32 pm
content: This isn't the first thread either...

author: Mr. Scram
author_rank: Super Contributor
timestamp: April 23, 2020, 08:28:09 pm
content: Start a thread, get lots of great information and nuance and become belligerent when that doesn't line up with your initial assessment. Insist everyone is wrong or in it to spite you.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 11:08:32 pm
content: When all you've got is name calling and personal attacks and blaming bugs it's...The Wrath of the Bruised Egos

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2020, 11:14:15 pm
content: Collectively, immensely more experience has been offered than you care to recognise or even accept.

author: Someone
author_rank: Super Contributor
timestamp: April 23, 2020, 11:30:59 pm
content: Quote from: engrguy42 on April 23, 2020, 04:58:31 pmWhat makes that a match for the "Test Equipment" forum you posted this in? As people keep mentioning to you there is no comparison or connection to test equipment. Consider what the response might have been in one of the other forum categories and how they may be more appropriate for your thread which you seem to insist is nothing about test equipment.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 11:32:06 pm
content: Quote from: tautech on April 23, 2020, 11:14:15 pmWith a one or two exceptions, all you guys have provided is vague, unsupported generalizations and personal attacks. I've provided actual test data, schematics, simulations and analysis of an actual power supply's performance. Actual data. And nobody has even attempted to dispute any of that data, only finger wagging that ATX supplies suck.Your experience is irrelevant if you can't discuss, or don't know, the facts.Shameful.

author: engrguy42
author_rank: Frequent Contributor
timestamp: April 23, 2020, 11:33:44 pm
content: Quote from: Someone on April 23, 2020, 11:30:59 pmSERIOUSLY?? That's all you have? I posted in the wrong forum???Unbelieveable.

author: Shock
author_rank: Super Contributor
timestamp: April 24, 2020, 12:14:45 am
content: It belongs in computers.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 24, 2020, 12:31:37 am
content: What's with the -boots in- on OP engrguy42 ?FYI to disbelievers > there's some great ATX psu and eye candy mods on Youtube posted on there for yearsthe tubers go into great detail as well as emphasis on safety etc and show the unit/s operating long term etci.e. an ATX is just a power supply, and an unloved or orphaned one can be open season for re-purpose, hacks and mods,just like most electronics gearGive it a rest and perhaps gab about protection and limiting mods that won't break the bank or require EE know how,and alternative uses or application suggestions that may appeal to 'have a go-ers'