Help building a lab power supply from an ATX - Page 1
Forum section: electronics / projects

author: zdelarosa00
author_rank: Regular Contributor
timestamp: April 03, 2015, 07:26:00 pm
content: Hi there, new guy here, I came across the EEVBlog forum because I asked Dave on twitter about a good electronics forum where I could post my issues (ultralol). I’m a Mexican Electronics Engineering Student.I recently lost my good-ol little 3A +12v power supply, I decided to gave it a clean from the inside and I found out that it was so crappy a 10yo kid with a beginner kit could have done better, the thing was always giving +17 and so but I put googly eyes in it when I got it so it kinda earned the love.So I decided to build one for myself, I was planning to design the circuit, the printed board, with protections, cool looking led’s and even a transparent acrylic case, and suddenly a friend brought me an old kinda-cheapy ATX, so i said “meh, it can work like a charm for the job if I give it a little love and caring”.Then I opened it up, cleaned it up a little (it wasn’t that dirty), and re-wired the main outputs, +5,+12,+3.3, and also re-wired -5,-12 and GND to banana connectors drilled in the case internet-style, I also added a +9 output with a 7809 getting input from one of the +12 rails and screwed it to the main heatsink, clean work, very cautious, but I can not get it to turn on.I got to the point where i think I have read too much on too many websites.I know the Green Pwr_On cable is pulled up internally by the +5stndby Purple wire and it must be pulled down to Gnd in order for the ATX to turn on, but when I do that nothing happens (both wires passed the continuity test to the main board).I didn’t know you must put a minimum dummy load in order to start it up correctly, and I turned it un a few times without it to test it after getting the job done and I measured nothing on the outputs, but now even with a 15ohm 10watt resistor in +5 or +12 it doesn’t turn on.I know the ATX works because before the treatment I used it to power up a couple of breadboards with +12 and it went ok, but the Pwr_On was grounded with tape and everything was messy, sense line cut, most of the main rails of +12 and +5 also cut.I get +5stndby on the purple wire (which has a led and resistor connected to it), and +5 in the Pwr_On, but I don’t get anything on the Grey Pwr_Ok wire, fuse is ok, the caps seem good, nothing burned, nothing shorted, 3.3sense connected to +3.3 main rail, and mains input switch in the correct position (120v here), but when i switch the Pwr_On wire to Gnd the psu does not start. Any of you guys have any ideas or previous experiences?, I have declared war on this thing, I can post more info or photos if your need it. The model on the label is LC-B300ATX

author: Ian.M
author_rank: Super Contributor
timestamp: April 03, 2015, 07:33:54 pm
content: You've just found out why its extremely dumb to put any money/time into modding a used ATX supply. If you have to turn an ATX supply into a bench PSU, use a connector salvaged from a scrap motherboard to connect it to the rest of your bench supply circuit so the unmodified ATX PSU can easily be replaced.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: April 03, 2015, 07:42:39 pm
content: I guess, but im pretty sure its something simple, and the thing was already modded before

author: Yansi
author_rank: Super Contributor
timestamp: April 03, 2015, 07:59:23 pm
content: Thats not about if it can be moded, but if it is worth it. (No, it is not.)Yes, you can "mod" the ATX PSU topology into a full featuring lab supply with nice spec, but in other words it means to rebuild it ALL from scratch. Designing a linear supply with switchmode pre-regulator is pretty possible and achieveable with some serious spec, but it has nothin in common with "hacking a PC ATX supply".There are many "plans" for "hacking the PSU", more or less crap, but you won't get good lab supply out of this.If you want to build some, go and get a schematic for proper supply. Or if you need a supply, buy one. Those last Korad's 3005 PSUs are relatively good, after modding the heatsinking you get cheap supply with good bang for the buck.To sum up: If you need at least some regulated supply in your shack, buy one. Designing one without equivalent knowledge and tools is not probable for success. And hacked ATX PC supply is really NOT much useful. And also dangerous.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: April 03, 2015, 10:32:54 pm
content: So yea... my view on this thing is that sometimes you need clean, well-regulated power, and sometimes you need a fuckton of power. The two cases very rarely overlap.ATX PSUs are very good at delivering a fuckton of power. They are very bad at that power being clean. Also, delivering a fuckton of power has its drawbacks in most lab scenarios. "Hey I can run my poultry hatching machine and my heated enclosure 3d printer off of this with power to spare" is, unfortunately, synonymous with "omfg this can so totally put a breadboard AND my cables on FIRE".They are also ground referenced. This can be a major drawback for a lab supply. I'm sure they can be hacked, but it's putting lipstick on a pig. It's still a pig.That said, I think ATX PSUs totally have a place in the lab. I do plan to lasercut a nice enclosure for an ATX PSU, with high amp rated connectors, and some polyfused 2.1A USB charging ports. I don't intend to open the bloody thing up, just cut off the connectors and replace them with crimp terminals. Also, I'll only use it to do heavy lifting. I have a Chinese linear supply for breadboarding and similar.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 03, 2015, 10:58:23 pm
content: Quote from: Yansi on April 03, 2015, 07:59:23 pmA "hacked" ATX PSU can be quite convenient when what you need is mostly 3.3/5/12V - I have one on my bench in large part for that. (And in part because its 5VSB regulator is busted, so it needs a little help to get started but still works perfectly fine otherwise.)

author: Yansi
author_rank: Super Contributor
timestamp: April 03, 2015, 11:05:42 pm
content: Quote from: Sigmoid on April 03, 2015, 10:32:54 pmStandard 30V 5A lab bench supply will deliver the same fuckton as the modified ATX shit.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: April 04, 2015, 06:12:00 am
content: Okay, I do need a power supply, and I definately gonna buy a good one on the future, but I'm not modding this because of the pleasing sensation, I do it because I could get it to work first, all I need is to turn it on, it obviously is not the first option for a sturdy bench power supply, but I'm here and more than half of the job is done, I definately gonna look for a good one while this thing gives me reasonable functioning

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: April 04, 2015, 06:32:18 am
content: The heatsink is either live or grounded. Make sure that the tab of the 7809 is isolated from the heatsink. Also, if you want an awesome bench supply (with a bit of output noise) fleabay has some ~$30 digital constant current/voltage supplies that can do up to 30v at 8a.

author: BradC
author_rank: Super Contributor
timestamp: April 04, 2015, 06:39:14 am
content: Quote from: Yansi on April 03, 2015, 11:05:42 pmUnless your 30V 5A Lab supply can magically do 25A @ 12V then you are pretty badly wrong.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: April 04, 2015, 07:48:01 am
content: Quote from: iamdarkyoshi on April 04, 2015, 06:32:18 amAwesome..! I feel so dumb ! .. you're a demigod dude..! thank u so much..! you helped me on my way to become an electronic wizard technomancer dude..! respect forever...! a story to tell to the kids :') .. i shat myself :'''D .. no smiley could express this but im going to put the clapping one anyway.. seriously dude thanks :') ... It was something simple at the end indeed.aaaand,, pink floyd by the way.... im definately gonna check up for one of those good bench supplies..!

author: DanielS
author_rank: Frequent Contributor
timestamp: April 04, 2015, 08:02:15 am
content: Quote from: BradC on April 04, 2015, 06:39:14 amExactly.A bench supply is great when you need (relatively) precise and clean power with variable voltage and current limits. An ATX supply is great when you just want to power something up that uses more power than what reasonably affordable bench PSUs can handle and that load just so happens to fit conveniently on one of the rails.An ATX power supply is safer, cheaper and lower maintenance than a car battery or other convenient high-power 12V sources.

author: nctnico
author_rank: Super Contributor
timestamp: April 04, 2015, 11:16:44 am
content: But still I'd like to have some (current) control over a high power PSU. There are good deals to be found on Ebay; last year I picked up a couple of 1000W HP6012A bench PSUs.

author: dom0
author_rank: Super Contributor
timestamp: April 04, 2015, 11:20:43 am
content: Quote from: Sigmoid on April 03, 2015, 10:32:54 pmThe isolation is insufficient for that. Don't do it.Also, EMI issues. The secondary is grounded to have a return path for all that HF feeding through the transformers.

author: Yansi
author_rank: Super Contributor
timestamp: April 04, 2015, 11:30:40 am
content: Quote from: BradC on April 04, 2015, 06:39:14 am

author: Sigmoid
author_rank: Frequent Contributor
timestamp: April 04, 2015, 03:54:48 pm
content: Quote from: Yansi on April 04, 2015, 11:30:40 amVery mature. Thanks for the contribution.

author: dannyf
author_rank: Super Contributor
timestamp: April 04, 2015, 07:39:41 pm
content: Another option you may want to consider is a laptop power source + regulators to step it down to your desired voltage. Easy, cheap and much smaller than an atx-based solution.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: April 05, 2015, 07:49:35 pm
content: Quote from: dannyf on April 04, 2015, 07:39:41 pmThats a good idea, never thought of it regarding the modded pre-made power supplies, you made me remember this thing...

author: Ian.M
author_rank: Super Contributor
timestamp: April 05, 2015, 08:16:16 pm
content: Start with a laptop PSU with at least 19V 2A output with a TWO pin mains lead.Make your power supply in a box with a DC in power jack to match the laptop PSU.Add DC-DC converter modules off EBAY for whatever output rails you need. Use isolated ones for negative rails. It will be easier if you pick modules with presets for output voltage and current limit that you can wire to front panel pots rather than digital controls.If the total output power possible from the modules you have chosen exceeds the laptop PSU rating, add a circuit breaker in series with the DC power in wiith a trip current of the laptop PSU rating.A master switch on the front panel that disconnects ALL the output rails except 0V simultaneously is very useful. Add a 2mm test probe socket for each rail *BEFORE* the switch on all adjustable rails + a 0V one so you can check voltages and current limits before doing the traditional smoke test.A LED + series resistor on each rail (On fixed rails add a series Zenerso Vf+Vzn is ~ 90% of the rail voltage so it will go out if the rail is being dragged down) is also good to have. You may want to use a constant current circuit for the LED on adjustable rails.If you are feeling flush, go wild with metering etc, but it soon reaches the point where it would be cheaper to buy a reasonable quality used bench PSU.

author: hamster_nz
author_rank: Super Contributor
timestamp: April 06, 2015, 04:54:57 am
content: You could also tryhttps://www.sparkfun.com/products/12867orhttp://www.seeedstudio.com/depot/atx-breakout-board-bench-power-supply-p-1222.html+http://www.seeedstudio.com/depot/ATX-Breakout-Board-V11-Acrylic-Case-V1-DP10080-p-1288.htmif you wanted to put it in a case.And if you don't want to order either of these, you can at least look at the schematics...

author: kjs
author_rank: Regular Contributor
timestamp: April 06, 2015, 07:10:08 pm
content: The Sparkfun board in my opinion is outright criminal. You have all the open metal things with power where a short can happen, no, will happen when used on a lab bench. The seeedstudio with case is better but you'll have a hard time to clamp wires into the binding posts.I didn't like all these ideas with the looooong cables from teh ATX supply crowding the desk and went with a second front panel mounted to the ATX power supply. A bit more work to change if/when the supply fails (20mins to cut and solder the wires) but at least a clean table.https://www.eevblog.com/forum/projects/a-pc-psu-%28atx%29-based-lab-power-supply-the-umpteenth-version/msg621374/#msg621374If you add fuses please read their datasheets. A fast blow 5A fuse will stand 150% (7.5A) for at least 1h and 210% (10.5A) for up to 30mins. That's a lot longer than your power supply will survive. We have several of them in case we need high current and no real need for a clean supply. They all had to withstand a short while in use and survived but they are fused conservative (10A outputs have a 5A fuse, 20A a 10A fuse).Just keep in mind that these aren't lab power supplies. If you want a lab power supply you will have to spend money for it. The cheap Chinese are switchmode too and have very noisy outputs especially if you use them in current limiting mode. I have one here from a friend who wanted to save money and it's interesting how you can make it freak out with the right pulse load. If you run it in current limiting mode set to 3mA it oscillates between 1.5 and 5mA, useless. You simply get what you pay for.Unless you need high current all the time it is often a better idea to get a small but good linear lab power supply (15 or 30V/1-2A) for day to day use and use an ATX for the few occasions where you need the current.

author: Cloud
author_rank: Regular Contributor
timestamp: April 06, 2015, 07:53:51 pm
content: Why would you bother to use ATX supply? Its better to use LM317 or 338. ATX don't have very fine regulation. Connect voltmeter to the 12V rail and measure voltage, than put a few ohm load on it and than measure voltage, you'll notice, that voltage will drop for about 0.5V. Or try to put on some load via pwm controller. You'll hear supply making sounds in frequency of PMW. If you pay shipping I'll send you all the components you need.

author: DanielS
author_rank: Frequent Contributor
timestamp: April 06, 2015, 08:37:12 pm
content: Quote from: kjs on April 06, 2015, 07:10:08 pmAny half-decent ATX power supply has over-current on their outputs and usually over-power protection on the primary side, as required by the ATX specification and those requirements got a tighter with the 2.xx revision. If you give them a hard enough short, they shut down and they are required to recover with no ill effects whatsoever after the short is removed, at most requiring power-cycling to reset the OCP/OPP lock-out. Even the ~12 years old Antec SL350 I have on my bench recovers perfectly fine from shorts from any rail to ground, which is usually the only type of supply-side short I need to worry about since I rarely need more than one rail at a time.The main problem with ATX supplies is there is a horrible lot of crappy models on the market, so you need to know what you are looking for or you very likely end up buying a cheap (or overpriced) PoC.Quote from: Cloud on April 06, 2015, 07:53:51 pmIt is convenient when you need a fair amount of current and do not care much about exact power regulation, either because the load does not care about 50-100mV worth of noise or +/-5% nominal value (3.3V/5V logic) or because the load has its own local voltage regulators so it only needs a supply voltage that is in the right general ballpark.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: April 08, 2015, 04:10:36 am
content: yes, the loads that are gonna be powered from now on are not so picky, in a bad case i will simply think to use a 78xx regulator or similar... in regarding to the fuse guy.. is that the fast blow one? then the fuck delays the slow one? what kind of fuse is that? even my multimeter blows at 20 secs max according to the labels.I dint know about the atx's having noise, aren't they supposed to be powering processors and sensible electronic components?, that means motherboards and pc stuff can take a reasonable amount of noise and variations of supply voltage?

author: Ian.M
author_rank: Super Contributor
timestamp: April 08, 2015, 04:17:07 am
content: Motherboards have on-board regulators for the CPU + digital circuits don't really care about supply noise that is several times less than their logic thresholds.However analog circuits, unless very well designed for high PSRR, *DO* need clean power.The only high current low voltage stuff most of us have is automotive electronics, and although its nominally 12V, its normally used with a supply of about 13.5V, as 12V flooded cell Lead acid batteries on float charge are around that voltage. That means you are under-powering the CB, or car audio or whatever by 11% if you use a PC PSU which is not exactly ideal.