Looking for a design: Mod to a Computer PSU that will provide Constant V / A  - Page 1
Forum section: electronics / projects

author: irakandjii
author_rank: Regular Contributor
timestamp: March 17, 2017, 06:50:31 pm
content: The Need : I would prefer to not to spend >$750 (Cdn) for a linear bench power supply if possible.I need a bench power supply for IoT and embedded system work. 12v max is fine (driving motors, relays) and most commonly I use < 5v to drive components, accurate control of 3.3v is a high priority (Most of my stuff is 3.3v).I have installed a ATX Breakout Board from Dangerous Prototypes that presents the PSU voltages to some binding posts. I believe this circuit ensures a proper "load" to the PSU to prevent it from overloading. This is ok but far from ideal.Materials on hand:I have some High quality (800+ watt) Computer Power supplies that I no longer use, but in great shape. These supplies are "Gold" standard efficiency, high quality etc. etc.They provide +12V, -12V, +5V and +3.3VI have 30+ Amps available on each 12V rail.Goal: I would like to build a project which will use the output from the PSU and provide:Some Points of clarification: (from post below)I envision a 3 part system:A) The computer PSU is the back end and is not opened or in any way tampered with, I expected to use the 24 PIN Molex as the Input connector (no wire cutting) it provides power to parts B & C.B) Power Rail (+12v) This is a fused rail (or current limited supply) that provides power to mechanical components motors, actuators etc. (would be a separate battery in the real robot) The bulk of the PSU power would flow this was up to say ~300 WattsC) Front end (the project) provides the variable voltages and currents for the electronics test bed1) I expected to come off the PSU immediately into a circuit breaker / polyfuse as protection into the project.2) The output of the project would be max 5 Amps on the 12v and 3 amps on the other output voltages. I could get away with less.3) I was looking for a way to make sure any noise/ripple in the PSU was reduced within the project, and not passed to the outputs4) I was hoping to find a way to make sure the front end was very precise since it controls the electronics5) "Emergency" shut down capability from the project to shut off all power via solid state relays. Example: Big red Button and/or triggered from conditions on the outputsSo I look at it as if we have a constant voltage source to multiple precision power supplies.Part C - The Front End cont'd1) Constant Voltage output set by a variable resistor .. highest possible with the +/- 12v sources.2) Variable Current limits on all voltages, essentially a variable fuse on each of the stock voltages (+12v, -12v, +5v, +3.3v)3) Constant current output using the +12v & -12v sources, Ideally with a settable voltage <= the source.4) Digital display of settings, (maybe digital control of voltages etc. via digital resistors.).5) Avoid voltage spikes at all costs (I have spent too much already on blown materials)6) Need through hole parts, I can't mount tiny things on boardsIf anyone knows of an efficient robust design for this please provide a link so I can find it.Background:It is not for lack of trying that I come to this request ...I made a few "Internet Projects" that havefailed dangerously, one in a spectacular fashion with an explosive mosfet! (parts bounced off my eyeglasses) others just overheated, smelt bad and ejected magic smoke.So just "googling it" has presented a lot of "solutions" but, I do not have sufficient knowledge to choose between the alternatives. As-in what is a professional safe design vs some young players daydream. I have seen many .. many .. internet solutions based on LM317 or 78xx voltage regs but just as many conflicting opinions about using them. I have also seen folks say just use a transistor with no further detail, quite frankly if I understood what to do I would not need to make this post.I have watched Dave's Videos for a PSU, but it seems the designs evolved so much that I am not sure what portions are applicable to my need. It may be that what I need is a sub-component of his design.Finally, I am interested any pointers to a really solid and current educational resource on precision power supply. I have become so "confused now" and I can't see the forest for the trees.Thanks in advance

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2017, 07:37:03 pm
content: The computer power supply is not a good starting point unless you want something like 12 V and 25 A with lots of noise. As a side note, the -12 V is usually only very low power, maybe 100 mA if you are lucky.A better starting point would be an old laptop supply - these are usually something like 20 V and 3 A. So an output up to 16 V or so might be realistic. Dave's design is no that great - using an voltage regulator and adding current limit as an afterthought is problematic.There are linear lab supplies for a reasonable price if you don't insist on high power and resolution. The main drawback with the cheap Chinese ones it that they tend to overheat at there rated current. So as long as you stay away from the maximum current they can be reasonable good.Building you own supply is more about learning, not so much about saving money.

author: Avacee
author_rank: Unknown
timestamp: March 17, 2017, 07:40:52 pm
content: One reason computer PSU's are not recommended as bench power supplies - even though there are a LOT of projects out there when you "google it" - is that if anything goes wrong a PC PSU can supply a lot of current. You mention 800W PSU's and the 12V rail having 30Amps. That's a LOT of current to blow through You and/or your IoT/Embedded system when something goes wrong.You mention a budget of CDN750 so may I suggest looking at a "proper" bench power supply with all those protections build in.You can then use that proper bench PSU with its protection as a source whilst you learn to build your own power supply.Something like the popularhttp://www.rigolcanada.com/products/dc-power-supplies/dp800/dp832/Converting a PC PSU into a bench power supply is a fun project, and I've done it myself, but if you're currently blowing stuff up whilst learning then blowing stuff up with a PC PSU behind it it can blow up big and bad and worse, hurt you at the same time.Learn how to do constant current, voltage, overload protection, voltage spikes,etc using a small power supply first (ie a wall wart,laptop charger/a proper bench psu).When you have those fully understood then move up to something can hurt youNot to put you on a downer as a self-built PSU is a fantastic project for learning and its highly satisfying to use your own gear - but please don't learn with something so dangerous.Another great project when building a PSU is a DC Load to test it with - how will you know your PSU control circuitry is working unless its tested with a known load?Peter Oakes, imho, does a good job of explaining the basics of building the control circuitry for a Power Supply and DC Load (as see Dave's EEVBlog #102).Work through learning PSU design with a wallwart/laptop charger/proper bench psu firsthttps://www.youtube.com/playlist?list=PL_atu5RtEPi4aNzoMtZ5_S6ruhFR98T_p

author: BrianHG
author_rank: Super Contributor
timestamp: March 17, 2017, 08:14:26 pm
content: Even a crummy supply like one of these is a safer bet than retrofitting a PC supply (I listed 10amp and 30amp units):http://www.ebay.ca/itm/Low-Cost-0-30V-10Amp-Bench-Power-Supply-CSI3010SM-/221720976295?hash=item339f999ba7:g:MqwAAMXQeW5TYsaxhttp://www.ebay.ca/itm/CSI3020SW-0-30V-0-20A-Switch-Mode-Bench-Supply-/381200355498?hash=item58c14fd0aa:g:3NkAAOSwofxUdggQhttp://www.ebay.ca/itm/Hi-Resolution-0-01V-0-001A-Mini-DC-Power-Supply-Output-0-15V-0-10A-AC110-220V-/302236440409?hash=item465eb21759:g:A~kAAOSwjDZYY8tOhttp://www.ebay.ca/itm/Industrial-10-Amp-30-Volt-Adjustable-Digital-DC-Power-Supply-Box-Kit-w-Clip-Cord-/201702004870?hash=item2ef6605886:g:bCwAAOSwB09YE5Ynhttp://www.ebay.ca/itm/Leader-718-10D-Laboratory-DC-Power-Supply-18-Volt-10-Amp-/142314771443?hash=item21229f13f3:g:wwsAAOSwLF1X2YKwhttp://www.ebay.ca/itm/Hi-Resolution-0-01V-0-001A-Mini-DC-Power-Supply-Output-0-30V-0-10A-AC110-220V-/282376251674?hash=item41beefa91a:g:KMEAAOSwjDZYY8w5Though most of these are Chinese crap, they are safer to develop with compared to the raw output of a PC power supply.

author: Monkeh
author_rank: Super Contributor
timestamp: March 17, 2017, 08:20:37 pm
content: Quote from: irakandjii on March 17, 2017, 06:50:31 pmOkay. How about <$100?https://www.circuitspecialists.com/bench-power-supply-csi1802x.htmlhttps://www.circuitspecialists.com/csi3003sm.htmlhttps://www.circuitspecialists.com/csi5003xe.htmletc etc.

author: farzadb82
author_rank: Contributor
timestamp: March 17, 2017, 08:44:27 pm
content: If you only need a single supply at any given time, I would highly recommend the Korad KA3005D (http://www.ebay.com/itm/120922717980?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT) as budget power supply. Dave has done a video on these, which is what prompted me to get one. I would not recommend the dual output version, however. The power supply itself is fine, but the UI is somewhat kludgy.Alternatively, if you want to go even more budget, you could go with something like the DPH3205 (http://www.ebay.com/itm/32V-5A-Adjustable-Digital-Programmable-Buck-Boost-DC-Power-Supply-Module-H9H0-/132030603424?hash=item1ebda320a0:g:60kAAOSwA3dYS5S6). I personally have not used them, however, there was an article on hackaday a little while ago about how "hackable" these were and a small community has started to grow around modifying them to provide nicer features. Pair this up with a nice laptop power supply that can provide 19v and you should easily be able to get the voltage range you need and for < $100.

author: BrianHG
author_rank: Super Contributor
timestamp: March 17, 2017, 09:00:56 pm
content: More 20 amp cheap junk:https://www.aliexpress.com/item/PS3020-DC-switching-power-supply-30V-20A/32757587746.html?spm=2114.01010208.3.45.ZWkm76&ws_ab_test=searchweb0_0,searchweb201602_6_10065_10130_10068_433_434_10139_10136_10137_10138_10060_10062_10141_10056_10140_10055_10054_10059_10099_129_10103_10102_10096_10052_10053_10050_10107_10142_10051_10106_10084_10083_10080_10082_10081_10110_10111_10112_10113_10114_10078_10079_10073_10070_10122_10123_10124-10102,searchweb201603_9,afswitch_1,ppcSwitch_5,single_sort_0_default&btsid=6db8291a-7e75-45cc-9549-d2388ba4201a&algo_expid=761181d4-f710-452f-9d3e-59a95f77e1a7-5&algo_pvid=761181d4-f710-452f-9d3e-59a95f77e1a7https://www.aliexpress.com/item/Precision-PFC-Compact-Digital-Adjustable-DC-Power-Supply-OVP-OCP-OTP-32V-20A-220V-0-01V/32328835922.html?spm=2114.01010208.3.46.XxWfZ7&ws_ab_test=searchweb0_0,searchweb201602_6_10065_10130_10068_433_434_10139_10136_10137_10138_10060_10062_10141_10056_10140_10055_10054_10059_10099_129_10103_10102_10096_10052_10053_10050_10107_10142_10051_10106_10084_10083_10080_10082_10081_10110_10111_10112_10113_10114_10078_10079_10073_10070_10122_10123_10124,searchweb201603_9,afswitch_1,ppcSwitch_5,single_sort_0_default&btsid=14268e1a-bde6-487c-9ad3-f3c370d96bc6&algo_expid=f1bac658-e2f9-4810-9f86-ba617608a05f-5&algo_pvid=f1bac658-e2f9-4810-9f86-ba617608a05fDont forget to verify your AC input voltage support.

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2017, 09:41:46 pm
content: My most used bench PSU is one that I built using a 32V power brick from one of those printer/scanner/copier things that I got for free and a $5 Chinese buck regulator module with current limiting. I mounted it all in a box I salvaged from work and put a cheap LED V/A meter on the front panel. Removed the pots from the regulator board and replaced them with panel mounted pots. Output is a bit noisy, expected since it's a switcher, and the current limiting oscillates a bit but does the job keeping the magic smoke from getting out. I have less than $20 invested in the whole thing and it gets used a lot because it's compact and very light weight.If you want a linear PSU there are some simple designs out there that have adjustable current limiting. Power supplies make good projects IMO, they're fun to build, not too complex and you can never have too many.

author: elecman14
author_rank: Unknown
timestamp: March 17, 2017, 09:50:59 pm
content: You could hook something like a DP20V2A (multiple current and voltage rating modules are available) to your computer power supply. Not sure how much headroom you would be left with if you used the 12V rail though. Might be better with a laptop power supply. You also could add fuses or buy a product with poly fuses like this:http://dangerousprototypes.com/docs/ATX_Breakout_Board

author: Smokey
author_rank: Super Contributor
timestamp: March 17, 2017, 10:38:08 pm
content: I'm always amazed that people think they can desolder a magic resistor or add a magic capacitor or something and turn a PC power supply into a high precision, professional, programmable, Ethernet connected, LCD displaying, bench supply. It's even more silly to think there is a magic circuit you can solder in somewhere that will only cost you $3.50 that does the same thing. And it only takes 5 minutes to upgrade!There are guys that spend their entire career designing circuits to do this stuff. There is a very fundamental lack of understanding on how power conversion works here. Might as well be asking how to make a Mars Rover.Buy a power supply and have fun building whatever you are planning on building.

author: irakandjii
author_rank: Regular Contributor
timestamp: March 17, 2017, 10:51:40 pm
content: Quote from: elecman14 on March 17, 2017, 09:50:59 pmThe DO20V2A is the sort of circuit I image, but with 12V in so it is the correct conceptThank you, the ATX Breakout Board is in fact the board I have now.Some Points of clarification:I envision a 3 part system:A) The computer PSU is the back end and is not opened or in any way tampered with, I expected to use the 24 PIN Molex as the Input connector (no wire cutting) it provides power to parts B & C.B) Power Rail (+12v) This is a fused rail (or current limited supply) that provides power to mechanical components motors, actuators etc. (would be a separate battery in the real robot) The bulk of the PSU power would flow this was up to say ~300 WattsC) Front end (the project) provides the variable voltages and currents for the electronics test bed1) I expected to come off the PSU immediately into a circuit breaker / polyfuse as protection into the project.2) The output of the project would be max 5 Amps on the 12v and 3 amps on the other output voltages. I could get away with less.3) I was looking for a way to make sure any noise/ripple in the PSU was reduced within the project, and not passed to the outputs4) I was hoping to find a way to make sure the front end was very precise since it controls the electronics5) "Emergency" shut down capability from the project to shut off all power via solid state relays. Example: Big red Button and/or triggered from conditions on the outputsSo I look at it as if we have a constant voltage source to multiple precision power supplies.I keep reading in the eevblog forum that lots of power supplies have big start-up spikes. I was hoping to avoid this.

author: irakandjii
author_rank: Regular Contributor
timestamp: March 17, 2017, 11:08:40 pm
content: Quote from: Smokey on March 17, 2017, 10:38:08 pmOh you assume too much my friend. I do not think this is easyas you should see in my post, nor do I think it is a $3.50 exercise. I expect that a proper and safe design would take some effort to do. As it is the PSU's are worth over $300 given what I want to do $100 in parts would be fine.I am hoping that one of the folks that has spent 30 years in the field would have some designs to offer up to the public domain as I have on the software front. Good folks are out there.But thank you for taking the time to express your opinion.I am sure your approach will help someone else justify throwing money at a problem rather than be open and learning of a way to do better.I prefer to learn and make something right.

author: djnz
author_rank: Regular Contributor
timestamp: March 18, 2017, 04:48:31 am
content: Have you taken a look at this:DIY SCPI programmable dual channel bench PSU 0-50V/3A (now EEZ H24005) by prasimixhttps://www.eevblog.com/forum/projects/diy-programmable-dual-channel-bench-psu-0-50v3a/

author: Avacee
author_rank: Unknown
timestamp: March 18, 2017, 08:36:25 am
content: Your requirement sounds like a smooth linear 0-12V,0-5V 0-xA with typical bench psu features and the reason you haven't found a design from someone with 30 years experience whilst googling is because those designs do not exist as people with 30 years experience recognise that a PC PSU is, quite simply, the wrong tool for this job. You are trying to fit the proverbial square peg in a round hole.If such an instructable / circuit existed you'd have found it.Safety aspect. If something goes wrong your 12V 30A rail is capable of 30A sustained. It's peak when something goes tits up will be much higher before the PSU's safety trips. That's beyond a slight tingly feeling in the end of your finger. Along with a melted/ignited breadboard. You mention an "oh-shit switch". It's going to have to be a bloody good switch. Wrong tool for the job.You can't do 0-12V from a 12V rail simply because you'll drop some voltage over the transistor/mosfet/whatever.So you would need to boost the voltage to higher than 12V and then drop it down again. Efficiency issues. Wrong tool for the job.The ATX Spec details a tolerance of 11.4V-12.6V with a max ripple of 120mV on the 12V rail land 4.75V-5.25V with max ripple of 50mV on the 5V rail.So your boost converter is getting a noisy rippletastic input and boosting it to something that's going to also be noisy which you then have to smooth.People with 30 years experience will just get a smoother power input at more than 12V and avoid having to deal with both the PSU's noise and boosting it. Wrong tool for the job.Mains Isolation. It's recommended that a bench psu is isolated from the mains which a PC PSU isn't. Wrong tool for the job.Right tool for the job for developing your IoT/Embedded systems = buy a proper bench PSU with all the protection built it that is isolated from the mains and make one of your projects building a power supply that isn't powered by a ripple-tastic massively over-powered mains connected PC PSU.12V 3A can be provided by a Laptop charger. I use an IBM one that's 16.8V 3.33A. The 16V can be dropped down to 12V in one go without needing boosting first and it handles 3A ok.When is a PC PSU the right tool? Powering motors/lots of LEDS/other high current devices/etc at 5 or 12V.There's lots of examples on how to put a PC PSU into an enclosure and wire up some banana jacks. You've probably seen them with your previous googling and that's what the breakout board provides.Those tutorials will also explain about the dummy load requirement that's needed for an SMPS to achieve regulation.Here's a project using a buck/boost converter if you are dead set on doing it:

author: irakandjii
author_rank: Regular Contributor
timestamp: March 18, 2017, 04:54:54 pm
content: Quote from: Avacee on March 18, 2017, 08:36:25 amThank you for this, I edited the quote so I could match my feedback to you comments.

author: alm
author_rank: Super Contributor
timestamp: March 18, 2017, 07:28:31 pm
content: Given the task of taking 12V from an ATX PSU to create multiple clean, floating 0-XV rails, I would probably just use the ATX power supply to power an inverter, and from that 110/230 VAC power a traditional bench supply. Going through a bunch of isolated DC-DC converters and filters seems like an awful lot of effort just to save a few transformers and caps.

author: Nusa
author_rank: Super Contributor
timestamp: March 18, 2017, 08:16:02 pm
content: Something like this is probably your easiest route to do what you want. The DPS line has gotten reasonable reviews. The hardware has been hacked, and there is an open source firmware you can install if you don't like the original.https://www.aliexpress.com/wholesale?catId=0&SearchText=programmable+power+supply+module

author: irakandjii
author_rank: Regular Contributor
timestamp: March 18, 2017, 09:32:33 pm
content: Quote from: alm on March 18, 2017, 07:28:31 pm.. I just don't know what to think, clearly you are trying to be helpful in some way. I just can't imagine what it is?