ATX Power Supply Modification - Page 1
Forum section: electronics / general

author: shawty
author_rank: Regular Contributor
timestamp: July 19, 2019, 12:00:40 pm
content: Hi All,Digital electronics I can do, more conventional stuff, well I'm probably closer to beginner than anything else.Hence why I find myself here asking this question.I have some specialized computers for powering industrial equipment, 4 serial ports on them a digital I/O system on them, 2 Ether, 4 USB and the whole thing just looks like a giant heat sink designed to be bolted onto the side of something.What I don't have however are power supplies.These things have a green 3 pin connector, which I've already sourced plugs for, the 3 pins are -v, grnd, +vThe voltage input to these things is from 9V all the way up to 40V so they can operate on quite a range.My problem however is the ampage.The - and + both require supplys that can support up to a 3 amp or above draw.I have a whole bunch of old but working ATX PSU's kicking about, and all of them will give +12v at up to 20A but I can't find a single one that will give more than about 0.3A on the -12V output.so, my question is.....Is there anyway I can up the ampage on the -12V rail?I don't mind having to build an external circuit with something like a boost converter if that's what I need, as long asA) The + & - voltage levels matchB) Both + and - can hold 3A or higherIdeally though, I'd like to modify an ATX to do this, or at the very least build an adapter to connect to the -12 that allows me to draw higher, is this possible?CheersShawty

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: July 19, 2019, 12:49:27 pm
content: http://planetimming.com/atx_mod/atx_mod.html-Have a look at this link. I did this and had some positive results increasing the current on all the rails of a cheap ATX supply.Works great.

author: shawty
author_rank: Regular Contributor
timestamp: July 19, 2019, 01:50:49 pm
content: Thanks Joseph, reading now :-)

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2019, 01:57:06 pm
content: You know you can buy power supplies that have higher current on both positive and negative, here's just a basic list:https://www.digikey.com/products/en/power-supplies-external-internal-off-board/ac-dc-converters/133?FV=ffe00085%2Cii2%7C2211&quantity=0&ColumnSort=1000011&page=1&pageSize=2517$ gets you 1.3A +/- 13.5v for example :https://www.digikey.com/product-detail/en/mean-well-usa-inc/RD-3513/1866-3982-ND/7705981But you know, there's toroidal and regular transformers out there starting from around 15$ for a 50VA transformer (about right for a +9v and -9v and around 2.5-3A on each rail). Add a plain bridge rectifier that costs half a dollar and a couple of plain electrolytic capacitors to smooth the output voltage and you have your power supply.

author: mindcrime
author_rank: Unknown
timestamp: July 19, 2019, 02:06:35 pm
content: You may be able to make your own -12V rail, based on the +12V rail, instead of relying on the -12V rail built into the power supply. There are voltage inverter IC's you can use for this, but there are other ways to do it. See:https://www.digikey.com/en/articles/techzone/2012/oct/generating-negative-output-from-positive-input-voltage

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 19, 2019, 02:47:07 pm
content: Quote from: shawty on July 19, 2019, 12:00:40 pmMaybe, but the approach I would take is to hang a buck-boost converter off the 12V rail. Note that the peak current in the switch for Vout = -Vin will be a little more than twice the DC load current, so keep that in mind if you restrict yourself to integrated switch controller ICs (which, to be fair, are all you should really be considering if you haven't built a switchmode power supply before).

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2019, 03:03:01 pm
content: Some switching power supplies have a boring 7812 linear regulator or a 34063 style switching regulator for -12v, because it's not critical - it's only used for serial ports, and most serial devices are happy with anything better than around -8v anyway.So you can't really "increase" the -12v output by a significant amount. It's pretty limited to that 0.3A .. 0.5 AYou CAN use switching regulators in inverting mode, a MC33063 or MC34063 will give you maybe up to 0.75A to 1A ... a LM2576 or LM2596 will maybe give you up to 2A ... but by the time you buy the psu AND the switching regulator and the inductor and the various tiny resistors and capacitors and make the circuit board, it's often easier and cheaper to just buy the actual psu with those outputs, or buy a classic transformer with center tap or two secondary windings.

author: Marian_elf
author_rank: Contributor
timestamp: July 25, 2019, 05:49:38 am
content: Quote from: shawty on July 19, 2019, 12:00:40 pmHi,Most of the low-end ATX supply units i have worked draw the -12V from the +12V main power transformer secondary winding trough some FR104 rectifier diode ( or others like it ) and it achieves regulation trough magnetic coupling on the main choke ( inductor ) with the main +12V output winding.The main current limiting factor is the wire gauge on the inductor for the -12V output, the leakage inductance referred to the main +12V winding on the choke, and the rectifier diode it's self.Still if this is the configuration for your ATX supply units than you really should get more than jut .3 amps...Maybe you could post some photos with the main board inside, just to be sure we know what and how...All the best.

author: Mr.B
author_rank: Unknown
timestamp: July 25, 2019, 06:08:01 am
content: My first question would be: Are you sure you need 3A on the -12v rail?I can understand the 3A requirement for the +12v, however on the industrial style PCs that I have worked on in the past, the -12v was only to support the serial comms and the current requirement was quite low.Just asking...

author: shawty
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:43:22 pm
content: Hi all, sorry there's been no immediate reply, work called and I had to take off for a week or 2 to a job site.I'll read through the replies thus far, and reply to them as appropriate.CheersShawty

author: shawty
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:46:57 pm
content: Quote from: mariush on July 19, 2019, 01:57:06 pmHi Mariush,Unfortunately owing to time constraints I have actually done this now, I've bought 4 of these:https://www.ebay.co.uk/itm/AC220V-to-DC5V-12V-24V-48V-Switching-Power-Supply-LED-Driver-Transformer-Adapter/391782456678?ssPageName=STRK%3AMEBIDX%3AIT&var=661152128164&_trksid=p2057872.m2749.l2649I would still have liked to make use of gear I already had available (I hate wasting stuff, especially electronics), I salvage and re-use pretty much everything I can.CheersShawty

author: shawty
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:48:39 pm
content: Quote from: mindcrime on July 19, 2019, 02:06:35 pmThanks, I'll have a read of that. Even though I've bought some PSU's now (See above) I'm still going to follow through and read up on this subject, as I have a load of spare ATX's that I can (and would like to) repurpose for other things.

author: shawty
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:51:43 pm
content: Quote from: Marian_elf on July 25, 2019, 05:49:38 amI've got lots of ATX PSU's so unless I knew exactly which was the best one to use, I wouldn't know which one to photograph :-)PS: Like the robin Williams quote ...

author: shawty
author_rank: Regular Contributor
timestamp: August 06, 2019, 09:58:43 pm
content: Quote from: Mr.B on July 25, 2019, 06:08:01 amAccording to the specs for the industrial PC's I'm trying to power, yes, there very strict about it. I did actually just try hooking one up without increasing the ampage, IE: -v to -v, + to + and gnd, both supply's at 12v and the PSU basically gave me the middle finger. It was happy as larry again once I disconnected the -v rail, but then the 4 serial ports on the device didn't power up and initialize correctly, and caused the Linux distro I was trying to boot with to crap itself and fall over with a kernel panic in the driver file.The devices are an older model "Arbor" (Actual model num escapes me at present and I'm not at my usual PC at the moment), in quite a thin in height case that looks like one huge heatsink. 4 serial ports, 4 USB, 2 Ethernet, serial ports are capable of RS485 and a row of Digital and Analog I/O along one side that I need to work out how to program.