Benchtop supply converted from an old ATX PC supply - Page 1
Forum section: electronics / general

author: LAZLOT
author_rank: Contributor
timestamp: November 02, 2012, 01:32:52 am
content: Just looking at the various conversions on the interwebs, but I have one question... If you connect across +12V(12A) and -12V(0.8A)- (Obviously the Current spec. will vary depending on the exact supply used, but the question is about the principal- not the spec's) to get 24V, then what current should I expect? Will it be the sum of the currents divided by two, as I expect... 6.4A? If I'm wrong, I'd love to know why... Ta (in advance), Larry

author: tom66
author_rank: Super Contributor
timestamp: November 02, 2012, 01:42:01 am
content: Nope, 0.8A. Max. And don't expect the -ve rail to stay in regulation if you push current into it.

author: Rerouter
author_rank: Super Contributor
timestamp: November 02, 2012, 01:44:31 am
content: agree with tom, that current rating is the capacity of the rail, not how much its providing or sinking, if you go over that you get into issues,note that 0.8A is more like 0.45A continuous unless its a fairly old supply, as for the past 10 years almost nothing has used the negative rail so they have progressively downsized it and rated it in its peak capacity,

author: LAZLOT
author_rank: Contributor
timestamp: November 02, 2012, 02:05:34 am
content: Thanks Tom and Rerouter... That was my second guess, which I was hoping was wrong, but obviously not a useful conversion if you want to combine any rails for different voltages. Looks like I'll be going down the path of mod'ing one into a basic supply (not the variable version), and not expecting to bridge for 24V (or other combo's)... But starting to think it's not worth the price of the required parts, as I already have two benchtop supplies, and one of them is variable 50V at 5A, but the only reason I was thinking about converting one is because I have a few PC supplies laying around, and the 5V may be handy for working with TTL. At least now I know to expect little versatility from it. Ta

author: eengineer
author_rank: Contributor
timestamp: November 02, 2012, 04:31:49 am
content: funny u mention this, my friend just purchased this kit:https://www.sparkfun.com/products/9774which takes a 24pin from computer PSU and makes it a variable (somewhat) dc power supply

author: Rerouter
author_rank: Super Contributor
timestamp: November 02, 2012, 07:22:58 am
content: i would still never recommend an atx supply for direct connection to ttl, the ripple on some can be downright horrendous,

author: casinada
author_rank: Frequent Contributor
timestamp: November 02, 2012, 05:33:14 pm
content: http://www.seeedstudio.com/depot/atx-breakout-board-bench-power-supply-p-1222.html?cPath=155

author: tom66
author_rank: Super Contributor
timestamp: November 02, 2012, 07:39:58 pm
content: I remember having gone through several ATX PSUs after blowing them up.One I used to short out all the time, eventually it just died (no pop, just wouldn't turn on, still had standby power), I suppose you can't blame it...Second one I shorted -12V to PS-ON (don't even ask how) and after that it would make a weird whining sound continuously but only have a few volts on each rail. I'm guessing I wounded the PSU controller or shorted a diode.In my opinion, too noisy/imprecise for a bench supply. Designed for computers with fairly wide tolerance to supply voltages.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 03, 2012, 08:56:42 am
content: How badly do you want a 24v supply?This is what I have done (not my thread but same as what i did) for a high current 24v PSU for battery charging;http://www.rcgroups.com/forums/showthread.php?t=1750411I Isolated 2 of them and series'd them up for 24v.Regulation is good, quiet (if you mod the fans), handles high current easily and cheap (i got both supplies + stuff to mod them for $55 incl shipping).I have considered making bench powersupply from them, using an adj linear regulator with a tracking pre-regulator.Actually, i was thinking of doing it a lazy way with a adjustable stepdown converter with a diode to drop the voltage to the adj pin of a linear regulator so it didn't dump as much power, would this work? I don't entirely see why it wouldn't...

author: LAZLOT
author_rank: Contributor
timestamp: November 04, 2012, 04:00:21 pm
content: Ta all for comments/links. I didn't expect a bad ripple factor from a PC supply... Looking more like they're good for spare parts only, but I might check the ripple before a teardown (have an old 150Meg Hitachi V-1150 CRO). I'll probably keep one just in case..., so far my two benchtop supplies have sufficed. I was mainly looking for something to with the spare 5 or 6 supplies I had laying around.

author: tlu
author_rank: Regular Contributor
timestamp: November 04, 2012, 04:09:35 pm
content: Quote from: peter.mitchell on November 03, 2012, 08:56:42 amWhen converting an ATX PSU to a benchtop unit, you mentioned it can handle high current. I'm curious as to how much current can a single ATX source?

author: T4P
author_rank: Super Contributor
timestamp: November 04, 2012, 07:13:54 pm
content: The cheap ones you can get for essentially free are rubbish, you get terrible regulation and poor noise as well as not being able to supply what is actually writtenEven at 20$ it's still pretty piss poor, i'll tell ya. Poor regulations can be a reason why your computer seems to be a little warmer sometimes and why it shutdown for no reason ...At 100$ things get interesting but hell you can buy a bulletproof rack mount power supply for 100 bucks and have a million times better noise figures as well as regulationHere's a typical SSI EPS 12V 2.91 PSU (taken from the PDF) I saw this roughly 100 bucks PSU Thermaltake T.power XT 675W in the stores and thought i decided to see what sort of regulation they require to have the SSI v2.91 standards-12v @ 10% That's terrible+12V1,2,3,4 @ 5% When was the last time you saw a bench supply have more than 3% typical regulation? NEVER.And check out the ripple ratings ... my jaw dropped50mVpp for 3.3V and 5V and 5VSB120mVpp for +12V and -12V When was the last time you wanted a bench supply that has 10% noise? It's like the caps never existedlike removing the muffler from your car and making it direct exhaust, all the noise in full glory! Desirable if you wanted noise anyway but it's as desirable as a V6 engine sound, none the more desirableThe PSUs might do better but i doubt they would really do

author: Bored@Work
author_rank: Super Contributor
timestamp: November 04, 2012, 08:20:32 pm
content: Quote from: T4P on November 04, 2012, 07:13:54 pmThere is at least one manufacturer, QJE, aka Ningbo Jiuyuan Electronics, who tries to pass a supply with 200 mVpp ripple as a bench supplyhttp://www.nbjiuyuan.cn/switching-model/products_30_en.htmlIf you want one, tryhttp://www.triosmartcal.com/dc-bench-supplies/2631-qje-ps3005-bench-power-supply-150w-0-30v-0-5a.htmlAlthough there the spec magically changed to "Noise & Ripple: 2mV RMS nominal".

author: tlu
author_rank: Regular Contributor
timestamp: November 04, 2012, 09:11:52 pm
content: Quote from: T4P on November 04, 2012, 07:13:54 pmI don't know about the regulation but couldn't you smooth out the ripple by putting a big cap across the output? I know with certain amount of load, you will eventually encounter the ripples again but if sized with a big enough cap, itwould help right?

author: Unknown
author_rank: Unknown
timestamp: November 04, 2012, 09:36:33 pm
content: Assuming you attached a huge low-ESR cap to the output, what would this cap do if you accidentally short the output, preferably through some expensive semiconductor? What would the short circuit current be?

author: T4P
author_rank: Super Contributor
timestamp: November 05, 2012, 10:08:26 am
content: Hey there Bored!My college has thousands of QJE PSUs which is labelled QJ3003EIII also BUT it isn't half as noisy because it's a linear but it's a still quite a noisy arseThat's why no one uses that thing for TTL

author: Bored@Work
author_rank: Super Contributor
timestamp: November 05, 2012, 05:53:25 pm
content: Quote from: T4P on November 05, 2012, 10:08:26 amThat QJ3003EII looks like one of those generic Chinese linear bench supplies.http://www.nbjiuyuan.cn/Mutli-Way-output/products_18_en.html.I am a bit surprised they don't manage to build it so they get a clean output.Regarding the PS3005, not to long ago I had a chance to look at and into the 3A version PS3003. I didn't want to write about it at that time, to not disturb Triosmartcal's business. They had that trouble with the Korad PSU before, and might not be too happy about my opinion. However, as I just saw before writing my previous posting, I am surprised about their "Noise & Ripple: 2mV RMS nominal" spec. My quick measurement at the PS3003 was 140 mVpp, which is close to QJE's < 200 mVpp.

author: tom66
author_rank: Super Contributor
timestamp: November 05, 2012, 06:16:35 pm
content: And that is a linear supply? Sounds like someone messed up the control loop compensation.

author: Bored@Work
author_rank: Super Contributor
timestamp: November 05, 2012, 07:25:51 pm
content: Quote from: tom66 on November 05, 2012, 06:16:35 pmThe QJE PS300x are switch-mode power supplies. Still, for a switch-mode bench power supply 200 mVpp or 140 mVpp ripple, is still ridiculous. E.g. the small bench SMPS mike once reviewed is specified as something like 30 mVpp.