Beginner: Need advice on ATX PSU > Lab PSU... - Page 1
Forum section: electronics / beginners

author: LeWidget
author_rank: Regular Contributor
timestamp: July 25, 2015, 11:39:32 am
content: Hey guys,So I found a couple of PSU's in storage and I'de like to turn 1x into a Power Supply that I can use (primarily) for Electrolysis. I watched this video, and it seems ok, but was hoping for general feedback on you opinions on how to go about doing this.https://youtu.be/z2oSFpKh_Uwhttp://jumperone.com/2012/12/converting-atx-power-supply-to-bench-power-supply-faq/#Q2There's one section in the tutorial where he uses 2x Resistors in a Prototype board. I wasn't planning on using 2x LEDs.. maybe just the 1x. Is this recommended?Other tutorials showed connecting a resistor between the 3.3v wire and the LED (though Im not sure what spec resistor to use.. or why use a resistor). Could I just use one of those little carbon type resistors and heat-shrink over it ?Thanks

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: July 25, 2015, 12:36:01 pm
content: For the 5V rail dummy load mentioned earlier in the video he's using 2x 10R 5W resistors in parallel instead of a single 5R 10W resistor.He used two LEDs; one to indicate the PSU has mains connected (5V standby) and one to indicate the PSU is switched on (5V). Each LED requires its own resistor, 330R is a safe recommendation @ 5V. Google is your friend RE: why

author: yeayeahchino
author_rank: Newbie
timestamp: July 26, 2015, 03:07:13 am
content: I have successfully made this conversion in a number of different methods. I found the dummy load with a resistor is problematic over time. The best way around this is by connecting the fan in the PSU case directly to the 12v rail. This will also help to cool the PSU. Trace the fan’s wires to the plug remove and attach to any 12v wire and ground. This is a more reliable approach to the load sensing, especially if it is going to be used a lot. Doing this conversion is one thing and actually using it is another!

author: LeWidget
author_rank: Regular Contributor
timestamp: August 07, 2015, 12:25:46 pm
content: Thanks guysRead a little about resistors.. they basically 'resist' current decreasing current flowing into a component ? Requiring a resistor on a 5v line as LED lights operate at less v ?If that's the case, can I use a 3.3 LED straight onto a 3.3v rail for dummy load? or can only use 5v or 12v ?Regarding having a fan as dummy, is it just a matter of connecting the live wire to the 12v wire and keeping ground as is?The PSU Im planning on using seems to power on (thought it was borked), but jumped the Green and Ground and away it went, fan and all.used the DMM and measured the 12v, 5v, 3.3v, and they all seemed to be outputting ok, though there is a cap inside that started to bulge which Im gonna replace to be safe.Thanks again, appreciate all feedback

author: McBryce
author_rank: Super Contributor
timestamp: August 07, 2015, 12:53:09 pm
content: Hi LeWidget,the resistor on the LED circuit is to limit the current. Without it the LED (essentially just a diode) will short the supply and take as much current as the supply can give, hence the LED will instantly burn out because too much current is flowing. To work out what resistor you need you just need ohms law.If your supply is 5V and the LED has a forward voltage drop of about 2.3V (check datasheet) and to light needs between 5 and 20mA (depending how bright you want it). So the resistor will be dropping the other 2.7V. So R = V / I = 2.7 / 0.01 = 270R (will limit the current to 10mA).That's how it works. You can do the calculations as above for 3.3V or 12V depending on what rail you intend connecting the LED to.McBryce.

author: LeWidget
author_rank: Regular Contributor
timestamp: August 07, 2015, 03:17:22 pm
content: Thanks for the explanation McBryceQuick question, when changing capacitors, can I go a higher uF & a higher Voltage? or does the uF have to be equal to the cap im taking out, but Voltage can be higher?I'm gonna replace about 4x Fuhjyya caps, 4700uF / 10v (10mm wide x 30mm high)

author: MarkF
author_rank: Super Contributor
timestamp: August 07, 2015, 04:31:31 pm
content: If you don't want to modify your power supply try thisATX Breakout Boardfor $14.00. You didn't say what your current requirement is but the board has 1.25A polyfuses one each output.

author: LeWidget
author_rank: Regular Contributor
timestamp: August 07, 2015, 05:45:35 pm
content: Hey MarkF, thanks for the suggestion. I had a look, though it appears it's around $14US, with the exchange rated + shipping I think financially it may be cheaper to stick with the PSU, as the PSU is already here not getting used + it's an interesting learning processI had a look at the datasheet for some LEDs I'm considering getting, but am having trouble figuring out what specs/requirements they are..http://pdf1.alldatasheet.com/datasheet-pdf/view/166116/KINGBRIGHT/L53HD.htmli think the above datasheet has details for all 3x LED's below ?Kingbright L-53HD - 25mA max? (0.025 Amp)Kingbright L-53ND - 30mA max? (0.030 Amp)Kingbright L-53GD - 25mA max? (0.025 Amp)According to RSOnline (where I'm looking to get from), they sayRED forward V = 2.25 VOrange forward v = 2.05 VGreen forward V = 2 VSo, say I put the RED LED on the +5v Standby and I want to run 20mA (0.020 amps) through it.RED LED has 2.25fV drop ? = 2.75v ?R = V / IR = 2.75 / 0.020R = 135.5ohms ? will limit current to 20mA ? = ok to use on that LED ?(orange R = 147.5)(Green R = 150)These are 133ohms,http://au.rs-online.com/web/p/through-hole-fixed-resistors/6833014/which meansI = V / RI = 2.75v / 133I = 0.0206... ?Is that right?If so, that would mean the 133ohm resistors would still be ok for the orange and green LEDs (about 0.022A) ?

author: MarkF
author_rank: Super Contributor
timestamp: August 07, 2015, 06:21:12 pm
content: LeWidget,Please update your EEVblog profile with your country code so we can provide more relevant options.

author: LeWidget
author_rank: Regular Contributor
timestamp: August 08, 2015, 08:12:37 am
content: *Bump*

author: LeWidget
author_rank: Regular Contributor
timestamp: August 31, 2015, 09:28:49 am
content: So got my 1st amateur psu up and running. few issues, but i considered this attempt a learning project.Ran into an issue with a few bits,1. heatshrink wrap too close to solder point, seemed to have heated up and cant move it over the solder joint2. Green LED doesn't seem as bright. used a 300ohm resistor.. maybe should lower it next time. Red LED seems ok (300ohm)3. Turns out I couldnt get the Blue LED on the power switch to work.. probably DoA (china), so installed the green LED instead.4. Couldn't fit all the Ground wires onto the Ground terminal... which lead me to ask, and hoping you guys have an answer, do I need all the ground wires? Does ground need to match Positive .. such as, if I have 12v10amp running out of the 12v rail, do I need ground to handle 12v10amp ?How does I find out the maximum V/amps that a single wire can carry? not only the wires inside the unit, but I've used wires with alligator clips on either end to connect from the PSU > probes... not sure if the full current is getting through the single clip, or whether I need to connect multiple wires?Question: How does electricity flow? I read Negative charged ions are attracted to Positive ions... so Negative flows to positive? How does that relate to 12v charge > ground? isnt 12v positive charge?

author: jeroen79
author_rank: Frequent Contributor
timestamp: August 31, 2015, 09:24:44 pm
content: 1. Well, redo it.2. Green LEDs have a higher forward voltage than red LEDs.So there will be less voltage over the current limiting resistor, which means that less current will flow. And less current is less light.3. What kind of switch is the power switch? Does the LED have it's own terminals?Does it work when you use it between GND and a 5V line?The on/off line may not be able to supply enough current to the LED to create enough light to see.4.Allthe current that comes outanyof the positive rails will go back into the ground rail.So the ground wires must be rated for the same current as the positive wires combined.Also, wires are not rated as 12V/10A.A voltage exists between two points.If there is a conductor between them then this conductor will carry current and that conductor is rated for a given amount of Ampere.Go and study electronics basics such asOhm's law.Seehttp://www.powerstream.com/Wire_Size.htmQ. Long, long ago people thought that the current consisted of positive particles and decided thatConventional currentgoes from the positive rail to a negative rail (relative to eachother).Later on it was discovered that the current consisted ofElectronsthat flow from the negative to positive.

author: MarkF
author_rank: Super Contributor
timestamp: September 01, 2015, 12:13:00 am
content: QuoteThis is not exactly true. There is a voltage and temperature rating in addition to the current rating. But these are related more toward the insulation properties and number of conductors in the sheathing.

author: jeroen79
author_rank: Frequent Contributor
timestamp: September 01, 2015, 01:11:01 am
content: Quote from: MarkF on September 01, 2015, 12:13:00 amYou're right, just like switches and fuses have a voltage rating as well.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: September 01, 2015, 12:35:04 pm
content: For the record, this gets asked again and again, and someone ends up posting something along the lines I'm going to post here: An ATX PSU makes areally, really, really shittylab PSU.Converting an ATX PSU into a benchtop lab supply is - quite literally - like converting a Caterpillar hydraulic excavator into a remote surgery robot. It begs the question of "WHY?!" The purpose of a benchtop lab PSU is a noise-free, clean voltage that lets you measure the characteristics of the actual system under test - as opposed to those of a bad PSU. The purpose of an ATX supply is to provide a crapload of power to applications that need a crapload of power (eg. gaming rigs with power hogging GPUs), and don't care particularly about how clean that power is (just think of those endless banks of electrolytic caps on a PC motherboard).If you limit the current the PSU can supply via polyfuses, you still have a PSU that has really bad voltage stability, gigantic ripple, and is altogether ridiculously noisy. If you don't limit the current, you have a box that can easily light anything on your bench onactual fire.That said, you sometimes do need a heavy lifting PSU (for 3d printers, heaters, high power lasers, etc.), and an ATX box is more than adequate for that. But that's not called a "lab PSU", and you shouldn't use it in the applications where you'd use a lab PSU.

author: LeWidget
author_rank: Regular Contributor
timestamp: September 02, 2015, 11:45:51 am
content: thanks guysjeroen79:1) This is the switch here (http://www.ebay.com.au/itm/201274888045?_trksid=p2060353.m2748.l2649&ssPageName=STRK%3AMEBIDX%3AIT). The switch works, but the LED wont illuminate, even with the 3rd post connected to ground. I just brushed it off as a faulty LED (they refunded the massive $1 price tag :p )Question: What's the difference between 'Ground' and 'Earth' ?2) Is there a way of finding out the limitation of a wire if there's nothing written on it? Or can i apply a voltage (until it gets hot), measure resistance then work out the amps with ohms law ?Sigmoid:You're right, I think it was poor choice of words for my heading, I really wasn't sure what to call it. I never really had the intention of using this as a Lab PSU thats used to test electrical equipment. More or less a little project to learn from and something that I can use for electrolysis (maybe something else down the road). i've seen those adjustable/variable style Lab PSU's, seen Dave use one, I'd certainly look into getting one of those for a more accurate and safe application when/if I get to that level of electronics.

author: jeroen79
author_rank: Frequent Contributor
timestamp: September 03, 2015, 11:47:14 am
content: Ground and earth are used interchangeably but sometimes ground may refer to device's chassis or a circuit's 0 volt line.You can determine a wire's rating experimentally by passing more and more current through it until something begins to melt or burn.If you know it's resistance per meter then you can calculate how much power it takes to make an ampere go through it. Then you can calculate at what temperature that power is transferred to it's surroundings. Based on what the wire is made of you can decide wether it is safe or not.Of course this is more complicated as the wire's resistance changes with temperature , each environment has a different capacity to absorb heat, wires in a bundle can radiate less heat than a wire in air, etcYou can also look at the thickness of the conductor and compare it to the AWG table.

author: tooki
author_rank: Super Contributor
timestamp: December 02, 2015, 10:41:54 am
content: Quote from: Sigmoid on September 01, 2015, 12:35:04 pmThis has to be some of the finest wordsmithing I've seen in a long time.Except perhaps for the "literally", which obviously isn't true. It does, however, inspire some truly amazing mental imagery, which I wish I had the visual art skills to convert to a drawing!!!Quote from: Sigmoid on September 01, 2015, 12:35:04 pmHow bad is it actually? And how do quality PC PSUs (e.g. the ones Apple uses, which everyone says are the creme de la creme of PC PSUs) stack up?Either way I find it amazing that CPUs, with their clocks running at gigahertz with small core voltages, can handle such dirty power and still work exceedingly reliably! (And meanwhile we have audiophools thinking they can hear noise introduced solely in the last 1m of power cord, ignoring the miles of wire from the power plant. Never mind that if that were true, it would be a damning condemnation of the power supplies in their expensive gear!!!)

author: McBryce
author_rank: Super Contributor
timestamp: December 02, 2015, 11:29:20 am
content: Quote from: tooki on December 02, 2015, 10:41:54 amThe answer is in the text you quoted "endless banks of electrolytic caps on a PC motherboard"McBryce.

author: fleuroman
author_rank: Contributor
timestamp: December 02, 2015, 12:04:10 pm
content: go to jaycar, for $200 you can buy a 0 to 30v (your not limited to +3.3v +-5v or +-12v) with adj constant current, or follow daves uPSU design

author: Cliff Matthews
author_rank: Unknown
timestamp: December 02, 2015, 12:54:44 pm
content: Quote from: tooki on December 02, 2015, 10:41:54 amI thought Seasonic was creme de la creme of PSU's.. FWIW, modern mobo's have their own multi-phase DC-DC converters filtering all that 12v dirty power, feeding 60 to 120 amps @ 1v into hundreds of power points on 2 sides of the package (which by-the-by, has it's own bank of MLCC's) leaving the other 2 sides for I/O.

author: timofonic
author_rank: Frequent Contributor
timestamp: December 09, 2015, 11:09:28 pm
content: Quote from: Cliff Matthews on December 02, 2015, 12:54:44 pmCan we cannibalize them from broken motherboards?

author: Cliff Matthews
author_rank: Unknown
timestamp: December 10, 2015, 02:32:56 am
content: Quote from: timofonic on December 09, 2015, 11:09:28 pmNope.. those outputs travel just 2 inches inside the multi-layer PCB - it's purpose made and besides, what great things can we do with 1.3 volts at 100amps? Here's an example of a 12-phase DC-DC convertor... sweet!

author: mikerj
author_rank: Super Contributor
timestamp: December 10, 2015, 08:09:32 am
content: Quote from: tooki on December 02, 2015, 10:41:54 amPretty bad.

author: tron9000
author_rank: Frequent Contributor
timestamp: December 10, 2015, 09:30:49 am
content: Quoteguess that's why ATX supplies are cheaper than lab suppliesand a testament to the DC-DC conversion on the Mother board!

author: allikat
author_rank: Contributor
timestamp: December 11, 2015, 09:45:06 am
content: Well, if you go poke around jonnyguru.com for some reviews, the real good ATX PSUs are capable of beating a cheap lab PSU for ripple and so on. However, you'll also pay about twice as much for the ATX unit that can do that against the cheap lab unit. The thing to take away from this is that the circuitry to get a nice stable output from a switchmode is not cheap, taking a top end ATX unit and turning it into a lab PSU will probably not cost any less than buying something workable that's actually designed to be a lab supply.$150 bucks will get a good ATX unit with sub 1% ripple on any rail, I'd bet you can get a decent lab supply for the same price.