ATX (bench) Power Supply Outputting Voltage but not Current - Page 1
Forum section: electronics / beginners

author: FrankvHoof
author_rank: Contributor
timestamp: July 22, 2019, 01:43:16 pm
content: I'm currently trying to create bench power supply (for small projects) from an old ATX-PSU..However, when I attach a load (I'm using a PS3-controller for testing purposes), I can see Voltage across it, but no current. At first I thought this was because I did not have a load-resistor across the 5V-rail. However, even after adding this resistor, I'm still not seeing any current.I'm using a Component Pro ATX-300TAF 300W power supply that I pulled from an old PC.Eventually (when I get it working), it's supposed to hook up to this front panel:My current test-setup can be seen below. I've tried hooking up my USB-Port (female) to both the 5V StandBy-, and the 5V Active-Rails, to no avail.Does anybody have any other ideas? (Or has my tinkering already destroyed this PSU?)

author: ledtester
author_rank: Super Contributor
timestamp: July 22, 2019, 04:05:25 pm
content: How are you measuring current? The PS controller doesn't required a lot, and the panel meter you are using probably won't register anything if the draw is less than 10mA.How about testing it with another power resistor - e.g. a 10R resistor across the 12V rail, or something that should draw 100s of mA.

author: FrankvHoof
author_rank: Contributor
timestamp: July 22, 2019, 04:47:03 pm
content: I'm measuring current (and voltage) using my cheap chinese digital multimeter (https://bit.ly/2OcfDXE) in series. I've tried it both at my USB-port, and at my load-resistor, but am not seeing any current on either.When the controller is drawing current (i.e. charging), it's light should come on, and start blinking.I've also tried hooking up my phone, which doesn't receive any charge either.Edit: Correction: After fully 'resetting' the PSU, I'm now seeing .5A at my load resistor, but still nothing at my 5V Sb hooked up to the USB

author: FrankvHoof
author_rank: Contributor
timestamp: July 25, 2019, 04:24:45 pm
content: Update: So.. apparently I am getting current...I can get a led to light: (Both on 5V active & 5V Sb)(Although sometimes it seems to 'short out' & shutdown when I power it up with the led attached)However, as soon as I attach something over USB, it doesn't actually power up.I have tried re-soldering another USB-port (making sure to get the polarity correct, and checking that it's not shorted to the outer sleeve anywhere), to no avail.I have tried attaching 3 different items:- A PS3-controller. Apparently these will never charge without handshaking (seehttps://gaming.stackexchange.com/questions/73919/what-wall-plug-chargers-can-be-used-to-charge-ps3-controllers)- A mobile phone (Oppo F7). As far as I know, these should charge at 500mA when not handshaking (or 1A when the datalines are shorted?)- A cheap chinese battery charger. I'm not 100% sure, but I'd guess this doesn't require any handshaking. (https://www.aliexpress.com/item/32957808873.html)I currently do NOT have a load-resistor on the 5V-Line (accidentally stripped the leg off of my last 10W resistor, but I DO have one on the 12V line..Does anybody have any idea as to what might be happening?Do I require a handshaking-circuit to even power anything?Edit: it appears to be shorting out sometimes even without anything attached, so I'm guessing it's at least partially broken (and I will be replacing it when I get everything up & running).. However, as I can get it to light a led, it should also be able to power the USB-charger, no? (charger runs at 2.1A max, but I was able to get it to power on a 1A usb wall-plug from a ChromeCast)

author: mariush
author_rank: Super Contributor
timestamp: July 25, 2019, 04:50:35 pm
content: Do you actually start the power supply?By default, the psu should only output 5v standby on the mauve wire. The label says maximum 2A on 5v.You must turn on by shorting the PSON wire to ground (or through a resistor (100ohm or somewhere around that value) resistor )Should be fairly obvious the psu works because the fan should spin when it's on.What digital multimeter / display are you using? Some of those can not be powered from the same power supply they're monitoring, the display may need to be powered from separate psu/

author: FrankvHoof
author_rank: Contributor
timestamp: July 25, 2019, 04:55:57 pm
content: I start it when checking the 5V Active, yes (by connecting green to ground directly).This is also how I realised it's shorting out sometimes (even with nothing attached), as the fan will not spin.I'm using a battery-powered hand-held multi-meter to check voltage & current, so it's definitely not interfering with it.

author: mariush
author_rank: Super Contributor
timestamp: July 25, 2019, 05:46:47 pm
content: Are you checking the voltage and current correctly?You check the voltage by placing the probes in the ground/common and voltage and by placing the probes on negative and positive.To measure current, on most meters you must move one lead from voltage to current position (i'll call it jack), and some multimeters may even have a low current and a high current jack. Then, the multimeter must be in series with the device powered... basically power has to go through the multimeter. You don't put the meter on plus and minus, you put one lead on positive output of psu, then connect the device on the other lead of your multimeter.Easiest to check if something works is with a fan, or with a simple incandescent bulb, they're super cheap as they're sold for various cars so any auto parts store has some in stock. They have a reasonable amount of power, 10w, 20w ... so you'll see 1-2 A on your multimeter.A fan will not consume more than 0.2-0.4A on 12v, once it's actually spinning.

author: mindcrime
author_rank: Unknown
timestamp: July 26, 2019, 03:33:14 am
content: What value resistor(s) are you using for the load on the 12V rail?

author: FrankvHoof
author_rank: Contributor
timestamp: July 28, 2019, 01:11:57 pm
content: @Mariush I'm measuring 5.09V and 0.03A when attaching a LED to the 5V SbWhen attaching the USB-batterycharger I'm measuring 5.07V and 0.00A (it should run at 2A)@Mindcrime I'm using a 10 Ohm 10W resistor on the 12V, and am intending to use the same on the 5V active.

author: mindcrime
author_rank: Unknown
timestamp: July 28, 2019, 06:22:13 pm
content: Hmm.. well, it is entirely possible that you have a dodgy power supply. If you can find another "known good" one to test with, that would help sort that part out.Beyond that, it seems like what you're doing should work. Since you can light an LED, you know the PSU is doing *something* so that's good. As an experiment, maybe ditch the charger for a minute and use a series of "dummy loads" at different resistance levels, calculate the expected current (Ohm's law) and then measure the actual current and see if things match up. IF they do, then the problem probably isn't the power supply.Also, double and triple check the obvious stuff... bad connections, double check the "pin out" on the PSU cable bundle, etc., etc.

author: GerryR
author_rank: Frequent Contributor
timestamp: July 28, 2019, 10:50:33 pm
content: Just thought I'd mention that you are going to fry the 10, 10 W resistor. Rule-of-thumb is 2-1/2 times the power being "consumed" by the resistor for a wattage rating.

author: FrankvHoof
author_rank: Contributor
timestamp: July 29, 2019, 02:34:13 pm
content: @MindcrimeTried a known good PSU (a Dell NPS-210AB I pulled from a NAS), and got the following results:- Battery Charger: 0A- Phone: 0A- Load Resistor (10 Ohm 10W): .5A- Go-Tcha (Pokemon Go Accessory): 150 mAI'm guessing the top 2 REQUIRE handhaking?Can I add a chip/circuit to do this handshaking? I'd love to be able to charge my phone at 1A+ (OEM charger is 2A) on the 5V Sb.@GerryROops.. Miscalculated that (yeah.. It's pulling 14W atm) I'll probably add another 10W resistor to bump up the resistance to 20 Ohm, which should drop it to 7.2W at .6A

author: ledtester
author_rank: Super Contributor
timestamp: July 29, 2019, 09:22:48 pm
content: The "handshake" could be as simple as placing certain voltages on the data lines.See this Stackoverflow answer, for instance:https://electronics.stackexchange.com/a/177792/95488

author: xani
author_rank: Frequent Contributor
timestamp: July 29, 2019, 11:36:37 pm
content: Few things:You might want to reconsider using actually not worthless electronic devices as load resistors, while unlikely with ATX supply, you probably don't want to accidentally burn it.Please do put some kind of fuses on the lines. PC PSUs can put out tens of amps on the 5V/12V line and you probably do not want to weld something accidentally with it.There is no real standard on how much/little load you need to put on ATX supply to be stable so that's basically trial and error part, but I'd at least double whatever I found to be a minimum just in caseHave you seen? This + ATX power supply is easy way to get features useful for lab power supply, like the current limiting.

author: FrankvHoof
author_rank: Contributor
timestamp: July 30, 2019, 07:02:13 pm
content: So, I tried hooking up some voltage dividers to the D- and D+ -pins, according to the StackExchange-Link (2.0V on D+, 2.75 on D-), and still got nothing..I'm done.. All other outputs (both fixed & variable voltage) work, so I'm just going to keep the USB's unplugged, I'll have to charge my devices another way.@Xani, all outputs are fused on the back of the front panel. Furthermore, I'm using a boost-/buck-DC-DC converter for the variable input, which has built-in (variable) current limiting, so if I expect to need it, I'll use that output.

author: soldar
author_rank: Super Contributor
timestamp: July 30, 2019, 08:20:12 pm
content: Pretty much the definition of 5 volts is "that voltage which will push 1 Amp through a 5 ohm resistor". If you have a 5 volt source and connect a 5 ohm resistor and get no current and you still have 5 volts.... I'd like to know how it's done.

author: FrankvHoof
author_rank: Contributor
timestamp: July 31, 2019, 12:01:12 am
content: @Soldar I can get current through a resistor or LED. I just can't get anything to charge over USB, even after adding voltage dividers on the data-pins

author: Brumby
author_rank: Unknown
timestamp: July 31, 2019, 12:36:48 am
content: My suggestion is to perform a range of tests with "dumb" loads - ie resistors - before trying anything with active electronics. Using simple resistors avoids any questions about what the load might do. They will respond according to Ohm's Law - pure and simple.Don't forget - if you put you meter in series for current measurement, you will get a voltage drop across it (burden voltage), so make sure you take that into account. (Random thought - If your meter has one, the current range fuseisintact ...yes?). Alternatively, measure each resistance you use before you place it in circuit, then simply measure the voltage across it when in circuit. Ohm's Law will tell you the current.Do this and maybe draw up a graph to see how things are behaving.

author: ledtester
author_rank: Super Contributor
timestamp: July 31, 2019, 02:33:31 am
content: I found this article, and it suggest something slightly different from the above SO answer:https://www.quora.com/How-can-I-use-a-laboratory-power-supply-as-a-USB-charger"... If your bench supply can output at least 1.5A, tie the white (D+) and green (D-) wires to either end of a small resistor, something in the 33–200 ohm range will be fine. This is the bus condition a USB device uses to detect Battery Charging mode. That specification delivers a minimum of 1.5A at 5Vdc, though a few devices can pull more. ..."Another person replied with:"... To signal to the device that it can draw up to 1.5A (maximum specified by the standard), short the other two wires together. Obviously make sure you set the current limit to 1.5A or above before you plug the device in. ...I presume the D+ and D- leads should be shorted.

author: soldar
author_rank: Super Contributor
timestamp: July 31, 2019, 08:47:57 am
content: Quote from: FrankvHoof on July 31, 2019, 12:01:12 amIt seems the problem has nothing to do with the power supply and all to do with the load in which case the thread title is misleading. The PSU is fine and giving 5V. The load is not taking in any current. Does it do thisonlywith this PSU? I suspect it would do the same with any 5V PSU.

author: rawrs
author_rank: Contributor
timestamp: August 01, 2019, 04:36:52 am
content: If there is voltage there, the device is either going to draw current, or it is not going to. The power supply is fulfilling its end of the obligation if you are seeing voltage. Some USB devices - actually, I'd wager, a lot of USB devices, are not going to draw current blindly if you leave D+ and D- floating. It's some Chinese standard (I can't remember which one) whereby tying D+ and D- together will let the device know that it's at least attached to *a* charger, even if the device may not know *what* charger exactly. Try doing this.Working or not, that power supply is really not going to be all that useful to you as a bench supply. Those power supplies date back to a day when onboard regulation was pretty much not a thing on motherboards, and the architecture actually dated back to the AT days, and ATX was kind of tacked on as a cheap addition to bring their old builds up to the modern standard. Why I believe this is the case, i that you have mega 3.3v and mega 5v but almost nothing on the 12v. 8A isn't *useless* - but for a notionally 300 watt power supply, it may as well be.I'd recommend using a real ATX power supply that has a decent 12v output if you want to be pulling meaningful amounts of power for stuff. Granted, for anything but really power electronics, 12v 8A is plenty - before conversion, that is. The second you need to step that voltage up, useful power drops at a rapid rate of knots.Hope this advice that no one asked for is of any help.EDIT: I feel like an idiot for not seeing the resistor, for some reason I saw differently. I want to leave the above alone, in case someone else comes across this. Now the problem you may be having, is that the device is seeing some weird resistance across the output terminals and is going "wtf is wrong with my port, I am not charging from here". This can particularly be the case if, for example, a PS3 controller usually authenticates with some sort of controller upstream of the port itself, and then negotiates charge, or, the PS3 has some obtuse, bespoke current-setting resistor. Being Sony, I'd imagine that any combination of any of the above is true. I'm out at the moment so I can't be arsed checking, but try removing the resistor and just tying D+ and D- together. If nothing else, this should make most any other device that was built to work with the Chinese standard (which is a whole metric buggerton of things made this side of the publishing of the Magna Carta).Hope this helps the even less folks who will find it handy!EDIT 2/P.S.Quote from: xani on July 29, 2019, 11:36:37 pmThrow that antiquated AT garbage out and buy one of these. Seriously. I have the higher power model of this (the fixed version of the one that Dave had drop its guts on him) and in addition to being a fun little project, it's a great switching supply. If you are going to use a switching supply at all for a bench supply, it might as well be a quarter-arsed decent one. You'll need to BYO power supply, er, power supply though. For 20v 3A or whatever the little self-contained module is, this isn't a huge issue. For 60v 20A... yeah I'm still trying to work that one out, myself. The smaller module requires almost no assembly work at all, or precisely zero assembly if you don't mind using it as just a bare module. For something that is so cheap, and gives you a safe, stable, and programmable, supply, you can't go wrong.

author: rawrs
author_rank: Contributor
timestamp: August 01, 2019, 05:16:15 am
content: I hope folks don't mind the double post, but I feel like I've edited my original post far, far too much. Plus I want this to be easy for folks to find.If you just wanted to charge stuff from the power supply - get a few of these. They'll run off basically anything over 5.5v or so, and they're really cheap, really easy, really elegant, and will actually do a half-arsed job at it. It might not be something you've built yourself, but it'll do the job far, far better.eBay auction: #352664636353