oem atx power suply noise and ripple measurment - Page 1
Forum section: electronics / beginners

author: kokodin
author_rank: Regular Contributor
timestamp: December 10, 2019, 08:13:16 am
content: HelloI am building a custom pc in old oem computer case, so i have to use certain form factor power suply, and it can't be anything more standard than what i could find already. I need to rate if the power suply is up to the task for long term use.atx standard specs saySupply (V) Tolerance Range, min. to max. (V) Ripple, p. to p., max. (mV)+5 ±5% (±0.25 V) +4.75 V to +5.25 50−5 ±10% (±0.50 V) −4.50 V to −5.50 50+12 ±5% (±0.60 V) +11.40 V to +12.60 120−12 ±10% (±1.20 V) −10.80 V to −13.20 120+3.3 ±5% (±0.165 V) +3.135 V to +3.465 50+5 standby ±5% (±0.25 V) +4.75 V to +5.25 50But i have no idea how to test it in real life.for instanceMy test computer 12V rail have idle voltage of 12,2V, but it sags under load (graphic card benchmark) down to 11,9-11,85V and sensor sweep on pc makes it looks like a very ugly graph, many dips and raises. This is the kind of load power suply will experience, and was measured from inside software on the pc itself, so it might not be very accurate. But i am still concerned that under bigger load it may even fall out of atx specs or die.Power suply didn't get even a little bit warm under load mind you, and i was benchmarking it for over an hour. And 2 of oem special sauce wires were not even connected (i suspect they are for fan speed controll).Test system i was using also might be at fault of wrong measurments, because it is an old computer that liked to kill power suplies in the past.By the way the power suply in question is made by delta and it is 300w fujitsu branded part, I also have 250w model. I have no schematics or specyfication sheat other than the label on them.Is there even a point hooking up a scope to my test system as it is and looking at the ripple and noise, or should i have linear load like some dumb 300w resistor or bulb? I mean cpu and graphic card are not really a good electronic load candidates, they are very spiky power consumers, very inconsistent. Then again if we consider fluctuations under load as ripple, it would be already out of atx specs by the factor of 2, still in range of min/max voltages though.I don't really have a way to directly measure power draw on each line . I can measure power draw of entire system with a clamp meter, but the power draw on each dc line is different and spreaded, so even scoping cpu 12v socket or atx plug, or molex connector can bring in different data.What do you think, how should i unpack this problem?

author: magic
author_rank: Super Contributor
timestamp: December 10, 2019, 07:55:55 pm
content: Current is going to be roughly constant if you apply constant load, such as 1 or 2 or N cores running Prime95 and nothing else. You could probably see the switching ripple by probing in some unused HDD plug. Be sure to connect the ground clip to the exact same plug. Use AC coupling, obviously.Dave recently did a video on scoping PSU noise, you may watch it for inspiration.But my first worry would be current capacity, I hope you have checked the label. And close second: cross regulation. As you load 12V, the output on 5V and 3.3V may rise up and possibly go out of spec. Such are the wonders of some ancient PSUs designed for systems utilizing primarily low voltage rails.

author: kokodin
author_rank: Regular Contributor
timestamp: December 11, 2019, 12:40:23 am
content: Well it isn't that ancient, it is 2007-2009 vintage . Labeled as maximum continuuous output 300W. And A*V all summed up come up exactly as 300W with 10 sec surge overpower of around 315W3,3V - 8(9)A5V - 15(17)A12V -17A-12V - 0,3Aaux/standby 2(2,5)AAnd for i could say, putting it under heaven benchmark made 12V jump around while 3,3 was rock solid in sensor sweep in aida64 set to 50ms probing.I was using it a lot last year with 98W phenom 2 x4 and the same gt1030 4 sticks of ram and 2 hdd's as a joke pc, and it was keeping up. But it was running paired oem board with special sauce bios to accept phenom 2 processors at all.i wish to turn it into something a little bit more modern (ryzen 3 1200 on b450 board) and it would be a disaster if it gone in smoke with the broken power suply. I might be overcaucious because it does run a test system of pentium dual core on nforce board with the same gt1030 with no problems. What shocked me was it being cold to touch while under load and i started checking things up.I hope analog 10mhz scope would do to visualise ripples and noise, but i did watch daves video and my scope doesn't even have bandwidth selection :] Watching Daves video might be the root of my interest in ti in the first place. I don't really have many engineering tools , only basic ones i use for work.As i understand i can do 2 things1 put psu back in phenom pc and put it to crunch prime or other syntetic cpu stress test and load psu to around 120W I wonder if there is similar thing for graphics, because i could het to around 150-170w this way , but still 3 phase cpu power would make more noise than psu itself2 try to do it on non oem board but then i have even less load capailitiesI am not really sure where am i getting with that, i am more interested in how it should behave in real life scenario, than testing it's ripple and stuff in a lab way.How much of a voltage fluctuations should i exect under wierd loads like that turning on and off constantly. stuff like that.

author: tunk
author_rank: Super Contributor
timestamp: December 11, 2019, 11:12:51 am
content: I believe Delta is a reputable brand. That said, the capacitorsmay be beyond their sell by dates. If they are, you should be ableto get an indication if the ripple is too high. I've also got theimpression that recent PCs mostly uses the 12V rail - if that's thecase, the PSU can provide around 200W.

author: kokodin
author_rank: Regular Contributor
timestamp: December 11, 2019, 11:51:37 am
content: Well yeah, new one was rated around 200W on 12V rail, that is obvious by a label. But it is quite small unit all things consider , so it have to run in quite high rfrequency to compensate for lack of weight and size. Recaping capacitors might be actually a good idea. There is not many of them there anyway. I doubt increasing the output caps capacity would help anything, but i could also do that (in reasonable amount). Don't know how would it play out in the long run with the switching frequency.I am just concerned if it is a good indicator that under random load of a game benchmark, 12V power rail jumps from 12,2 flat line, to 11,8~12,1 stairs just after launching the benchmark. I might test it with different power suply first , because it might be just a sensor glitch on the board. (i am dataloging voltages from the pc via software on that pc) I have comparable 350W atx psu with only difference being 22A on 3.3V rail and 4A on standby. I was busy remodeling the inside of that computer case yesterday so i didn't do much testing .

author: magic
author_rank: Super Contributor
timestamp: December 11, 2019, 12:04:25 pm
content: 10MHz is enough, those PSUs run at tens or hundreds of kHz. 50ms sampling, not really.11,8V is still within spec.Changing a 98W CPU to a 65W one is unlikely to make anything worse. Motherboard doesn't matter, all motherboards with the 4-pin connector power the CPU from 12V.

author: kokodin
author_rank: Regular Contributor
timestamp: December 11, 2019, 03:57:57 pm
content: 4 pinNew mobo has 8 pins 12v :] , but i get the point. Motherboard support more powerful cpu's so 8 pin might be better but in most cases they are connected internally anyway. Also tpd does not always mean the same thing nowdays from cpu to cpu.It isn't a power draw anymore but some crazy thermal equation with multiple variables.The thing with my test motherboard was, when i get it new it was rated to support 400w psu's and up. Even if my total system power draw was 90W at the time (idle), it destroyed 2 350w power suplies (modecom brand) within a month and i didn't even had a graphic card. It was 2 computers ago though.So this motherboard might be a very funny test rig by itself (does not monitor 5v rail though).

author: magic
author_rank: Super Contributor
timestamp: December 11, 2019, 04:49:45 pm
content: It's 2019, I bet you can find reviews with precise measurements of power draw of that CPU if you look around.Even considering creative marketing, I doubt that 65W rating today is much worse than 98W ten years ago.

author: kokodin
author_rank: Regular Contributor
timestamp: December 12, 2019, 01:07:53 am
content: Ok i was tired after work, but i did some extra tests and i might know where the problem is.I didn't have time to pull out a scope yet, but counting on onboard voltage monitoring isn't much help either. So i hooked up my everyday uni-t139c to molex and set it to min max mode.Meter showed only 0,04V min max , so the load and ironious measurments might not be related to power suply side of things.Also prime95 set to maximum power draw does not move the voltage graph from the base level, only graphic card load does.Going further i made an oem atx 24 to standard atx 24 extension cable. But it was really like making atx 20 to atx 24 extension. There is still only single yellow cable going from power suply to the motherboard (not counting cpu power), only it splits in 2 on the connector, so i guess this might act like a resistor. Graphic card might just be draining what little juice motherboard gets.Tomorow i will try to desolder oryginal oem atx power cable from the pcb and replace it directly with full atx 24 cable, Extension plug was looking ugly anyway. Then i will check again if i still have voltage drops in the software . If i still have them i bring out a scope from storage and play around with that on the weekend. But it might just be that either i had bad connection in extension cable or that 1 to 2 cable split wasn't enough to carry enough current without saging a bit. There is always a chance that the motherboard itself is badly designed.

author: kokodin
author_rank: Regular Contributor
timestamp: December 13, 2019, 10:31:22 am
content: Multimeter measurment after swaping power connectorsmolex12.05V idle12.01V full loadon atx connector on the board under load11.95Vmin max 0.1Vseems to be in specsBut i must say replacing all the cables was a nightmare. I had to drill them out instead of desoldering because they bere crimped into groups.