Common ATX PSU faults - any ideas would be great, thanks... - Page 1
Forum section: electronics / general

author: iamwhoiam
author_rank: Contributor
timestamp: May 07, 2012, 07:55:43 pm
content: Hey guys :-)I have a Dell PC here - a "Dimension 5000", to be precise, and the PSU has died (tested the MOBO on another supply and it comes on). I just wanted to ask if there are any common fatalities within a PC SMPSU, as it's all too easy to toss it and buy another (I likely shall), but whilst I have the free time, I would like to try and diagnose the "cause of death" of this one.What's been done------------------There is 240VAC going into the supply okay, rectification and reservoir caps are fine. There is 300VDC going right up to the primary side of the transformer... but nothing coming out of the secondary winding onward.Any ideas? Thoughts? I have no schematic, but if there are things I should consider, I would appreciate the help :-)Thank you.

author: T4P
author_rank: Super Contributor
timestamp: May 07, 2012, 09:49:35 pm
content: Remove the "transformer" it's actually a common mode choke, whatever, and check the winding resistance

author: Unknown
author_rank: Unknown
timestamp: May 07, 2012, 09:57:26 pm
content: The most common appears to be leaking electrolytics, both the secondary filter caps and the small cap that is part of the start-up circuit. You can often see the bulging top/bottom seals and sometimes even leaking electrolyte.

author: rsjsouza
author_rank: Super Contributor
timestamp: May 07, 2012, 10:36:24 pm
content: If you are into disassembling boards and reusing parts for your projects, keep in mind these power supplies have high-power MOSFETs and Schottky diode pairs (among other things) that can be reused in several projects.I do this for fun all the time (obviously testing everything after desoldering) and using them in my projects. The advantage is that, if you happen to blow one of these, they were "free" anyways...

author: Psi
author_rank: Super Contributor
timestamp: May 08, 2012, 01:38:20 am
content: The two faults i see most often are- Bad capacitorsOutcome A)The PC starts crashing or rebooting due to noise on the supply rails, replacing the bad caps normally fixes everything just fine.Outcome B)As the caps fail more and more they cause the stress on other components to increase until it just cant take it and something explodes. In this situation you have to replace both the caps and all the other parts that blew up too.Often its not worth doing.- Bad bootstrap resistorThere is a small resistor that provides power from the high voltage side to the low voltage side in order to run the logic enough to kick the powersupply into operation when it first starts. This resistor takes quite a big wack of power for a very short time and often fails. It's normally underrated wattage-wise due to the very low duty cycle but over a period of time the repeated strain on it gets too much and its resistance increases or it goes open. At that point the power supply wont start.

author: SeanB
author_rank: Super Contributor
timestamp: May 08, 2012, 05:50:04 pm
content: I will add that an ATX supply has 2 separate switchers inside. There is a small one, generally a self oscillating one where the capacitors on the primary side go high resistance, or the resistors go open, and where the output is around 18V before going to a 7805 to provide the always 5V rail and the start up supply to the main switcher controller. the secondary caps or the diode often goes open circuit or short, and then no operation at all from both.the main supply generally suffers from capacitors going high ESR, easy to see as they will be dark brown and bulging, or burst. the smaller caps in the controller also do the same, just without showing any physical signs. Not often the diodes fail, though they can have dry joints, or will have desoldered themselves. The power devices are easy to see if they have failed, the fuse will be broken or a mirror finish, and the input bridge will be dead short, with the 2 transistors having blown open their cases.

author: FenderBender
author_rank: Super Contributor
timestamp: May 09, 2012, 02:36:46 am
content: Electrolytic caps would be the first thing. Usually only on the secondary side since the PFC caps don't get too much stress.Second would be a faulty transistor or IC. Sometimes on cheaper units they use cheap silicon, or lack of protection circuits, which can end up with blown parts.Fuses? There would probably be a root cause for that though...

author: SeanB
author_rank: Super Contributor
timestamp: May 09, 2012, 06:05:54 pm
content: The fuse generally blows last...............