Whine noise in ATX power supply - Page 1
Forum section: electronics / repair

author: macboy
author_rank: Super Contributor
timestamp: September 18, 2017, 05:56:38 pm
content: I have a working ATX power supply that is making a moderately high pitched acoustic noise, I'd estimate 4 khz. The frequency and level vary a little with load.I first suspected the output filter electrolytic capacitors, since I have fixed a few small wall-wart type SMPS supplies with similar symptom, and it was always bulging/leaking caps. I desoldered and check the caps, and all are good: no leaking, in-spec capacitance and ESR.I would think this is not an uncommon problem, and maybe some experienced person might be able to say, "Oh yeah that's probably the ...". One can hope. What would be on your short list of suspected components?

author: Toasty
author_rank: Regular Contributor
timestamp: September 18, 2017, 06:27:22 pm
content: Several items can cause this. May we have the brand & model number and perhaps a top-down photo showing the component side?With it on and the noise present, use an insulated, non-metallic tool and begin touching components. If a change in the sound occurs, you have localized the problem.You can also use your ear and a straw or some tubing to find the noise. Much like a stethoscope.Suspects are coils and transformers. Although, 4kHz seems low as the switchers usually run in the 30kHz and up range.T

author: keymaster
author_rank: Regular Contributor
timestamp: September 18, 2017, 08:59:05 pm
content: if all fails , then change the output caps with i slight bigger value. that will change the frequency ,hopefully to a non acoustic one.

author: macboy
author_rank: Super Contributor
timestamp: September 18, 2017, 09:17:42 pm
content: Quote from: Toasty on September 18, 2017, 06:27:22 pmThe drinking straw stethoscope is brilliant. I had thought of attaching a microphone to a dowel or something, but the straw is so much simpler, and probably safer.I'll also try keymaster's suggestion of physically dampening components to isolate the culprit.I hope to find some time tonight or tomorrow to look at it. Thanks for the suggestions.

author: wraper
author_rank: Unknown
timestamp: September 18, 2017, 09:23:53 pm
content: Probably there are no faulty components. Does it whine when PC is shut down? This could be standby PSU. Many ATX PSUs emit some high pitched noise out of the box, often it's related with achieving high efficiency.

author: RayRay
author_rank: Frequent Contributor
timestamp: September 18, 2017, 09:51:22 pm
content: If you ask me, the noise is not necessarily electronics related!It could just be it's fan being whiny.FYI, computer PSU's regulate the fan speed depending on the load (they increase/decrease it's voltage to do so) so that would explain why it's more noticeable at higher loads. You could try replacing it.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: September 18, 2017, 11:39:48 pm
content: Quote from: Toasty on September 18, 2017, 06:27:22 pmYes indeed.Acoustic emissions = something vibrating. A fan is the obvious source. But inductive components can vibrate, due to the varying magnetic forces on the windings and the cores.Telcommunications power supplies, which have to meet acoustic emissions specs., commonly glue transformer core halves, to stop any movement. A near-invisible speck of dust, between two unglued core halves, acts like a pivot. When energized, the core halves can rock back and forth on the speck, causing an annoying whine.For really quiet operation, the windings are also glued, to stop any movement there.

author: dave_k
author_rank: Frequent Contributor
timestamp: September 19, 2017, 02:38:14 am
content: Quote from: keymaster on September 18, 2017, 08:59:05 pmexplain how?

author: Toasty
author_rank: Regular Contributor
timestamp: September 19, 2017, 03:56:27 am
content: Quote from: macboy on September 18, 2017, 09:17:42 pmErm..?Quote from: Toasty on September 18, 2017, 06:27:22 pmQuote from: RayRay on September 18, 2017, 09:51:22 pmOnly if the fans are running at 240,000 RPM...Quote from: dave_k on September 19, 2017, 02:38:14 amIndeed, I too am curious...T

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2017, 04:56:17 am
content: Quote from: RayRay on September 18, 2017, 09:51:22 pmCould be fan PWM or blade or bearing noise, but perhaps unlikely. (Usually sounds different, or others more likely. Not ruling it out though.)Quote from: thermistor-guy on September 18, 2017, 11:39:48 pmNot widely known, but well understood: magnetic materials are usually magnetostrictive, i.e., they shrink (by some ppm) when a magnetic field is applied. This is magnitude dependent, thus the acoustic signal is twice the magnetization frequency. Or, a bias must be present to make a minor oscillation apparent.The latter is the most common case: the control loop becomes unstable, due to increasing capacitor ESR (and decreasing value), causing oscillation.The oscillation is audible through a few possibilities:- Power transformer (ferrite cored)- Filter choke (ATX supplies usually use #26 or #52 powdered iron, which is lower magnetostriction than the ferrite core transformer, but it's still nonzero)- Film capacitors (very unlikely in electrolytics, but quite common in film capacitors, which exhibit the analogous electrostriction property; a film capacitor is typically used for coupling and/or bypass in the half bridge topology)Quote from: dave_k on September 19, 2017, 02:38:14 amOld capacitors have high ESR, the output capacitors being the most critical. ATX supplies are usually voltage mode, so the capacitor ESR is a necessary zero in the loop stability criterion. Too much and, not only is the output ripple way beyond spec, but the loop goes unstable and you usually get a whine at the loop cutoff frequency, which is usually pretty low, a few kHz. So, there you go.Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: September 19, 2017, 10:28:22 am
content: There is a tiny fly-back stand-by PSU that can get whiny at low loads, if you locate the it's decoupling capacitor and increase the capacitance it could lower the frequency to a less annoying whine, while a resistor across the decoupling capacitor would increase the frequency to where it's no longer audible.Also if you want to find out whether or not it's the fan you can disconnect it and then listen to the PSU, at no load the PSU won't get hot enough to require a fan.

author: macboy
author_rank: Super Contributor
timestamp: September 19, 2017, 01:02:55 pm
content: Thanks for all the comments.Rest assured that it isnotthe fan(s).Although I checked the value and ESR of all the main rail output capacitors, I didn't check the standby supply. I'll have a look at that.

author: Armadillo
author_rank: Super Contributor
timestamp: September 19, 2017, 03:38:48 pm
content: Quote from: macboy on September 19, 2017, 01:02:55 pmI suspect the magnitude of the Whine should be proportional to the loading on the power supply. Hence if unloaded but enabled the whine should be abated somewhat.What I want to say is when the capacitor turns to resistor like, it dissipate energy than store it, hence extra loading onto the power supply. So the capacitors checking should extends to all the secondary load boards if those were hooked up to the power supply.I suspect the "glue" should have been deteriorated by now separating, not holding the coils and the core thus vibrating as others has said. If this is true, then the only way is to change the switching transformer or try to re-shellac it.I don't know if you have experienced it, but its akin to the fluorescent ballast choke [not the electronic type]. Over times, it hum at 50/60 hertz and the only solution is to change it [because it's cheaper that way]. ]