Squealing transformer inside a UPS - Page 1
Forum section: electronics / projects

author: Cozzmo
author_rank: Regular Contributor
timestamp: October 23, 2012, 07:15:11 am
content: Howdy all !First post here, i've been watching Dave's videos for a while now, and after bumping into him one day a month or so ago at the North Rocks computer markets, figured i should get myself on the forum.Timely as it turns out because i bought a couple of used APC Back-UPS CS 500VA UPS units. One is fine, however one was squealing when plugged in. I disassembled and found a small transformer on the board was making the noise. Removed and inspected it, couldn't really "see" anything wrong physically. Put it all back together and it will still making the noise so i took the unit back the next week and swapped it for another one ... which does exactly the same thing !Two questions here.1. Does it matter? the UPS will be placed in a location where the noise will not be an issue, but if there's any risk of fire/catastrophic failure, i will not use it.2. If it does matter, what are the chances of being able to find a suitable replacement part?Cheers !

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2012, 08:08:17 am
content: How loud is it? Some noise from inductors is normal and even more so when the UPS is active and running from batteries.Replacing the inductor/transformer most likely wont fix the problem. Its often due to the transformer vibrating at a rate within human hearing.Does it sound like a 50/60hz buzzing or a high pitched tone?If its something you can only notice in a quiet room then its probably normal and fine to leave but if you can hear it all the time, even when other sound are present then it maybe a symptom of a bigger problem.

author: Cozzmo
author_rank: Regular Contributor
timestamp: October 23, 2012, 08:30:04 am
content: The noise is loud enough that i can hear it over my TV (annoyingly) and the tone changes slightly as the front panel lights blink. Its part of the charging circuit from what i can see from the schematic so when its running off the battery, the noise goes away.Its a high pitched noise, certainly not mains buzz.Given that i have two of the same model, one is quiet as a mouse (except for a slight humm which is to be expected) and this one is making the noise, its surely not a 'design feature'.Schematic attached. Offending part is on page 4 "T1".

author: Codemonkey
author_rank: Regular Contributor
timestamp: October 23, 2012, 08:36:52 am
content: You could try painting it with some varnish to stop the windings/core from moving, or even better remove it and dip it then bake it in an oven to dry it out, thats what the manufacturers do anyway.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 23, 2012, 09:02:56 am
content: yeah, just paint it with clear nail varnish a few times.

author: Cozzmo
author_rank: Regular Contributor
timestamp: October 23, 2012, 10:18:06 am
content: OK thanks guys. I assume any kind of clear nail varnish will work ?

author: robrenz
author_rank: Super Contributor
timestamp: October 23, 2012, 11:49:13 am
content: It sounds like the alarm sound on mine. Are you sure that thing is a transformer?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 23, 2012, 01:47:32 pm
content: my UPS doesnt make any noise. i think it is a bad indication whether its poorly designed, poor part brand or something bad is going to happen soon.

author: robrenz
author_rank: Super Contributor
timestamp: October 23, 2012, 02:45:18 pm
content: https://www.google.com/#hl=en&sugexp=les%3B&gs_nf=3&gs_mss=apc%20500%20ups%20alarm%20sounds&tok=9bKsvf4w79TfFCqNkyCMhw&pq=apc%20ups%20alarm%20sounds&cp=37&gs_id=1y&xhr=t&q=apc+500+ups+alarm+sounds+continuously&pf=p&sclient=psy-ab&oq=apc+500+ups+alarm+sounds+continuously&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=cfe7a5b78bb5c473&bpcl=35466521&biw=1280&bih=626

author: SeanB
author_rank: Super Contributor
timestamp: October 23, 2012, 03:41:19 pm
content: Check C12, C48 and R12, C45 by substitution. These are the supply and snubber components for the power supply, and if they are faulty then it will make a noise. Also check Q2, Q18,Q19 preferably by substitution for leakage.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 23, 2012, 06:31:30 pm
content: Quote from: SeanB on October 23, 2012, 03:41:19 pmdont! send it back to the retailer. test all their stock and only replace with the one who doesnt make noise, since the OP mentioned he got one unit which doesnt make noise. i dont care if its by design, if the two behave differently, i'll pick the one which i prefer and replace the other until i satisfied.

author: penfold
author_rank: Frequent Contributor
timestamp: October 23, 2012, 07:29:40 pm
content: If you have the option, definitely return it for a 'good' one.However, from personal interest. Would be nice to know if there's any more details about IC4, whilst switchmode transformers can squeal, its very rare in a modern power supply where switching frequency will be somewhere above 100kHz, could quite possibly be some sort of control instability on that power supply section.Dan

author: poptones
author_rank: Frequent Contributor
timestamp: October 23, 2012, 07:42:22 pm
content: I gotta agree with taking it back. It very likely is a poorly implimented snubber, which means it is doomed from the start. I would actually be more concerned about the one that isn't doing this as it seems the design is flawed and the "good one" probably won't be "good" for long. I'd take all of them back and buy something else.

author: Cozzmo
author_rank: Regular Contributor
timestamp: October 24, 2012, 03:03:59 am
content: Quote from: robrenz on October 23, 2012, 11:49:13 amQuote from: robrenz on October 23, 2012, 02:45:18 pmIts not the alarm. The tone of the noise changes when thetransformeris physically interacted with while operating, that's how i found which component was making the noise.Quote from: SeanB on October 23, 2012, 03:41:19 pmThanks for the suggestions Sean, i'll see what i can do over the weekend.Quote from: Mechatrommer on October 23, 2012, 06:31:30 pmThey were used/second hand parts. As far as i'm aware the guy doesn't have any more of them so a refund would be the only option and at this stage, i can't find the receipt.Quote from: penfold on October 23, 2012, 07:29:40 pmI wish i had the presence of mind on the day that i took the first one back to swap to check the replacement, but i didn't. Once i got home and realised it made the same noise i was too buggered to bother going all the way back.Quote from: poptones on October 23, 2012, 07:42:22 pmBeggers can't be choosy sometimes.

author: penfold
author_rank: Frequent Contributor
timestamp: October 27, 2012, 02:38:10 pm
content: Trying to think about what causes audible in normal SMPC's (not snubbers) it did occur to me that some integrated switchers like that that runs the small flyback converter on the charger have the tendency to operate in 'pulse skipping' under low load conditions. The bursts of skipped pulses can often repeat at audible frequencies,Could it be that one of the batteries is in a higher state of charge than the other and the power converter charging the more charged one is in a low load condition and the transformer windings are vibrating at the audible frequency,Just a theory... would mean it isn't a problemI highly doubt that it is anything to do with snubbers, if the supply is operating any above audible frequency, they won't make any audible noise, and no fault with them would cause any audible noise. Electrical noise: certainly

author: Cozzmo
author_rank: Regular Contributor
timestamp: October 27, 2012, 09:12:26 pm
content: This UPS has only a single 7.2ah 12v battery. Swapping the battery with the "good" ups doesn't make a difference.