3
\$\begingroup\$

i have an APC brand UPS installed inside another project, and have just recently powered it up for long durations for the first time. and i've picked up an annoying buzz in a small transformer on the main circuit board :<

i'm certain this little jerk is the only thing buzzing, but i've failed to find it online for R&R.

buzzing transformer

the text reads : 430-0271-Z. VIKING F-1. LEI-4 BJ08.

the buzzing is present intermittantly when the mains power is connected, and transitions to a steady sound when battery is unplugged. it goes away completely when battery is plugged in and mains is disconnected, so i conclude it's part of the charging circuit.

could a faulty battery cause this problem?

if it's a custom part for APC, and i can't source a replacement, is it safe to muffle the buzzing with some self-adhesive duct foam? i mean the black mat material used to insulate commercial air conditioning ductwork. it's a dense heat-resistant rubber foam.

\$\endgroup\$
3
  • \$\begingroup\$ Flyback transformers seem to have a tendency to make noise. \$\endgroup\$
    – Steve
    Dec 11, 2015 at 21:46
  • \$\begingroup\$ Are you sure it is the transformer? I ran across quite some APC UPSs that had the FETs make noises. Does it appear when the UPS is running some mains appliances too? I have seen a lot of H bridges in UPSs creating awful noises on all occasions, all which was basically "by design". Caps getting bad and distorting the driving waveforms made this worse. In one I was lucky to be able to hack the firmware to increase its swithicng frequency into non audible ranges. \$\endgroup\$
    – PlasmaHH
    Dec 11, 2015 at 22:09
  • \$\begingroup\$ Use an insulated rod, pressed against the component and your ear, to exactly pinpoint the source. \$\endgroup\$
    – rdtsc
    Dec 16, 2015 at 14:57

5 Answers 5

4
\$\begingroup\$

You have given no indication that the transformer is broken, and that replacing it will therefore do anything useful.

Options are:

  1. Live with it.

  2. Get a different UPS.

  3. Muffle the whole UPS somehow, keeping in mind its need for ventilation.

Adding foam to the board, or doing anything else inside the unit is a bad idea. You don't know what each part does, and what thermal considerations it has. You shouldn't be inside something with dangerous voltages without really understanding it anyway.

Perhaps you can put the whole UPS inside a cardboard box with sufficient vent holes to allow air flow. The holes can be arranged to muffle sound in the direction you care about. If you do this, at least at first, put a thermometer in there and monitor the ambient temperature the UPS will now see. Check the UPS datasheet and make sure you're not exceeding its requirements. You need to check this under sustained maximum load, both while operating from the line and from the battery.

I'd probably just get a different UPS and use the first one in a different place where noise is less of a issue, or sell it, or something.

\$\endgroup\$
1
  • \$\begingroup\$ good evening. as i mentioned, in my original post, the UPS is installed inside another enclosure, and has beeen removed from the original retail enclosure. i'm certainly not exceeding the load capacity of the UPS while putting under an idle load of zero amps. i don't need to muffle the entire circuit board, just the single component of the transformer. if i could find a data-sheet for the transformer i would know the thermal limits... which is why i posted a picture of the transformer. \$\endgroup\$
    – ijason
    Dec 11, 2015 at 19:04
3
\$\begingroup\$

The best tool to help identify noisy components is a good-old-fashioned stethoscope. You sometimes find cheap ones with a plastic chestpiece, which is much safer when listening to off-line power supplies (mains + metal = danger)

LEI on a magnetic may refer to Leider, an Asia-based contract manufacturer of magnetic components. (My previous employer did business with them.) Most likely this part is custom made for the application and will not be commercially available. Not sure of the significance of the VIKING marking.

If the part isn't varnish impregnated (or isn't impregnated properly) it is more likely to buzz under stress vs. a part which is impregnated. So as Olin said, there isn't much you can do about it safely.

\$\endgroup\$
3
  • \$\begingroup\$ thank you for your reply, i was actually able to identify the specific component with the aid of a wooden spoon. but the method is exactly the same as a stethoscope -- hollow of the spoon over the ear, and physically touch each component with the far end. you'll hear plainly which piece is making the noise :) i've seen other forums where people tried to heat up (removed) transformers, and varnish them at home, but noone met with much success. if i can't find a model for this specific transformer i'll just have to try dapening the sound. i can take thermal images during some load trials. \$\endgroup\$
    – ijason
    Dec 11, 2015 at 19:09
  • \$\begingroup\$ if you know what component is vibrating / causing noise, could you try to dampen it by adding mass to the component / clamping it? It has to be mechanically vibrating for it to make sound, right? \$\endgroup\$
    – bathMarm0t
    Dec 11, 2015 at 23:38
  • \$\begingroup\$ "Varnish" impregnation has to be done in a vacuum to eliminate all of the voids. Not surprising home-brews have little success; quite a feat. \$\endgroup\$
    – rdtsc
    Dec 16, 2015 at 15:01
1
\$\begingroup\$

If this is a high frequency buzzing (i.e. not 50/60 Hz, maybe > 10 kHz ?), it's probably the inverter PWM frequency. If the buzzing pulses on and off, then that's the regulation loop; perhaps that also depends on the state of charge of the battery.

If it is the transformer, try clamping it -- use a Vise grips to (temporarily) clamp the core tighter, or to clamp the windings tighter. If that works, then use a small C clamp for the core, or tie string tightly around the windings.

It may not be the transformer -- ceramic (not electrolytic) capacitors also have some electrostriction characteristics (basically a piezo effect). Replacing those with higher voltage rated ones might help (they will be physically larger). Alternatively unsolder one, place it vertically ('tombstone'), and manually add wire from the top to the original PCB location will minimize the coupling from the ceramic to the PCB.

\$\endgroup\$
1
  • \$\begingroup\$ Could check the ESR of the in-circuit caps, perhaps they are out-of-spec. \$\endgroup\$
    – rdtsc
    Dec 16, 2015 at 15:05
0
\$\begingroup\$

Since this is for your own personal project and you imply you're handy, I'd say why not try to do something. Yes, the engineers have engineered this and that, but people like us make modifications all the time that sometimes do and sometimes don't work; that's the risk we take. I'll give you an answer that might actually help you instead of just saying "live with it".

We're going to submerse the transformer in a box of oil. This is done all the time by pros and amateurs alike for various purposes.

I would:

  1. Try some more to find a replacement part, order it, and then keep it on hand for if these noise reduction mods blow it up.
  2. Google "electrical RTV" to find an electrical sealant you can buy in your region.
  3. Find some sheet metal that you can make a box to fit over the transformer with.
  4. Get a short, sheet metal screw in a diameter of 1/4" to 1/2".
  5. Cut a square out of the metal for the bottom and another the same size for the top.
  6. Drill holes in the bottom plates for the wires. Make them bigger than the wires, we will fill the gap with the RTV.
  7. Drill a hole slightly smaller than the sheet metal screw in the top plate. This is your fill hole.
  8. Cut and bend a rectangle of sheet metal for the four sides of your box. Make the walls of this box as tall as will fit in the case.
  9. Feed the transformer leads though the holes in the bottom plate. In between the bottom of the transformer and the bottom plate, apply a (~3/16") layer of the RTV covering the entire plate.
  10. Place and solder the four walls to the bottom plate. Solder good because it needs to be waterproof.
  11. Place and solder the top plate to the box.
  12. Fill box with mineral oil and check for leaks. (Don't do this until the RTV is cured.)
  13. Apply a few small drops of RTV to the PCB where the transformer sits, but not in contact with any electrical components.
  14. Feed the box's leads through the circuit board and solder.

The transformers leads may not be long enough to reach through the two layers of RTV to the circuit board, so before you start, account for this.

The cooling surface area should already be dramatically increased so there's no need for fins or anything.

\$\endgroup\$
4
  • \$\begingroup\$ How about potting it in wax? \$\endgroup\$
    – Kaz
    Dec 11, 2015 at 19:36
  • \$\begingroup\$ @Kaz I am unfamiliar with that technique so I can't speculate. \$\endgroup\$ Dec 11, 2015 at 19:54
  • \$\begingroup\$ Basically we melt some paraffin wax, and submerge the part in it, allowing the wax to permeate the windings. Then let it cool so the wax solidifies. This is done with electric guitar pickups, to squelch microphonic feedback which can occur when the amplifier is cranked loud, and high gain is used. \$\endgroup\$
    – Kaz
    Dec 11, 2015 at 19:56
  • \$\begingroup\$ I would think wax may possibly melt, depending on the load. This is a power transformer, not a low-current guitar pickup. \$\endgroup\$
    – Steve
    Dec 11, 2015 at 21:50
0
\$\begingroup\$

good evening,

thank you everyone for your suggestions :)

sapping the transformer in paraffin is a very interesting suggestion! unfortunately, it seems unlikely to find wax with a melting point reliably higher than 45... and that's a fairly low temperature. so i think i'll need to try that on a lower voltage application.

i decided to try and muffle the transformer with some dense foam-rubber, commonly used to dampen vibration in commercial ducting. it does wonders to absorb sound, too - but not in this case!

i made some small aluminum sinks to help dissipate any trapped heat from the foam.

enter image description here

even wrapped in two layers of the foam, and strapped fairly tight, even with a large block of the foam under the circuit board to prevent reflected sound... ugh, simply too noisy.

not to mention, looking like the dog's lunch.

enter image description here

i'll have to omit the UPS from this enclosure and add it further up the line for the supply current. there's another enclosure out of the listening space, so the buzzing won't reach any ears.

thanks again!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.