Online UPS silencing

[NiHaoMike]

Many PSUs with PFC will run fine from 80V DC, albeit at much reduced power. That can help find a more useful way to put a constant load.

[taydin]

Did you locate the source of chirping?

If it is a coil, KONTAKT Chemie PLASTIK 70 or a similar laquer varnishing the windings and core might silence it by preventing movement. (Source: How i learned it during my TV repair stint.)

Ceramic capacitors can exhibit a piezoelectric effect, you might either isolate or exchange against non-hissing types.

I haven't looked whether the issue is the charger SMPS transformer or caps. But I'm suspecting that potting the transformer would only decrease the chirping, not eliminate it.

How would one go about determining if a cap is making noise, other than removing it and putting another one in? I suppose one would need a vibration meter. Or maybe a microphone that works by contact ...

[taydin]

Many PSUs with PFC will run fine from 80V DC, albeit at much reduced power. That can help find a more useful way to put a constant load.

I thought about changing the charger with another SMPS that won't chirp, but the existing SMPS is very tightly integrated with the main controller. There are many optocouplers, sending information to the main controller. Also, the main controller does closed loop control of the output voltage. I would have to reverse engineer and draw the schematic of that part of the circuit.

Or maybe you are talking about attaching an EXTRA PSU to the output of the charger, which will guarantee that 1A is drawn at all times. Either the PSU itself draws it, or the batteries when they are being charged ... That would really be the perfect solution.

[schmitt trigger]

First of all, I want to compliment you on your wood and metal working skills. They are an order of magnitude better than mine. 

To your transformer "chirping": Having worked fin a company that manufactured them, I can tell you that the source of the noise is the air gap between the pair of core pieces.
Fully encapsulating the transformer in two-part epoxy will eliminate the chirping.
Unfortunately, you would have to remove the transformer from the board to do that. 

[taydin]

First of all, I want to compliment you on your wood and metal working skills. They are an order of magnitude better than mine. 

To your transformer "chirping": Having worked fin a company that manufactured them, I can tell you that the source of the noise is the air gap between the pair of core pieces.
Fully encapsulating the transformer in two-part epoxy will eliminate the chirping.
Unfortunately, you would have to remove the transformer from the board to do that. 

No problem with removing the transformer actually I should try that. Worst thing that can happen is, it will continue to chirp.

[jbb]

How would one go about determining if a cap is making noise?

Never tried it, but I heard you can glue a probe (e.g. thin wooden stick) to a paper cup. When the probe is touched to a component, the vibrations shake the cup and make audible noise.

[taydin]

Never tried it, but I heard you can glue a probe (e.g. thin wooden stick) to a paper cup. When the probe is touched to a component, the vibrations shake the cup and make audible noise.

Hmm, that's clever!

Given that i'm at an advanced stage of GAS, I was immediately looking for some kind of equipment to buy 

[BradC]

How would one go about determining if a cap is making noise?

Never tried it, but I heard you can glue a probe (e.g. thin wooden stick) to a paper cup. When the probe is touched to a component, the vibrations shake the cup and make audible noise.

Nice trick. I poke a bit of 1/4" pvc hose in my ear and use the other end as a stethoscope. That responds to direct contact just as well as proximity.
I used to balance motorcycle carbs with a piece in each ear and the other ends down both carbs. Adjust until the hiss is equal in volume.

[jbb]

Oh - I forgot you’re working on mains powered stuff. I recommend you don’t probe  live surfaces!  If your wooden stick is damp it may conduct, and it’s way too easy for your fingers to slip down a plain stick.

I’m not sure how to safely construct a probe now. Maybe you could use a (suitably insulated) multimeter probe in place of the stick?  They have plastic insulation and a ring in the front to stop your fingers.