Electronics > Repair

Power Supply - Transformer Buzzing

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2X:
Hello,
I have a PCB from a power supply where I have a hearing buzzing problem. When I just have connected the power supply on the mains (without load) a transformer/inductor (Picture 1) makes a little hearing buzzing noise, but when I connect a device/load on it the buzzing noise is quite big (it gives the feeling that something is going to explode). I placed an oscilloscope on the output of the power supply and I saw some spikes on the output waveform (Picture 2), where I beleive this voltage ringing is the source of buzzing.

Also, I measured with an oscilloscope a same one power supply (this don't make a hearing buzzing when is working with or no load on it) and on the output waveform (Picture 3) there is no the existence of the voltage spikes like in Picture 2. One strange thing is that I observed is when I disconnected the C56 capacitor the buzzing stops and the voltage spikes dissapeared from the waveform. I took the same  good one capacitor from the good one PCB and I replace it but the same buzing again.

I made a schematic (Picture 4) from a part of the PCB and I can't understand what causes the voltage ringing. There is a chance that the buzzing problem caused by the transformer? What else in combination with the capacitor C56 can cause the voltage ringing?

Jwillis:
Transformers will vibrate due to Magnetostriction. I most cases this isn't profoundly noticeable unless there is something loose within the transformer. Broken adhesives on the core or loose winding's can produce noise. Also the amount of noise depends on the resonant frequency of the material used, the frequency of the field applied and the strength of field. Loose core halves can have gaps which can cause spikes because of the electromagnetic energy stored in those gaps. It's really a mechanical issue that can't be remedied without changing the frequency and/or fixing any loose transformer core elements and winding's.   

2X:

--- Quote from: Jwillis on March 24, 2024, 02:18:10 am ---Transformers will vibrate due to Magnetostriction. I most cases this isn't profoundly noticeable unless there is something loose within the transformer. Broken adhesives on the core or loose winding's can produce noise. Also the amount of noise depends on the resonant frequency of the material used, the frequency of the field applied and the strength of field. Loose core halves can have gaps which can cause spikes because of the electromagnetic energy stored in those gaps. It's really a mechanical issue that can't be remedied without changing the frequency and/or fixing any loose transformer core elements and winding's.   

--- End quote ---


So theoretically if I replace the transformer from the good one PCB the noise will be stop (I will make a try). Also, could I check the loosing windings or the other defects that you said of the transformer with an Insulation Tester Multimeter (I don't have one yet).

The bad luck is tha the transformer resonates with this certain capacitor and if I remove it the noise stops. As I know this C56 capacitor is an X type capacitor which helps only to reduce EMI due to differential noise and the designeer selected this certain value for the optimal EMI reduction I beleive, so if I to try to put another value or to remove the capacitor at all is not acceptable in my opinion.

https://www.fluke.com/en-us/product/electrical-testing/insulation-testers/fluke-1587-fc

Thank you for the detailed reply.

inse:
Where is the measurement taken?
The waveform looks like what “modified sine“ inverters put out.
Many electronic devices don’t like this waveform as it causes huge ripple currents in capacitors.

2X:

--- Quote from: inse on March 24, 2024, 09:21:33 pm ---Where is the measurement taken?
The waveform looks like what “modified sine“ inverters put out.
Many electronic devices don’t like this waveform as it causes huge ripple currents in capacitors.

--- End quote ---

I measured these waveforms with probe of the oscilloscope between P1 & P2 pins on the schematic.
The picture 2 is from a device that buzzing and the picture 3 is from another exactly same device that doesn't makes any noise.

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