I am trying to repair the power supply of the sewing machine. This card I think, which is in SMPS structure, is an LCC Resonant feeding card. I have repaired this board and I get the output voltages healthy. When I plug in the 220 AC sockets from the card's input points, there is a buzzing sound from the small transformer. When I searched a bit on the internet, I learned that they called it "coil whine". The system works well, but this noise gets even more severe, especially when the card's output pins are open. I am affraid of exploding this power board.
I added a video (the video link) to better explain what I wrote above.
in the first case:
card's outputs are open and energized. There is a noise,
in the second case
When I put a magnet on the transformer, the sound is cut off. I saw this in a video. However, this is decresing the output voltage 24v to 12 volt. I think this transformer goes to the mosfet's Gate.
In the third case,
I plugged a fan into the 24 V outlet and again the noise was gone and the system works healthy. What is the reason for this noise and how can i fix it?
Best Answer
It doesn't have to be the transformer producing the noise, it could also be a capacitor (like the big red one next to that transformer).
Using a magnet or a metallic item to check the noise on a transformer (especially on a high frequency transformer) is neither recommended nor helpful, because it could affect the electro-magnetic fields in it and change the operating mode of the switching power supply, which might change its operating frequency or the electrical pulse shapes through the circuit, making the noise disappear for the moment and give you a false impression that the transformer is making the noise.
You should use a sturdy piece of plastic and press (not too hard to break something) on the transformer (different part of it) to see if the noise changes/reduces/stops.
If it's the transformer, you should push some hard thick paper in between the coils and the core to dampen the vibrations.
If it's not the transformer, you should try pressing other components like the big film capacitor(s). The electrolytic capacitors are less likely to produce whine because their paper insulator inside is a natural dampener.
You could also press the capacitor or other components with an eraser as that could dampen its vibrations if it is the one making the noise.