Wednesday, April 3, 2013

Singing capacitors

Today I have been repairing a 12 year old 15" LCD monitor. It had no apparent fault, everything worked nicely, but the monitor produced a extremely strong high pitched tone (around 18 kHz) enough to produce headache in only two minutes.

Usually high pitched noises come from the switched power supply. In the past I have found switched power supplies with loosen transformer cores that produced noise just like an speaker.

But this was not the case. The monitor has a single transformer in the power supply and it seemed rock solid. With the aid of a plastic trimming tool, I added pressure in some parts of the transformer but the annoying noise were still there. By chance, I touched a capacitor with the trimming tool and the noise got attenuated a lot. It was a small 1nF, 1kV blue ceramic capacitor (the one at the photo), and effectively, the noise seemed to come from the capacitor itself.

This was new to me. A singing capacitor? Yes!

Ceramic capacitors can exhibit piezoelectric effects and act like a tweeter (speaker). Even small SMD capacitors can produce noise. Just look for "piezoelectric effect in capacitors" in Internet for more information.

This time the solution was easy: I replaced the singing capacitor with another one, and the annoying noise just disappeared. So if you have a switching power supply that produces high pitched tones, check for inductors, transformers and also ceramic capacitors: Both SMD and ceramic ones!

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