We have dimmers on many of the incandescent lamps in our house. When the lamp is dimmed, a faint buzzy whine can be heard. The sound doesn’t come from the dimmer but from the light bulb itself. Why does the dimmer make the bulb buzz?
• A tungsten lamp is dimmed when the controlling electronics switches off the power to the bulb each time the alternating current of the mains voltage crosses the zero point in its sine-wave cycle, restoring it at a later point in the half-cycle. The longer the power is switched off, the lower the average current passing through the bulb. This reduces its brightness.
The loss and restoration of current happens very quickly, generating a spiky waveform through the lamp that is rich in the harmonics of the mains frequency. Bulb filaments often have a natural resonant frequency of vibration of a few hundred hertz, so can be excited into resonance by these harmonics. When it reaches this frequency, the bulb will buzz.
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Chris Collins, Llandrindod Wells, Powys, UK