What causes the high-frequency, ascending-pitch sound coming from a camera's flash unit when charging.

Asked by: Robert Smith

Answer

The electronic flash uses a solid state oscillator to step up a low voltage to a much higher one. The oscillator uses a transistor(s) to chop the low voltage DC into pulsating DC which can be stepped up by a transformer, the output of the transformer is then rectified (converted to DC) and used to charge a capacitor. The whistling you hear is mostly due to the transformer vibrating at the oscillator frequency although some capacitors are also capable of making some sound. The pitch rises after each flash because the oscillator isn't very well regulated and the discharged capacitor presents a heavy load to the oscillator, as the capacitor charges the load decreases and the frequency rises.
Answered by: James Hess, President, Hess Electric, Inc.