I haven't examined all the photoflash units out there, but I have
examined many, and I have even built one. In the ones that "sing" I
wouldn't say they are using a "switching power supply" like JD called
it. I would say they are using a DC-DC converter based upon an
inverter-rectifier design. The battery drives an oscillator, the
oscillator drives a transformer, the stepped-up voltage from the
transformer is rectified and charges the flash capacitor.
I believe the flash units that "sing" are working the way I described
above. JD is correct that there are other ways to accomplish voltage
conversions, and some of these work more like a switching power supply.
I suspect they don't sing because the oscillation frequencies are
generally higher than audible frequencies.
Also, I am not keeping up on electronics as much as I would like, but my
old fashioned self is inclined to reserve the wording "switching supply"
to units that achieve a lower DC voltage from a higher DC voltage by
varying the duty cycle of the output transistor. Has the wording taken
on broader meaning?
Regardless of the wording, JD's basic message is correct... we have an
oscillating circuit and there are ways for the electrical oscillations
to produce sound, magneting fields from a transformer or inductor being
one of those ways.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu