I’m curious, how many people are aware of these sounds. I have designed, etched, and built my own switching power supplies along with winding my own transformers. I am aware of the source of the noise. So, does anyone else hear these high frequency sounds regularly?
It depends on the AC-DC converter used.
The cheapest one is a “half wave rectifier”
That uses a diode to block the half of the AC wave headed in the wrong direction. Then a capacitor to charge and release the wave headed in the right direction in something nearing a flat output.
It does vary it’s output and flicker at mains frequency.
The smaller the capacitor or larger the load, the bigger the dropouts. The more pronounced the flicker.
Full wave rectifiers are better, they flip the direction of the other half of the wave, so will have a shorter dropout to smooth over. If they’ve cheated out on the capacitor or they’re overloaded they’ll also have flicker though.
I don’t think there are any half-wave rectifiers used for those things since the 4 diodes needed for a bridge rectifier are cheap and you can even get the whole bridge as just one package.
The difference is mainly on how much that abs(sin()) wave remains untouched or not.
There are really good AC-DC conversion methods that will output a very steady DC voltage with very low voltage ripple, but from what I’ve seen cheap LED lights just have a bridge rectifier, a big capacitor, enough led filaments/chips in series to have a large combined voltage drop and a resistor to limit the current, so the LEDs still see a ton of voltage oscillation (transformed into a current oscillation by the resistor that’s mean the limit the current, which is what affects LED brightness) hence the flickering.