2016-12-07, 21:02
What is the current rating of the USB power supply? Did you populate through-hole LEDs? That will increase the current draw.
Your first USB supply sounds like it was overloaded if it can only provide 4V.
Coils can make noise if the wire is a little loose and is excited at the resonant frequency. A poor power supply might be running in a mode that would result in a lot of noise. This could be audible in the coils. I would look for a cleaner USB supply with higher current capability. If a friend has an oscilloscope you could observe the output and check for electrical noise that could be causing the acoustic noise from the coils.
Greg
Your first USB supply sounds like it was overloaded if it can only provide 4V.
Coils can make noise if the wire is a little loose and is excited at the resonant frequency. A poor power supply might be running in a mode that would result in a lot of noise. This could be audible in the coils. I would look for a cleaner USB supply with higher current capability. If a friend has an oscilloscope you could observe the output and check for electrical noise that could be causing the acoustic noise from the coils.
Greg