1. Cause of the sound
The pulse load like the motor and the indicator generally will produce the sound. In most of the case, when the frequency of the pulse load is audio frequency which is a few kHz to 20kHz, the sound is produced. The part producing the sound would be choke coil. The vibration of core by the magnetic field produced by the pulse current flowing through the coil produces the sound. Please refer to the following for the mechanism of producing the sound. If the current flows through the magnetic body like the core, the core is magnetized. And then, if the current changes, the magnetostriction is generated. It is the phenomenon that the magnetic body expands and contracts. The air vibrates by this phenomenon, and makes a sound. Because there is no nick, crack and the change of the magnetic characteristic by generating the magnetostriction, because this is common phenomenon to the magnetic body, there is no problem in the electric characteristic.
Even if there is a sound from the power supply, it is no problem to the electric characteristic. Besides, the sound from the power supply will be reduced if external LC filter is assembled to the output of the power supply.
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