There are a few reasons why some AC adapters and power supplies make a whining noise. One reason is that the adapter or power supply is trying to find an outlet to connect to. If it can’t find an outlet, it makes a whining noise. Another reason is that the adapter or power supply is trying to create a voltage difference between itself and another device. If the voltage difference is too large, it makes a whining noise.
Most of the time our AC adapters and power supplies tend to be quiet, but what does it mean when one makes a whining noise? Should you be concerned? Today’s SuperUser Q&A post has the answers to a worried reader’s questions.
Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.
Photo courtesy of Bart Everson (Flickr).
The Question
SuperUser reader Rishat Muhametshin wants to know why some of his AC adapters and power supplies make a whining noise:
Why do some AC adapters and power supplies make a whining noise?
Why do some AC adapters and power supplies make this whining noise? Why do some not make this noise? Is there anything that I can do to suppress it?
The Answer
SuperUser contributors DragonLord and Daniel R Hicks have the answer for us. First up, DragonLord:
Followed by the answer from Daniel R Hicks:
Most modern AC adapters are switched-mode power supplies. The internal switching frequency of an SMPS is typically low when unloaded and increases with a load (up to a certain point depending on the design). The no-load frequency is often low enough to be within the human hearing range.
In addition, in low or no-load situations the PWM (Pulse Width Modulation) used to regulate voltage at the inverter stage will be at a low duty cycle and create a “spikey” output profile which is more prone to causing vibration in coils, and the transformer itself will tend to vibrate as well. Together, these can lead to audible noise especially in cheaper units which fail to suppress this noise.
Under a load, a properly functioning SMPS should operate at a frequency well above the human hearing range, typically 50 KHz or higher (although some older designs operate at 33 kHz). However, the same noise can occur under a load with a poorly designed or defective power supply as the coils may vibrate under electrical stress at a sub-harmonic frequency.
This is why you sometimes see a “glue” of sorts on coils inside electronic devices. The glue helps reduce the vibration and noise the coils generate during normal operation. Of course, this means that a user can apply glue onto coils using a glue gun to suppress coil whine—and yes, people have done this successfully with PC motherboards, graphics cards, and power supplies. However, you generally cannot do this easily on small wall chargers of the sort you mentioned without risking damage to the charger or exposure to potentially dangerous voltages.
In conclusion, a whining noise is not necessarily a sign of trouble in cheaper wall chargers when they are unloaded. However, a computer PSU or laptop charger that generates coil noise, especially when under a load, may be defective and you may want to consider replacing it.
More information on coil noise can be found in this Wikipedia article.
Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
