Eli5 Why chargers make high pitched noise when battery is full?
In: 25
Some of the other answers here explain why a phone charger can make noise in the first place–parts inside the charger can audibly resonate at the same frequency as the changing current flowing through these parts, which can range from hundreds of kilohertz (inaudible) to just a few kilohertz (audible and often annoying).
Why does the current flow need to change? Because this is how the charger takes the high voltage from your wall and turns it into a small voltage for your phone. Imagine you’re in a room with a painfully bright light. You can’t switch it on and leave it on (it’ll burn your eyes), but you can switch it on and off really fast to reduce the average light of the room to a manageable level. If you want more light, you “pulse” the light on for longer while shortening the time that you leave it off. The charger does something similar.
As your battery nears “full”, the charger must provide less power–the light needs to spend less time on and more time off. This is where we run into a problem–flipping the light switch takes time. If you switch the light on and then instantly start switching it back off again, there is a minimum amount of time that the light will be on and you can’t do anything to make that “on” time shorter. At that point, the only way you can reduce the light level further is to just increase the “off” time–you have to lower the frequency of the light pulses you were making. The charger must do the same as the power supplied to the battery is reduced. If the charger’s operating frequency drops low enough, it can dip into the audible range for humans.
Why do only some chargers do this? Most chargers can “flip the light switch” so quickly that even at low power output, they’re still operating outside the human hearing threshold. They can do this thanks to MOSFET technology. Slower chargers sometimes rely instead on BJT technology, which is cheaper but also much slower.
Hopefully I wrote something coherent. It is very late for me.
Latest Answers