Why do a mobile phone charger (without mobile phone attached to it) constantly hums when kept plugged into A.C. outlet and the moment mobile phone is plugged into charger, humming goes.
Why so ?
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Related: http://electronics.stackexchange.com/q/51206/2028 – JYelton Feb 04 '14 at 08:20
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Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization.
The hum goes away when you plug your phone in because you've changed the load on the transformer. The phenomena is variable; some wall warts hum only when something is pulling power from them.
The hum likely means your charger is constantly using a couple watts, just to make noise. The cure is to get a different charger. Some are better built than others. Apple and Nokia make/made chargers that 1) don't hum 2) aren't watt vampires.
Wayfaring Stranger
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thank you for the answer.Well, the charger is that of Nokia Lumia. – user2888999 Feb 03 '14 at 16:59
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@Wayfaring Stranger. Are you sure that inside of this charger there is a transformer? – GR Tech Feb 03 '14 at 18:42
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1@GRTech I've never met one that didn't, but it's possible to build a low volt DC supply wo one. Cell phones don't seem exotic enough to need such a fancy charger. – Wayfaring Stranger Feb 03 '14 at 19:22
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I have an apple charger right now that's buzzing for a good while after it is plugged out. What's going on there? – DavidC Jun 01 '17 at 21:25