Charger Noise when unplugged

[Athosworld]

Firstly im not saying this is a problem, i just want to know what exactly causes it.
When you unplug a charger its still making the transformer pitch sound but it’s instead going high and high in pitch until a buzz is heard and then the sound stops. This lasts 10 seconds. What causes it?

 

[Nigel Goodwin]

The reservoir capacitor discharging.

 

[Sophia Zeng]

I think it might because there is still a lot of electricity in the capacitor of its internal circuit when you are unplug a charger, it will be released through the circuit components after the charger is powered off.

 

[Athosworld]

The reservoir capacitor discharging.

The reservoir capacitor is used for momentary (milliseconds) mains outages, right?

But the capacitor is typically a low rating (my charger is a small mobile one) around 2 to 5 microfarads, how could it last too long?

 

[rjenkinsgb]

The reservoir capacitor is used for momentary (milliseconds) mains outages, right?

The input cap after the AC rectifier has to be more than large enough to maintain the working DC voltage in to the switched mode section, through AC half cycles, at full load.

If the device is not loaded, the power used internally could be eg. a hundredth or a thousandth of the maximum through power, and decreasing as the cap voltage falls, so they can take a while to fully discharge and shut down.

 

[Nigel Goodwin]

The reservoir capacitor is used for momentary (milliseconds) mains outages, right?

No, it's to convert the rough DC to 'smooth' DC - it's size is dependent on the power requirements

But the capacitor is typically a low rating (my charger is a small mobile one) around 2 to 5 microfarads, how could it last too long?

It has to be large enough to do it's job, it's only small because the power requirements of the charger are very small - hence when you unplug it, it slowly discharges, and as the voltage drops the switch-mode controller tries to maintain the output voltage, so the frequency changes as it does. Eventually the voltage is too low, and the switch-mode controller switches off.

 

[Athosworld]

No, it's to convert the rough DC to 'smooth' DC - it's size is dependent on the power requirements

Yes, i know.
I meant that it has the second function of keeping the oscillator running if there’s a momentary outage of mains.

 

[Nigel Goodwin]

Yes, i know.
I meant that it has the second function of keeping the oscillator running if there’s a momentary outage of mains.

Not really, that's just an 'unexpected' bonus of the way it works.