LED doubt.. sometimes i see that when i switch off a device (Which has some LED indicator to show its switched on).. it takes about 3 or 4 seconds before the LED goes off..
how can there be light when there circuit is broken?..

I think.. its because it takes time for the diffusion current to die out.. and for the depletion region to be formed again.. it would be awesome if thats true.. but anyone has some opinion? :P

Question

LED doubt.. sometimes i see that when i switch off a device (Which has some LED indicator to show its switched on).. it takes about 3 or 4 seconds before the LED goes off.. 
how can there be light when there circuit is broken?.. 

I think.. its because it takes time for the diffusion current to die out.. and for the depletion region to be formed again.. it would be awesome if thats true.. but anyone has some opinion? :P

mrnood · Answer

Please take this reply as a 'laymans' explanation - it is not entrirely within my expertise.

An LED can operate at voltages down to just a few volts (3-4 V I believe)

Most power supplies (PSUs)  for electronic equipment are supplyin more than that (5 - 24V is typical for many applications)

IF your PSU is beased on ACmains input (110 - 220V AC) then it will have a transformer or voltage regulator. These devices normally have a capacitance across the output.

When th einput voltage is remove the output voltage is still present on th ecapacitor - and may take some time to decay below the 3-4 volt level. During this decay time th LED will glow.

Any electrical engineers with better detail???

anonymous · Answer

ah.. yea.. that explains it.. 

i even saw it in the most simple device.. the phone charger.. 
even that has a transformer and then a rectifier with capacitor filter circuit.. so it must be the capacitor itself!