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I've noticed occasionally that some LEDs stay on, even when the circuit they are connected to is broken. For instance: the "power" indicator on my computer stays on for a few seconds after I unplug it, and I have a another plug with a built-in LED that stays on at least 10 seconds after it is removed.

Why is this? I thought that if a circuit was not complete, current could not flow through it, and so an LED wouldn't work? And why is this specific to LEDs - I've never seen any other kind of light or motor stay on without power.

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  • \$\begingroup\$ Capacitance across the LEDs coupled with only small amounts of required current to light an LED. \$\endgroup\$ – M D Jan 26 '17 at 14:17
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    \$\begingroup\$ You're assuming that the LED is switched off as soon as the power is switched off. That is not the case. Most LEDs are connected to the internal supply lines (5 V, 12 V etc). These lines carry a voltage even after you switch the device off because there are smoothing capacitors in the supply which store some energy. Also: the circuit is not "broken", the (mains) power is disconnected. \$\endgroup\$ – Bimpelrekkie Jan 26 '17 at 14:18
  • \$\begingroup\$ Simple enough question that I'm surprised it hasn't been asked before. \$\endgroup\$ – pipe Jan 26 '17 at 14:39
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No electronics runs on mains power, it is all running from a DC power supply of some sort. A power indicator LED needs to be simple and reliable, the easiest way to do this is an LED and resistor connected across the output of that DC power supply.

In order to avoid the output voltage of a power supply from changing significantly if the load changes and to reduce noise in it's output all DC power supplies will have some fairly large capacitors on their output that act as a short term power reserve.

When you turn something off that DC power supply will turn off and stop supplying power[1] but so will most of the things using power. e.g any displays / backlights / LEDs under control of the electronics will normally be turned off and processors will normally get put into a halt or sleep mode rather than just waiting for them to crash due to a low voltage. So you end up with the power supplies output capacitors being fully charged but there being hardly anything other than the power LED using up that power.

Since the power LED doesn't use up much power the end result is a power LED that remains on for a reasonable amount of time as the internal voltage slowly drops.

So the LED isn't remaining on after power has been removed, it is a fairly true and fast acting (a lot faster than the response time of the human eye) indicator of the internal voltage. What you are seeing is the internal power taking a while fade away after external power is removed.

You won't notice this sort of thing nearly as much with something like incandescent bulb power indicators because they take a lot more power and so the power indicator itself will drain the remaining power very quickly.

[1] Depending on exactly where the off switch is in the system the power supply may carry on running for a short while. The supply will also have input capacitors, if the off switch disconnects the electricity into the power supply rather than signalling the supply to switch off its output then it will keep running until those input capacitors are drained. You can sometimes guess at whats going on based on the brightness of the power LED, does it stay full brightness, fade out or stay full brightness for a while and then fade out.

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You are quite right that a LED remains lit while current flows through it. Where you see equipment remain alive after a power-down switch has been flipped "off", stored energy in its power supply is depleted until it is consumed. The stored energy likely comes from a charged capacitor. In this case, the power switch disconnects the power supply from its source (at its input, rather than its output). Since all the circuits (including the LED) are always connected to the power supply output, any stored energy in the supply is available to that LED after the switch disconnects the supply from its source.
However, it is becoming more common that power supplies remain alive while plugged into the wall. The power switch operates differently than the scenario described above...it tells the supply to stop accepting power, rather than crudely disconnecting the power source. A computer or display can use a tiny inexpensive switch to perform this function. In this scenario, it is possible that shut-down might be delayed for a very short time while house-keeping functions are run. It is also possible that any delay is due to the same energy-storage in the power supply capacitor(s).
Incandescent lamps and motors do remain running after power is denied (for a very short while). A lamp has stored energy in the thermal mass of its filament, which gets quickly radiated away. A motor has stored energy in its rotating mass, which spins down quickly when power is denied. Energy is conserved...at power-up, both lamp and motor require a surge of power equal to the "free energy" dissipated at power-down.

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