Side Question -- Why do button lights flicker on when power goes off?

[outsidequery]

I have a LED lamp with some lighting buttons that glow when pressed (that control the actual light). I noticed that if I have this plugged in to a power bar and turn off the power to the plugin, the buttons glow for a second (supposing they were previously dark.) (This is the case for Furman PST-8D and for an old Tripp-Lite surge protector.)

What might this mean? Is it likely something the lamp does simply when the power goes off, or does something else happen at the powerbar outlets when I switch off the power bars? Not that I can get any definitive answer here.

The LED looks vaguely like this
https://www.amazon.com/TaoTronics-Charging-Lighting-Brightness-Licensing/dp/B00APAQSP6/ref=sr_1_21?dchild=1&keywords=led+lamp&qid=1610419107&sr=8-21
but it is not the same model; perhaps it is an older version. The light-buttons are on the base.

[helius]

This is an effect of sloppy power sequencing. The power to light the LEDs comes from capacitors that hold a charge for a while after power is removed. The actual LEDs themselves would be turned on and off using a transistor as a switch, with the base (or gate) controlled by a MCU. When power is removed, the MCU should go into shutdown mode and turn off its outputs. If it doesn't do this in the correct way, the transistors can be left in the ON state instead of the OFF state as desired.

[Brumby]

There doesn't have to be an MCU involved.  Ordinary logic gates or even linear circuits can result in the same thing... capacitors hold power which can illuminate the lights and the signals controlling the lights drop into "indefinite" states.  Depending on the specific nature of these states, the lights can be switched on for a moment.

Normally, such behaviour is completely ignored as it does not represent a valid condition of such circuitry (You know ... it's just been turned off!)


There is the situation where such light flickering (of lights or whatever) IS an issue.  In those cases, special design considerations are made to ensure the circuit behaves as required in all circumstances.

In a number of situations, power sequencing - both powering up and powering down - is vitally important.  Some lab supplies have such functionality.