Befuddling LED – SOLVED
The mystery of the really long running has been solved, but i’m glad I asked about the observation and thank you for the kind responses…perhaps others would be stuck for an answer too were they to use a similar setup.
Mostly, its been a case of being so used to sub 1.5V circuits.
The efficiency of the LED and heavily restricted input to it are the major factors. With no resistor on the input, the expected and usual time of lighting would be a few seconds if that. Instead, the run can be up to 20 minutes with a 2200uF cap. The wall adapter will have a cap inside and the effect of dielectric absorption allows the kicking in of a topup when left for many minutes. The switch could be left on for minutes before being switched off, yet would still kick when switched back on many minutes later on.
Unfortunately the galactic alignments of the day weren’t a factor 🙂
Original description below –
I was wiring up a simple LED ‘ON’ indicator for a project and when it was switched off it didn’t go out…carrying on running and running, for 25 minutes before I thought to make a video of it.
The oddest part, is that on switching the power switch back on, the LED brightens again.
All the while, the circuit has been unplugged from the wall.
The LED stays on for about an hour, with 2x 25 minute gapped switches on and off of the power switch.
The circuit comprises the following:
7.5V 300mA wall adapter – model YL-35-0475300D
One input wire goes to the input of a 10A bridge rectifier, the other wire goes via an on/off switch to the other input connection.
The outputs of the rectifier run to a 2200uF 16V capacitor, for smoothing the input.
The LED has a 56K resistor on the Positive leg (measured 55.3K), the other leg connects directly to the smoothed Negative input.
I understand that a high enough resistor and large enough capacitor will significantly affect the draw of a very efficient LED…but, an hour long lit LED with just a 2200uF cap ? and then what about the way that switching the on/off switch will brighten the LED again for many minutes each time ?
All dielectric absorption from an unknown capacitor in the power supply ?
I might expect a 1F supercap to produce such a long run, but not a 2200uF.