There could be some sort of filter capacitor that stores a bit of charge and fades slowly. I have some 60 and 75w equivalent LED lightbulbs that definitely are nowhere near instant off (which suggests maybe not too good fro enlarger use).
Both the phosphorescence and filter cap theories sound reasonable.
Much more likely to be the capacitor. I have never known an LED to continue glowing without any current flowing through it (and I have had many years experience with LEDs).
Just for interest, here is the website of a company producing domestic and architectural LED lighting. The company I work for makes the flexible LED circuits and light guides for them: http://designledproducts.com/
Steve.
So on my electronics bench, I powered up a component super bright white led of 700mcd, 25mA, and aimed a fast photodiode at it.
Just with a mechanical switch, the oscilloscope indicates a fast fall of < 1 millisec of light emission to something less than 5% brightness, then a slow decay of that 5% to zero in about 800 millisec.
These are approximate, i will have to set up a switching transistor to get more accurate results.
But there is definitely an afterglow and filter capacitors are upstream of the switch in this test.
I'm fooling around with a Cree LED PAR 38 replacement to power my Beseler 4x5 condenser head, and it looks just like what you describe. You turn it off and most of the light dies instantly, with a fading afterglow that feels like a 1/2 second. I guess as long as it is consistent there wouldn't be any problem using it.
I actually found circuit diagram for such LED bulbs by googling "LED bulb circuit". The circuit is very VERY simple.
AC main goes into a bridge rectifier. It then goes into a filter capacitor. Then there is a current limiting resister, then into a series-parallel connected chain of LEDs. Seems most of them do not involve PWM.
The part that could cause persistence is the capacitor right after the rectifier.
Just with a mechanical switch, the oscilloscope indicates a fast fall of < 1 millisec of light emission to something less than 5% brightness, then a slow decay of that 5% to zero in about 800 millisec.
These are approximate, i will have to set up a switching transistor to get more accurate results.
But there is definitely an afterglow and filter capacitors are upstream of the switch in this test.
Are you sure the night lights aren't the type that have a battery and are designed to light up when the power goes out
So.... Let's smash that LED lamp and find out! Anything for science and to settle an Internet argument, right?? hehehe....
So.... Let's smash that LED lamp and find out! Anything for science and to settle an Internet argument, right?? hehehe....
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