Thread: LED afterglow question

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Okay, I'm a noob at this stuff and my self-proclaimed ADD prevents me from completely understanding the ins & outs of LED lighting (i.e., Resistor values, forward voltage, forward current, blah, blah, blah). Many of you in the forum are WAYYYY smarter at this stuff than I can ever claim to be - and I appreciate and respect that!

Anyhow, after reading several posts here about others who created their own spotlights, I decided to purchase some 5mm Ultra Bright LED's off of ebay and put together 6 spotlights for my yard haunt. Some of them have 3 LED's, others have 4 LED's. All of them use 1 resistor, which came with the LED's (330 Ohm). They all work perfectly.

When I was testing them, I used a simple 9V battery. I decided that for my yard, that would be impractical, so I wired them together with a regulated wall adapter I had lying around. I noticed that after I plugged them in, then unplugged them, the LED's continued to glow. While I'm not really worried about it, as I figure that all it means is that they will burn out quicker than I expect. I bought plenty of the LED's so this isn't really a problem, except that I would much rather spend my time on other props than continuously changing LED's!

All that said, I am curious as to the cause of this? Is my resistor value too low? If it helps, the values that are listed on ebay are:

Forward voltage: 1.8 - 2.0 or 3.2-3.6 (depending on color - blue LED's hold higher values)
Forward current: 18 or 20
Lumination: 10000 - 14000 mcd

All the circuitry inside the AC adapter / wall wart holds a little charge. So when you pull it out, the LEDs quickly drain that down to nothing, but not instantly. I'm sure if you were to put a switch on the cord between the LEDs and the AC adapter, they would turn off instantly. However, you are better off pulling the AC adapter from the wall anyway, as they eat power even if the thing they're supplying power to is switched off. Better yet, use a power strip or an outlet switch (usually 2-prong, just a little "extender" of the plug with a switch on top of it). I don't think you did anything wrong!

Originally Posted by Xane

I don't think you did anything wrong!

I certainly like hearing that!

Thanks for the input Xane. What you said makes sense to me. I appreciate your help!

http://led.linear1.org/led.wiz
Input all,your values you have along with source voltage and it will tell you what resistor to use so your leds last