so i was able to get a grabbag of LED's from my local electronics junkyard. thankfully the superbrights were seperated from the regular ones. anyway a couple of hopefully easy questions. one of the bags had about 50 to 100 led's in it that didn't light up. i figured that it was a bag of UV led's. so i plug in my webcam, try out a few of them and nothing. is there a way to determine what the led is without knowing anything about it. about 1/4 of the red superbrights appeared to have a very bright initial color then dim down a bit. i am thinking that some are 5v, but never really seen a 5v superbright red before. so basically same question as above, is there a way to determine what voltage these suckers really need? thanks
Measure the voltage across one connected in series with a 470-1000 ohm resistor to a 9V battery. Or if you've a 5V supply handy, measure with a 180-470 ohm resistor in series. I suspect your test method is breaking them.
Have a look across the "filament" if you've really over volted them it'll probably looked burned if not they might be ok but they might pack in after only a few hours.
The ones that didn't light up might be higher voltage? If you have a half-watt pot or rheostat you could try using that and a 12v supply to test them, starting at full resistance. Also if the light you get when you turn them on is very bright and off-colour (eg a green LED would appear yellowy), you're over powering them.
well the ones that dimmed down right away only were hooked up for an instant for checking purposes. none at all were marked so hopefully i didn't trash too many if any of them. if i was overvolting a superbright at 3v, what could their possible voltage be? I'm used to most led's being 3/5/12v thanks
LED brightness (and death) is a function of current, so if you connected them straight across e.g. a battery with no resistor there's a chance they could have expired, even at a low voltage, as the current through them would be largely unregulated (you only have the LED's internal resistance and the ESR of the battery to control it).
Any LED claiming to be 5V or over has a built-in resistor to suit that voltage; the basic LED is around 2V for red, orange, yellow, most green, about 3.5V for blue, white and true green, and around 1.3V for IR LEDs. You can buy 5V, 6V, 9V, 12V, 24V, etc, LEDs to run straight off a power supply such as a vehicle's but they will have the built-in resistor and they're more expensive to make - I wouldn't expect to find them in a cheap mixed bag. Testing straight across a 5V or higher supply with no resistor will kill an ordinary LED almost instantly. The only exception is using lithium coin cells like cheap key-ring torches, where the high internal resistance of the coin cell is enough to prevent excessive current; even so only one 3V cell is used with 2V red LEDs, two cells (6V) with blue and white 3.5V LEDs.
thanks for the help, i have no idea why i was thinking most were 3v, probably because i use mostly blue and green superbrights. anyway i volted it down to 2 to test them and was able to sort them out. now i just have to find my ir's. i have a ton of those suckers to play with and don't know where they are at. anyway thanks again for the help.
When you are testing LEDs it's best to use a power supply with adjustable current limiting. Just set the limit to about 15-20mA, and slowly turn up the voltage. Super bright red LEDs tend to be below 2V, BTW Aren't we all?
Or just use any convenient resistor in the 470R-1k range and any DC supply from 5V to about 12V, 5V for preference so you don't blow the LED if you have it the wrong way round. If you don't have a resistor use your body on a 12V supply, hold one LED wire, touch the other wire to a terminal and a finger to the other terminal (I've just lit a blue on a Nokia charger ). Using a set voltage with no resistor is asking for trouble.
