Could some electronics guru please clarify this a bit ? I know that 3mm white light LEDs have a rated current of 20 - 25mA, and a voltage of 3.0 - 3.5 Volts. What are the corresponding values for 5mm LEDs ? Are they the same ?
the same
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About the same, maybe more it depends on the LED, You'll probably need to check the manufacturers website.
usually one only puts enough current through the led to make it bright enough. under-driving them makes them last longer.
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The voltage drop across the LED depends on the semi-conductor junction being used to generate the light and essentially varies with the colour of the light being generated. This is quite independent of the size of the package.
The rated current is usually limited by the capacity of the package to dissipate the heat generated in the semi-conductor junction - as you put more current through the junction it gets hotter, and at some point it will get too hot to work right.
Bigger packages can dissipate more heat for the same temperature rise, which can mean that you can put more current through them, but there are lots of other factors involved.
The main advantage of the bigger package is that it's easier to get the light-emitting junction properly aligned with the lens formed by the top of the package, which can mean that they have a narrower emission pattern, but this isn't always true either.
In practice you always have to get hold of the data sheet for the device you are thinking of buying, and read it carefully.
-- Bill Sloman, Nijmegen
I expect most of the heat comes out through the leads, so the size of the package is not so important (TWSS).
As Jason says, "Ratings" of 20-25mA should be reduced depending on ambient temperature (that doesn't mean the temperature outside a box when you try to pack a bunch together), or the life can be significantly shortened. Red LEDs could probably be run at a higher current than white LEDs because they drop less voltage, but the specs (if available) and a cynical and careful interpretation should be the final arbiter.
Best regards, Spehro Pefhany
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