A commonly encountered question on message boards and in my email inbox deals with the elusive “5V LED,” and goes something like this:
I have a 5V blue LED, can I hook it directly to the 5V supply in my computer?or alternately:
I hooked up my 5V LED directly to my computer’s PSU, why was the magic smoke released?
The fact is, there are simply no LEDs with a forward voltage of 5V. So how does this confusion arise? There are three possibilities. We’ll go over them all.
Type one: confusion about specs
The specs on LEDs often include a reverse voltage spec, and 5V is not an uncommon value to see for this. But this is not an operating voltage, this is the maximum reverse bias voltage you can apply before the LED fails. This sort of confusion accounts for most of the “sightings” of 5V LEDs in the wild.
Type two: ambitious claims by the seller
The fine folks at Radio S*ack offer a 300 mcd blue LED (part number 276-311), for which they claim a forward voltage of 5V, 6V max. Their website also lists the wavelength as 430mm [sic]. This (430 nm) is a short wavelength blue compared to the more common 470nm blue LEDs available now. Most makers of 430nm blue LEDs use GaN on a SiC substrate. These LEDs do actually seem to tolerate being driven hard, for some length of time, and 30 mA drive currents are listed in several manufacturers datasheets for this type of LEDs. But I find that keeping to 20mA drive current or below is the soundest way to ensure the long life of your LED. Hooking an LED directly to any voltage source is a poor choice, regardless of specs.
Type three: LEDs with integrated resistors
They are far more common in 12V ratings (for vehicle applications), but you will occasionally find 5V rated devices. These are usually clearly labeled “LED with integrated resistor.” Or as one German site poetically puts it: “Die bei uns angebotenen 5V-LEDs besitzen einen integrierten 5V-Resistor, der (winzig klein) direkt im Epoxygehäuse der LED vergossen ist.”