"Steve Lusardi" wrote in
:

None of the responders have addressed the concern I raised, life span.
Larry is correct, in order to dim the light output of LEDs, you limit
the duty cycle, not the current. Yes Bruce, resisters limit the LED
current, but to what level? The greater the current, the brighter the
output, but the cost of this is durability. The brighter the burn, the
shorter the life. What is the correct device current? Normally it is
whatever the LED draws .2V more than on voltage, which differs
depending on the LED color. Anything more than that affects device
life. It is that voltage you should regulate to for maximum life. It
is not to the manufacturer's advantage to tell you that. They are
perfectly happy to sell you replacements. This rule is also valid for
Halogen lamps. Voltage regulators are prudent, but not required, just
understand the trade off in longevity.
Steve


The LED is first a DIODE. It's a very non-linear device that conducts
in one direction. You MUST, repeat MUST use a current limiting resistor
which is very easily calculated by the simple series circuit it creates
as a minimal circuit.

Now, these lighting LEDs don't need any resistor. They aren't just
diodes. It is just SO easy to add solid state devices to the same chip
the LED is made of what they do is add a constant current regulator to
the chip. To keep the regulator from making the chip hot, they use a
switching regulator, not an old analog transistor acting like a
resistor.

To find out if your LED is this kind of device, light it up on a
variable voltage power supply. Turn the power supply up until the LED
lights, much lower than the rated applied voltage. Bring the voltage up
while watching the light. At some point, the self-regulated LED will
get just so bright and then no brighter. If you move it back and forth
above the point where it stops getting brighter, a switcher regulator
will start pulsing the light way faster than you can see on and off.
Moving it rapidly sideways you can see it strobing on and off. If it
keeps getting brighter with voltage, it's a simple LED with series
resistor. If you advance the voltage from zero to 0.2V and it suddenly
gets very bright, indeed, and the current jumps up really fast with
voltage applied, it's a raw LED you must provide an external resistor to
protect.

There are all kinds of LEDs produced now, not just plain LED diodes.
The clear ones you can use a loupe and see the regulator circuitry
inside of next to the diode that lights up. Some of the better
regulator ones will run on any AC or DC voltage from 3V to 50V
continuously with life spans of several hundred thousand hours. The
ONLY thing that kills any LED is HEAT. Like any IC, heat causes
migration of the various doping in the silicon, destroying the chip.
Kept at a reasonable temperature, much higher than you expect, it'll run
nearly forever.

It all runs on magic smoke. Any solid state device will work fine
unless the magic smoke escapes. Once the magic smoke escapes, there's
no more magic to keep it performing miracles like they do.