Why aren't many / most LED light bulbs dimmable?

[Don Klipstein]

I'm thinking that illuminating a largish living room with one of my favorite green or blue LEDs at .1-.25 mA will be low enough on blasting my night vision for me to still have a majority of it, probably around/over

75% of it.

Then again, I can illuminate all rooms and the basement of a McMansion that brightly with 50 milliwatts for the whole house. :) (Not that I get a good ROI for doing so, in comparison to getting commercially available LED nightlights using ~100 times as much power and producing a few times more light.)

[clare]

No, an LED will light very nicely with no resistor if the voltage is close enough to the forward drop of the led. Excede that voltage by very much and the current goes up real fast, very quickly exceding the maximum power dissipation of the LED, and popping it.

If I set my lab supply to 15MA and connect a standard red LED I get

1.9 volts drop across it. I can stiffen that power supply so it can supply 20 amps of power, and as long as I do not increase the voltage above 1.9 volts, the current does not excede 15ma Below 1.45 volts, the LED does not light at all, and it varies in brightness quite linearly from 1.5 to 1.9 volts

With an 82 ohm resistor in series,changing the input voltage from 1.5 to 14, the voltage drop across the LED goes from 1.5 to 1.9 volts, with the current reaching 15ma. (roughly.02 watts) while the poor resistor is shedding 1.875 watts.

Without the resistor,25 ma of current produces a 1.91 volt drop,30ma produces 1.98 volt drop,35ma is 2 volts, and going to 50 ma goes to

2.06

So doubling the current from 25ma to 50ma only raises the voltage drop by .08 volts, which is roughly 4% on this 20ma rated device (I believe that is the spec)

[JIMMIE]

The yellow lights didnt look good at all to me, of course this may have something to do with the color of my counter top too. I have been thinking of experimenting with combining different color LEDs in the same cluster. Replacing one of the LEDs with a yellow one might not be a bad idea. LEDs arent ideal for task lighting so why use them for that. I used to have some light fixtures that had fluorescent, 60 watt incandescent and night light type bulbs all in one fixture. I can see where combining fluorescent and LED in an under-cabinet light would have some advantages.