Originally Posted By: Brian Barnhart
You can dim LEDs by adding resistance, but how much depends on a number of variables. I did several LED light designs for aircraft lighting over the last 10 years.
As stated, single LED die have a voltage drop of about 2 (red and amber) to 3+ (white, blue, and green) volts. If the "LED" you wish to dim has a single die (many do), you can add resistance in series with the LED and dim it a little. If it draws roughly 20 mA (as many do), you will probably need at least 1000 ohms of additional resistance to reduce the brightness to the level you are looking for. It may even take several thousand ohms.
You can usually dim an LED linearly (by using a dropping resistor) to about 5 to 10% of it's rated brightness. If you try to use a resistor to dim more than that, brightness starts to become inconsistent. A dim level of 5 to 10% sounds like a lot, but the eye's response is non-linear so that level of dimming doesn't appear to be 1/10 to 1/20 of the rated brightness to the human eye. In fact a 50% reduction in LED brightness is only slightly perceptible to the eye.
One of the lights I designed needed the ability to dim to 1% of full brightness, and another required the ability to dim to .1% of full brightness. Achieving those levels required PWM dimming. And the customer for the .1% dimming light (1000:1) was left wanting an even greater range of dimming.
Oh, and one more thing. Most white LEDs are actually blue LEDs with a phosphorous coating over the die (that looks yellow), and emit very little light in the red/amber spectrum. So the use of colored filters with a white LED to achieve the color/brightness you wish may not work very well.
Good post...yeah, lots of companies want 10,000:1 PWM dimming these days. That's a big PITA when they also want to run the PWM dimming at a relatively high frequency, there is so little on time that it is nearly impossible to control the base current level accurately.
One of the big problems with analog dimming is that the perceived color of the LED changes if you go much below 10%...the color doesn't appear to change if you PWM dim at a fixed base current level. So, some level of analog dimming is sometimes used to tweak in the base current during a calibration cycle and then PWM dimming off that level is used in the application. The PWMing is usually at 200Hz or above to be well past the frequency at which the human eye will perceive flickering.
