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Old 12-19-2012, 06:03 AM
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John Firestone
Location: Bremerhaven, Germany
 
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Quote:
Originally Posted by dc_wright View Post
Sorry but incorrect! The light output from an LED is directly proportional the current through it. You can control the brightness of the LED using PWM also but it's done to reduce the power dissipation of the dimming controller not because of the LED.
In practice, you can treat LEDs that way from, say, 25% to 400% (pulsed) of their static, maximum current. The further you stray from that range, however, the more an LED diverges from a linear ideal, because of its semiconductor non-linearity and its inexact manufacture. This is especially true of white LEDs. They must be matched or "binned" during manufacture (into perhaps 4 x 3 = 12 different bins) to have any hope of producing a group of them with a consistent hue and intensity. Even then, the hues and intensities within a group will diverge if you decrease their drive current much below a minimum value, their so-called "grouping current". This might be 5 mA for a 20 mA white LED which is not much dimming! Dimming an LED with PWM not only saves power, it also maintains its hue as you go lower, and dimming a string of LEDs with PWM keeps their intensities consistent.

As a practical example, I have a couple, series-wired, white LEDs in my dome light that illuminate the center console. I pulse width modulate them: with a 100% duty cycle, to light a map or a list of directions, and with a 1% duty cycle to provide ambient lighting. If I am unlucky, one of the LEDs might not even stay lit, if instead, I were to reduce their current from 10 mA to 100 uA to say nothing of how their hues might change.

Quote:
John, you can do the same with an audio taper potentiometer for LED control. The issue when you replace an incandescent with an LED is the huge difference in currents required for illumination. If you put an LED in a circuit design for incandescents your control range for the LED is at the very end of the control so you essentially have none. You can put a resistor in series with the control to rescale the current and get good control range.
The issue I have had with LEDs vs. incandescent lamps is how much more linearly a LED's brightness changes with voltage and current compared to an incandescent filament, as in somewhat linearly vs. very non-linearly. (I think that is what you mean.) I like your idea of using an audio taper potentiometer in series with a resistor, which will also protect the pot from shorts. If we set aside everything I wrote earlier, I suspect, however, it might only be a simple solution for fairly low LED currents. I get the impression that rather few use audio taper potentiometers as power rheostats. I can't find any over 2W (and those are pricey), and the pot datasheets I have browsed give no guidance for the maximum current the pot can carry approaching zero ohms.

Last edited by johnf; 12-19-2012 at 10:09 AM.
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