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Old 12-14-2012, 08:16 PM
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dc_wright dc_wright is offline
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Mein Auto: '96 328iC, '04 325Ci
Quote:
Originally Posted by petriej View Post
Surprised no one brought this up, but a dimmer switch does not work the way you would think on LEDs.

LEDs are like switches, either ON or OFF. To properly dim LEDs you need to modulate the voltage very quickly (called Pulse Width Modulation). .

Cool idea, though.
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.
If you use a linear control the power NOT delivered to the LED has to be dissipated by the control device. When the LED is fully dimmed the controller dissipates all the power. When you use PWM the only dissipation in the controller is due to the losses in the internal switching circuits. You are actually turning on the LED at full brightness but for a short time and off for a time at a rate fast enough that your eye can't discern the flicker. As you increase the ratio of on to off the LED appears to be brighter and when you decrease the ratio of on to off it appears to be dimmer.

Quote:
Originally Posted by johnf View Post
What you write may apply to the Z3 and Compact. A E36/2/4 dimmer unit, however, does use Pulse Width Modulation, but with a linear, ca. 20-100% dimming curve meant for incandescent lamps. To provide uniform and sufficient dimming, a PWM dimmer for LEDs needs a non-linear (exponential or square-law) dimming curve that ranges from nearly 0 to 100%. I couldn't find a suitable automotive LED dimmer when I looked a few years ago, and ended up designing one to dim a pair of 10 mA LEDs in series.

You face the same issue when you change the interior lights to LED. I square the linear, PWM, ramped lighting signal from the ZKE (and change, say, 10% on to 1% on) which seems to do the trick.
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.
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Last edited by dc_wright; 12-14-2012 at 08:28 PM.
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