LED dash light replacements? - PeachParts Mercedes-Benz Forum

raymr · #1 01-08-2006, 05:36 PM

I read about folks trying this a while back. It seems it would be good to update the tired yellow lighting with nice bright white lights. But as far as I know, they can't be dimmed. Any updated experiences with these?

Strife · #2 01-08-2006, 08:03 PM

Quote:

Originally Posted by raymr

I read about folks trying this a while back. It seems it would be good to update the tired yellow lighting with nice bright white lights. But as far as I know, they can't be dimmed. Any updated experiences with these?

Only the very brightest LED's would really equal the brightness of the bulbs. Generally, they are very directional, although you can buy them with different light cones. I have, however, experimentally replaced the red,yellow, and green lights along the bottom of my console with LED's (super-bright and very concentrated types). For this application, having a narrow cone of light is actually a good thing. This helps, because I can't see the turn signals with the top down on my car (A common complaint, can't hear 'em either) and I don't want to look like the other idiots making a left turn around the world.

You're right, LED's can't be "dimmed" in the conventional sense, but they could be modulated with a duty cycle ranging from 5% to 100% at a fairly high frequency, giving the illusion of dimming. The regulator, voltage/duty cycle converter and switch could be built into the base of a replacement bulb. But nobody's done this...yet...hehe...you read it here first.

raymr · #3 01-09-2006, 07:58 PM

So the rheostat would have to be replaced with some kind of duty cycle gizmo? Sounds like something I would screw up for sure without expert guidance. Oh well, I liked the idea anyway ..

sbourg · #4 01-09-2006, 08:30 PM

Quote:

Originally Posted by Strife

You're right, LED's can't be "dimmed" in the conventional sense, but they could be modulated with a duty cycle ranging from 5% to 100% at a fairly high frequency, giving the illusion of dimming.

This is not correct. LED luminance is proportional to the dc drive current. You can adjust smoothly from 'off' to full brightness via a dc current source. Brightness is NOT proportional to forward voltage, however.

Steve

skiier3_9 · #5 01-10-2006, 01:36 PM

chuni959 replaced the lights in the CC with blue LEDs and they are adjustable with the rehostat. He put three LEDs in series (then three sets of these) to give ~3.5V drop across each LED (at 12V). To do the same thing with white (or any other color) you would just need to put enough LEDs in series (or add resistors) to ensure that the voltage drop across each LED at 12V is near the maximum (recommended) voltage for the LED.
here are night/day pics:

skiier3_9 · #6 01-10-2006, 01:37 PM

wiring:

another day shot of the center console:

Strife · #7 01-10-2006, 07:05 PM

Quote:

Originally Posted by sbourg

This is not correct. LED luminance is proportional to the dc drive current. You can adjust smoothly from 'off' to full brightness via a dc current source. Brightness is NOT proportional to forward voltage, however.

Steve

Well, you can dim them, but they don't respond nearly as proportionally to voltage as do incandescents (not counting spectrum shift, etc). So, if you made a circuit with a pot input (maybe the existing rheostat) and a proportional current out, it might work. But I tried essentially this on some LED's and I got somewhat non-linear, non-matching results from LED's I had bought from the same lot. I think that pulse width modulation would yield better results. BTW, this is how at least some OEM LED tail/brake lights work - how they are "dimmer" for illumination than they are for braking. Look at one at night, (while you aren't the driver, of course) and avert your eyes quickly. You will see several taillights, becuse they are being pulsed at a rate faster than your eye can usually see. Another reason that they do this is that LED's usually have a much higher pulse current rating than a continuous rating.

sbourg · #8 01-11-2006, 09:59 AM

Quote:

Originally Posted by Strife

Well, you can dim them, but they don't respond nearly as proportionally to voltage as do incandescents (not counting spectrum shift, etc). So, if you made a circuit with a pot input (maybe the existing rheostat) and a proportional current out, it might work. But I tried essentially this on some LED's and I got somewhat non-linear, non-matching results from LED's I had bought from the same lot.

This is the mistake often made with LEDs - i.e. an attempt to control brightness with a variable voltage source. Forward voltage across an LED is fairly linear with brightness, once above the 'knee' of the diode. However, a very tiny voltage change will cause a huge change in current, so control this way is not easy nor is it ever done.

Current control makes good sense, since the luminance is essentially an electron-photon exchange process. The more electrons per second (i.e. more current), the more photons per second (i.e. more brightness). Controlling current with a linear regulator is a lossy process, though, and this is what you do with a rheostat in series. This also has problems because you cannot shut off the current, and the one pre-existing in the dash was not designed for controlling the lower current requirement of the LEDs, nor will this circuit limit the maximum current - a series resistor must be added or the LEDs will burn out rather quickly.

Higher efficiency is possible by using a switching regulator to adjust average brightness, but this is only significant for very high brightness LED assemblies, such as those you mentioned. For dash LEDs, a simple linear current regulator using a control pot instead of the rheostat would be a good choice.

Steve