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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
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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.