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

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