When the built-in incandescent light source of my venerable Olympus microscope failed after many years of use, I decided to design a reliable modern replacement. A 1-W white LED (SEOUL X42182, 350 mA max, Vf = 3.25 V) was the obvious choice to provide high brightness and full-spectrum light without the heat of incandescent or xenon arc lamps. The microscope lamp brightness needs to be adjustable, however, to accommodate the different objective lenses, which offer magnifications from 40× to 1000×.
This simple circuit allows full-range dimming by driving the LED with a stable current source while generating little heat (see the figure). Shunt voltage regulator Q2 sets a stable 2.5-V reference that is divided by R1 and R2 to give a maximum voltage of 0.66 V at the top of R2. Different values of R1 and R2 may be used as long as the sum of their values is greater than 20 kΩ, to keep Q2 in regulation.
As the nominal end-to-end resistance of potentiometers may have wide tolerances, measure the value of R2 that you are using and then calculate R1 to provide the 0.66-V maximum voltage at the non-inverting input of U1.