I am replacing a microscope lamp with a motorcycle LED headlight. The light is 20W, 9-85V DC; the driver built in and a complete black box to me. I power it with a 65W power supply (18.5V/3.5A). For as long as I have tested it (short), this seems to work well.
Now the original housing had a potentiometer to adjust the brightness of the lamp. Naively, I soldered that in serie with the headlight. For as long as it lasted it worked functionally well to reduce the light, but of course, within a minute, smoke, smell, and the potmeter even emitted red light (it had to endure 20W).
If I understand it well now, I am to reduce the output power fed to the light. Similar questions here suggest a FET is the way to go. What type of FET should I select here (and why?). I sketch below how I understand it now:
Is this the idea? It doesn't need to be very precise as it is only a light dimmer.