Is it ok to mix LED with different Vf connected in series? For example, can I connect white 3W LED (3.5 Vf, 700ma) and amber 3W LED (2.2 Vf, 700ma) in series and power by one 700ma driver?
if the leds are driven via a linear driver where the current going through the leds vary with their forward voltage, the current *may* be off spec.
if the leds are driven via a constant current source, the answer is *generally* yes, assuming that the voltage going through the diodes are sufficient.
buckpuck uses a step-down topology. so replacing an LED of higher Vf with a LED of lower Vf will always work.
most LED drivers use a step-up topology. there, replacing an LED with another one of lower Vf may not work. but replacing an LED with one of higher Vf will always work, within reason.