Discussion
What is the typical forward drop of a red light emitting diode?
Answer: D
A red emitter drops about 1.8 V, and the drop rises through the colours to well over three volts for blue and white, because the photon energy and hence the band gap increase. 0.7 V and 0.3 V are silicon and germanium rectifiers. Knowing that the drop tracks the colour is what lets a series resistor be sized without a data sheet, and it explains why a blue emitter will not light from a single cell.
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