Very elementary question, I think... I need to measure the Vf for some LED displays I received that have no datasheet so that I can size the resistor appropriately using the equation (Vsupply - Vf )/I. So I apply 12v to the Common Anode and run a resistor to ground for one of the segments. I measure the voltage across the LED (and before the resistor) and get 7v. So here's my question: Is the value I plug into the equation Vf = 7? or is it equal to the drop of 5V? Thanks!