You wish to use an LED (D1) which provides essentially zero resistance and is rated to accept either a maximum of 75 mW of power or 30 mA of current. In the following two circuits, what range values for resistor R1 will be acceptable in each case if the voltage provided by V1 is 12 V?
R = V / I
V = P / I
The Attempt at a Solution
Rmax = V / I = 12 V / 30 mA = 400 Ω.
V2 = P / I = 75 mW / 30 mA = 2.5 mV
So I need a resistor that will reduce the voltage from 12 V to 2.5 mV, requiring a voltage drop of 11.9975 V:
Rmin = V / I = 11.9975 V / 30 mA = 399.916667 Ω
Rounding to one or two significant figures (as mentioned in the instructions) would remove any sort of "range" from my answer. Furthermore, a 400V resistor would leave no energy for the LED. Is the problem in my answer or the large voltage supply?