When a red light-emitting diode (LED) is forward biased, it gives out red light. In this state, the voltage across its terminals will always be 1.4 V. Bob wants to drive this red LED with a 5 V power supply so that the driving current is approximately 13 mA. What is the value of the current-limiting resistor he would need? If a resistor of such value is not available at electronics stores. What is the closest commercial value he can use instead? Also find the power rating of the resistor and LED he would need to specify when buying them at the stores.
Solution
Since the potential +5 V is higher than 0V, the red LED tends to be forward biased.
KVL;
0 V+ 1.4 V + (13 mA)(R) = +5 V
R = (5 V - 1.4 V) / 13 mA = 277 Ω
The resistor of this value is not available at stores. Bob would have to use the closest one which is 270 Ω.
KVL;
0 V+ 1.4 V + (I)(270 Ω) = +5 V
I = (5 V - 1.4 V) / 270 Ω = 13.33 mA
The actual current flowing through the 270 Ω resistor and the red LED is 13.33 mA.
P270Ω = (270 Ω)(13.33 mA)2 = 48 mW ==> A 1/4 W resistor would be enough.
Pred LED = (1.4 V)(13.33 mA) = 19 mW ==> A 1/4 W LED would be enough.
Bob needs a 1/4 W 270 Ω resistor and a 1/4 W red LED. Ans
