FCC Exam Question: 8-10A4

What is the correct value of RS in Fig. 8A7, if the voltage across the LED is 1.9 Volts with 5 Volts applied and If max equals 40 milliamps?

A. 4,700 ohms.
B. 155 ohms.
C. 77 ohms.
D. 10,000 ohms.2009 FCC Commercial Element 8 Question Pool (approved 25 June 2009) PAGE 15
Correct Answer: C

Explanation: In this series circuit, the resistor (RS) limits the current through the LED to its maximum safe operating level. First, determine the voltage drop across RS. Since the supply voltage is 5 Volts and the LED drops 1.9 Volts, the remaining voltage across RS is 5 V - 1.9 V = 3.1 Volts. Next, apply Ohm's Law (R = V/I) to find the resistance of RS. The current (I) through RS is the same as the LED's maximum current, which is 40 milliamps (0.040 Amps). RS = 3.1 Volts / 0.040 Amps = 77.5 Ohms. The closest answer among the choices is 77 ohms.

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Includes Elements 1, 3, 6, 7R, 8, and 9.