The diode used in the circuit shown in the figure has a constant voltage drop of 0.5V at all currents and a maximum power rating 100mW. What should be the value of the resistor R connected in series with the diode for obtaining maximum current


A) 1.5 ohm
B) 5 ohm
C) 6.67 ohm
D) 200 ohm

Given:    
- Diode's Voltage Drop, Vd = 0.5V
- Maximum Power Rating, P = 100mW = 0.1 Watts 
- Battery EMF, E = 1.5V
- Resistance = R

1) Finding Current in the Circuit:

    So it's given E = 1.5V and Vd = 0.5V
    Formula: P = I*Vd           =>   I = (P/Vd)
                   
    I = (0.1/0.5) 
      = 0.2A

2) Finding Potential Drop Across R

    V = E – Vd
       = 1.5 - 0.5
       = 1V

3) Finding Value of R

    From above, V = 1V and I = 0.2 A

    V = IR
    R = V / I 
       = 1 / 0.2 
       = 5 ohms

Hence the answer is B. :)