Question

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

• A. $1.5\Omega$
• B. $5\Omega$
• C. $6.67\Omega$
• D. $200\Omega$

Answer 1

The correct option is B $5\Omega$

Voltage drop across diode $V_D=0.5\text{V}$
Maximum power rating of diode $P=100\text{mW}=100\times {10}^{-3}\text{W}$
Source voltage $V_s=1.5\text{V}$
As power across diode is $P=\frac{V_D^2}{R_D}$
The resistance of diode
$R_D=\frac{{0.5}^2}{100\times {10}^{-3}}=2.5\Omega$
Current through diode ;
$I_D=\frac{V_D}{R_D}=\frac{0.5}{2.5}=0.2A$
Total resistance of the circuit $=\frac{V_S}{I_D}=\frac{1.5}{0.2}=7.5\Omega$
Total resistance $=R+R_D$
$7.5=R+2.5$
$R=5\Omega$