Question

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 of $100$ milliwatts.What should be the value of the resistor $R,$ connected in series with the diode for obtaining maximum current?

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

Answer 1

Answer: (A)

Voltage drop across diode, $V_D=0.5V$

Maximum power rating of diode, $P=100mW=100\times {10}^{-3}W$

Source voltage, $V_s=1.5V$

The resistance of diode,

$R_D=\frac{V_P^2}{P}=\frac{{0.5}^2}{100\times {10}^{-3}}=2.5\Omega$

And

Current in diode,

$I_D=\frac{V_D}{R_D}=\frac{0.5}{2.5}=0.2\Omega$

Therefore, Total resistance in circuit,

$R=\frac{V_s}{I_D}=\frac{1.5}{0.2}=7.5\Omega$

And,

Value of series resistor $=$ Total resistance of the circuit $-$ Resistance of diode

$=7.5-2.5=5\Omega$