Question

The threshold voltage of a silicon diode is 0.7 V. It is operated at this point by connecting the diode in series with a battery of \( V \) volt and a resistor of 1000 \( \Omega \). Find the value of \( V \) when the current drawn is 15 mA.

Hint:

In a series circuit with a diode, the supply voltage is the sum of the voltage across the resistor and the diode.

Answer 1

Step 1: The circuit. The total voltage across the series combination of the diode and the resistor is equal to the supply voltage \( V \). The current through the diode is 15 mA.

Step 2: Applying Kirchhoff's Voltage Law. The voltage across the resistor is given by Ohm's Law: \[ V_R = I \times R = 15 \times 10^{-3} \, \text{A} \times 1000 \, \Omega = 15 \, \text{V} \] The voltage across the diode is the threshold voltage, \( V_D = 0.7 \, \text{V} \). 

Step 3: Total supply voltage. The total voltage \( V \) is the sum of the voltage across the resistor and the diode: \[ V = V_R + V_D = 15 \, \text{V} + 0.7 \, \text{V} = 15.7 \, \text{V} \] Thus, the value of \( V \) is 15.7 V.