Question

In the figure shown are $2$ LED's that can be used as a polarity detector. Apply a positive source voltage and a green light results. Negative supplies result in a red light. Packages of such combination are commercially available. Find resistor $R$ (in $\Omega$ ) to ensure a current of $20mA$ through the ON diode for the configuration. Both diodes have reverse breakdown voltage of $3V$ and average turn on voltage of $2V$.

Answer 1

Answer: 300

$i=20mA=\frac{E-V_{LED}}{R}=\frac{8-2}{R}$
$R=\frac{6V}{20mA}=300\Omega$

Also reverse voltage across red diode is $2V$ which is fine for LED with reverse breakdown voltage of $3V$.