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  4. 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 Ω ) to ensure a current of 20 mA through the ON diode for the configuration. Both diodes have reverse breakdown voltage of 3 V and average turn on voltage of 2 V.

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Q. 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 $20\, mA$ through the ON diode for the configuration. Both diodes have reverse breakdown voltage of $3\, V$ and average turn on voltage of $2\,V$.Physics Question Image

Semiconductor Electronics: Materials Devices and Simple Circuits

Solution:

$i=20 \,mA =\frac{E-V_{ LED }}{R}=\frac{8-2}{R}$
$R=\frac{6 V }{20 \,mA }=300\, \Omega$
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Also reverse voltage across red diode is $2\, V$ which is fine for LED with reverse breakdown voltage of $3 \,V$.