Question

Problem 4 You are designing a circuit to drive LED1, using the following circuit. The datasheet for the LED specifies that VF = 2 V. Treat LEDs as diodes with the constant voltage model. (a) Find the expected current through the LED I(LED) when VF = 2 V (b) After testing, it is determined that some LEDs within a batch do not light up correctly in the circuit for part a. It is suspected that this is because there is some variation in among the LEDs in the batch. Find the maximum value of VF such that the circuit above stops working I(LED) drops to zero so you can characterize this problem and report it to the manufacturer.

Answer 1

Answer:

a) I_LED= 1/6 A  b) Vf= 2.5V

Explanation:

Consider circuit in the attachment.

a) We will simplify current source in paraller with resistor to a voltage source in series with a resistor(see attachment 2)

Solving the circuit in attachment 2 using mesh analysis

-9+2I1+4(I1-I2)-4+2I1=0

8I1 - 4I2= 13 ............... eq 1

4+4(I2-I1)+ I2 + 2=0

4I1- 5I2 = 6 ............ eq 2

I1= 41/24 ;  I2 = 1/6; I2= I_LED

b) Solving the circuit in attachment 2 again, this time I2=0

8I1 - 4I2= 13

8I1- 4(0)=13

I1= 13/8

Vf= 4(I1- I2) -4

I2=I_LED=0

Vf= 2.5 V