Introduction to Biasing

In this lab we need to find the correct values of resistors in order to bias 2 LEDs so that they will not burn out when we put them in series and in parallel.

LED 1 is rated for 5V and 22.75mA

LED 2 is rated for 2V and 20mA

We model our system with laboratory equipment.

Our 9V battery will be our power supply. LED1 is rated to be at roughly 220 ohms and LED2 at 100 ohms. Using nodal analysis we find the following data:

I_R1 = 22.75 mA

I_R2 = 20 mA

VR1 = 4

VR2 = 7

R1 = 175.8 ohms

R2 = 350 ohms

P_R1 = .09W

P_R2 = .14W

We build our circuit and both LEDs are biased correctly.

Measurements from the circuit.

Configuration 1:

ILED1 = 14.20+_0.005mA

VLED1 = 5.39+_0.005V

ILED2 = 32.0+_0.05mA

VLED2= 2.41+_0.005V

IBATT = 47.0+_0.05mA

Configuration 2:

ILED1 = 10.0+_0.05mA

VLED1 = 5.41+_0.005V

IBATT = 10.0+_0.05mA

LED1 removed:

ILED2 = 37.0+_0.05mA

VLED2 = 2.42+_0.005V

IBATT = 37.0+_0.05mA

Questions:

a. The circuit can operate for .2Ahr/.047= 4.3 hours

b. LED1 had a percent error of ~38%. LED2 had a percent error of ~60%. The LEDs are not rated to be within strict tolerances as their function is to produce light. they are not good resistors.

c. The efficiency = p_out/(p_out+p_lost) = ~36%

d. The efficiency would increase as the voltage would be closer to LED1, 5V. The optimal efficiency would increase until you get to a point where the voltage becomes too much of a difference from LED2. I have not done the calculations to find this voltage.