Engineering44B.Ellison

Introduction to Biasing

The goal in this lab was to create a circuit that would allow for the functioning of two LEDs with different Voltage and Current ratings. To accomplish this we put the two LEDs in parallel and a resister in series with each LED. Shown bellow is the set up to the circuit.

The requirements for the Red LED was 5V and 22.75mA while the Green LED was rated for 2V and 20mA

Given this information we can use V=iR to find the resistance of the LEDs

R_ledred = 219.8 ohms R_ledgreed =  100 ohms

I_R1 = 22.75mA

I_R2 = 20mA 

Since they are in series with LEDs they will share the same current

V_R1 = 4V

V_R2 = 7V

Since the two series of resisters and LEDs are in parallel the voltage must be 9V. Since we know the voltage we want to obtain across our LEDs we can simply Subtract 9V-2V and 9V-5V

R_1 = 175.8 ohms    R_2 = 350 ohms

P_R1 = 0.091W     P_R2 = 0.14W

Assuming that we are only using cheap commercially offered resistors we will be using R_1 to be 220 Ohms and R_2 to be 470 Ohms.

Circuit set up

Data 

The above is a list of currents and voltages across the LEDs and also shows what happens to the current and voltage when one of the two LEDs is removed.

Additional Questions:

If the 9V battery had and useful life 0.2 A-hr then the circuit would operate for 0.2A-hr/28.04mA = 7.13Hours

(14.78-22.75)/22.75*100 = 35.0 percent error across the RED LED

 and 26.3 percent error across the Green resistor

Most of the error can be attributed to the Resisters since we used resisters that were cheaply available.

Bonus:

If either LED were to be removed then the Voltage and current across the remaining LED would be higher then its rated for, which will cause the LED to burn out.