Introduction to Biasing

This laboratory assignment introduced us to the concept of biasing and also introduced us the structure and operation of a breadboard.  We were instructed to make the necessary change required for the given circuit to operate properly since the 9-volt battery exceeded the voltage rating of two LED lights (LED 1 and LED 2) connected in parallel with this power supply. Basically what we did was add some resistors (R1 and R2) in series with these LED lights in order to correct this problem.

Our first step was to model the circuit from a theoretical standpoint in order to compute the current, voltage, and power of each element. We were given that LED 1 and LED 2 had voltage and current ratings of 22.75mA/5V and 20mA/2V, respectively. From this information we used Ohm's Law to compute the resistance of each LED light, which were found to be 220 ohms and 100 ohms, respectively. Since resistor R1 was connected in series with the first LED, then by KCL, resistor R1 must have a value of 22.75 mA. Likewise, R2 must also have a current of 20 mA, since R2 was connected in series with LED 2. Next we used KVL to compute the voltage across resistors R1 and R2, which were found to be 4V and 7V, respectively. So these current and voltage values allowed us to compute the theoretical resistance values of R1 and R2, which were determined to have values of 178 ohms and 350 ohms, respectively. From here, we easily computed the power that should have been absorbed by R1 and R2 as 91.5 mW and 140 mW. It was also interesting that we were shown that resistor value are only made for certain discrete values. But since only five resistor types were available in the classroom, we determined that the closest values of R1 and R2 were 150 ohms and 470 ohms, respectively.

After when we done making the required computations, our next step was to take out the materials needed to perform the experiment. As with every other laboratory experiment, we had to use a multimeter to measure the resistance of each resistor separately in order to determine how accurate their resistance were in comparison with their rated values, which were determined by their color code. The nominal values in comparison with their measured values were 220/212 ohms and 470/464 ohms,  respectively. Since their nominal values were indeed close to their measured values, we deemed suitable for our experiment. Their power values were both deemed to be 1/8 W. We then took out our voltage supply and set it to 9 V, in order to simulate the 9 V battery. We were then given instructions regarding the structure and usage of breadboards and jumper wires. It was then time to perform the experiment.

In performing the experiment, we were instructed to set up three different configurations. Configuration 1 had us use both LED's, whereas Configurations 2 and 3 had use LED's 2 and 1 separately. We were then instructed to record the current through and the voltage across these two LED's. In Configuration 1, the measured current and voltage values for LED's 1 and 2 were found to be 12.8 mA/6.17 V and 14.7 mA/2.12 V, respectively. In Configuration 2, LED 1 had measured current and voltage values of 12.8 mA and 6.22 V, respectively.  Finally, in Configuration 3, LED 2 had measured current and voltage values of 13.2 mA and 2.12 V, respectively. Configurations 1, 2, and 3 had supply currents of 27.6 mA, 31.6 mA, and 56.2 mA, respectively.

The experiment was now over and we were told to disassemble the circuit and to answer the analysis questions. Given that the capacity of the 9V battery is 0.2 A-hr, we found that the circuit could operate for a maximum of 4.8 hours. We found that the percent error between the achieved LED current and the desired value with both LED's in the circuit was very high. The percent error for LED 1 was -43.7%, and the percent error for LED 2 was -26.5%. We determined that the percent errors were caused by the added resistors. As a result of these high percent errors, we ended up with a low circuit efficiency of 30.7%. Through theoretical computations, we noted that the circuit efficiency would rise if a 6 V battery were to be used. Although we recorded high percent errors and a low circuit efficiency, this lab helped us understand the concept of biasing and helped us practice to use the breadboard and how to cut jumper wires and to measure the resistance of these wires. Hopefully our next experiment will produce better results.