Introduction to Biasing

We are looking at two different LEDs in parallel connected to the same battery source. Each LED has a different rating. In order to put the correct voltage through each LED, we use a technique called biasing to establish these predetermined voltages by wiring resistors in series with each LED.

Our setup consists of a 9 V power supply and LEDs rated at 5 V, drawing 22.75 mA and 2 V, drawing 20 mA. We calculated that we need 175 Ω resistor for the 5 V LED and 350 Ω for the 2 V, however, we only had access to 150 Ω and 330 Ω for the experiment.

The setup

We measured voltage and current for three configurations for the LEDs as well as the current in the power supply.

Configuration 1 - Both LEDs in the circuit

I_LED1 = 12.86 mA

V_LED1 = 6.14 V

I_LED2 = 18.85 mA

V_LED2 = 2.18 V

I_Supply = 31.05 mA

Configuration 2 - Remove LED 2 from the circuit

I_LED1 = 13.06 mA

V_LED1 = 6.19 V

I_Supply = 13.05 mA

Configuration 3 - Remove LED 1 from the circuit

I_LED2 = 18.99 mA

V_LED2 = 2.21 V

I_Supply = 18.96 mA

With a 9 V battery operating at 0.2 A-hr, we calculated that the circuit could operate for 6.4 hours before the battery voltage gets too low.

Our percent error between the achieved current in the LED and the desired value of the two LEDs was 25.63%. The reason for this error was that we did not use the required resistors in our theoretical calculations.

For the efficiency of the circuit with both LEDs, we found the power supplied by the battery (P_in) and the power dissipated by the two LEDs (P_out) to be 279.45 mW and 120.1 mW, respectively. The efficiency is the ratio between the power dissipated and the power supplied which we found to be 43 %. 

If we changed the design to a 6 V battery while keeping the LED parameters, the efficiency would increase because we will use smaller biasing resistors while keeping the current the same, thus reducing the power dissipated in the load. Using a 5 V supply would give the best efficiency because it will provide the minimum voltage needed to run the circuit while demanding the smallest biasing resistors.