Introduction to Biasing

We have two LEDs in parallel, each with unique voltage ceilings. Since we have only one power supply (also in parallel), we have to find out how to differ the voltage through each of the LEDs. This is called biasing. We can limit voltage across the LEDs by adding resistors to the circuit. These resistors "soak up" some of the parallel voltage so that not all of it is going through our sensitive LEDs.

Given a 9V source and two parallel LEDs with voltage limits of 5V and 2V, respectively, we could calculate that a 220 ohm resistor is needed in series with the 5V LED, and a 100 ohm resistor with the 2V LED. So we set up our circuit as such, and connected a voltmeter in parallel + ammeter in series with the LEDs to take some measurements. We also set up configurations with the individual LEDs.

The model

Here are our measurements:

Config 1
LED1 Current = 14.5mA
LED1 Voltage = 6.86V
LED2 Current = 21.0mA
LED2 Voltage = 1.65V
Supply Current = 35.4mA


Config 2

LED1 Current = 14.6mA
LED1 Voltage = 6.81V
Supply Current = 14.4mA


Config 3
LED2 Current = 20.7mA
LED2 Voltage = 1.65V
Supply Current = 20.7mA

Based on these results and a hypothetical 9V, 0.2 amp-hour battery, we calculated that the LEDs could be powered for 4.68 hours.

We then calculated the actual vs. theoretical LED current. Our actual current was lower due to the fact that we were using a non-ideal voltmeter in parallel.

Finally, we calculated the efficiency of our setup, which was quite low due to the resistors stealing a lot of our power.

Power out: 0.13412 W

Power in:    0.3186 W

Power lost: 0.18448 W

Efficiency: 42.1%

We were asked about what would happen to the efficiency if a 6V power source had been used instead of a 9V. I reasoned that it would increase, because less power is now going to the resistors (which aren't doing any work). 5V would actually correspond to the most efficient setup, since we're now using a value that allows us to eliminate the most resistance possible (one of our LEDs has a 5V max) and run a resistor only alongside the 2V max LED.