Most LED circuits require a current limiting resistor.
R + LED on 5 V supply.
The loadline calculator makes resistor selection very simple.
Loadlines for various resistors on 5 V supply.
Example 1: Chose a resistor to give 20 mA on a yellow LED and 5 V supply.
Loadline for 20 mA yellow on 5 V supply.
Solution: Choose 20 mA on the If axis. Slide across to the yellow LED curve. Find the nearest loadline: in this case it’s 120 Ω.
Example 2: Chose a resistor to give 10 mA on an infrared LED and 5 V supply.
Loadline 10 mA IR on 5 V supply.
Solution: Choose 10 mA on the If axis. Slide across to the IR LED curve. Find the nearest loadline: in this case it’s 390 Ω.
Example 3: Chose a resistor to give 30 mA on a white LED and 5 V supply.
Loadline 30 mA on a white LED.
Solution: Choose 30 mA on the If axis. Slide across to the white LED curve. Find the nearest loadline: in this case it’s between 50 and 68 Ω. 56 is the nearest standard value.
Ohm’s law states that the current through a resistive conductor (a resistor) between two points is directly proportional to the voltage across the two points.
\(I={\frac {V}{R}}\) where I is current (amps or A), V is voltage (volts or V) and R is resistance (ohms or \(\Omega\)).
Example 1 – resistor calculation
Figure 1. Series connection of a resistor and LED. The voltmeters show the voltage drop across each element of the circuit.
I have a 5 V supply and I want to run a red LED at 8 mA.
We can see from the LED’s IV curve that at 8 mA the red LED will have a forward voltage of 1.8 V. That means there’s 5 – 1.8 = 3.2 V across the resistor. Using Ohm’s law we can calculate the resistance.
390 \(\Omega\) is the closest standard value and that will be fine.
Don’t forget to check the power rating of the resistor! See Power calculations.
Notes
Note that the order of the components in this example doesn’t matter. The resistor can go between the +5 V supply and the LED or between the LED and ground. The same current will flow through the circuit and the same voltage drop will occur on the LED.
