what is the load line and operating point used for in practical?
Load lines are useful in understanding and calculating the operating point of a circuit which would otherwise require complex calculations. Diodes have a non-linear relationship between I and V and we often use an approximation such as "2 V drop across the LED" when making calculations. We can do better if we have a load line chart.



Figure 1. A simple LED circuit.



Figure 2. Loadlines for various resistors on 5 V supply. Image source: LEDnique.

Here we can quickly calculate the operating point of an LED.


If we used a 180 &Omega; resistor on a RED LED we would expect to get 18 mA through the LED.
If we wanted 10 mA through a blue LED we can see that the 270 &Omega; resistor would do the job.


No calculations involved (once the graph has been created). To create the graph draw the lines from the supply voltage point on the V-axis and the \$ \frac V R \$ point on the I-axis.