with a proper graph, on can see what size resistor to

use, with a non-linear device.

What defines a proper graph?

I ask because look at the current vs forward voltage graph for

this red LED.On the graph it shows 20mA and 2V matching pretty well.

But above in the tables of the datasheet it states the forward voltage can be as high as 2.5V at 20mA.

So to me that graph may not show my particular LED well at all. It may have a forward voltage of 1.8 or 2.3V at 20mA. So how can that be factored in creating a load line?

If one draws a straight line from the supply voltage at 0 current

Going to call that point A.

through the desired current point on the selected diode line,

Going to call that point B

to the current line, at 0 volts,

Point C

you can use that current

divided into the supply voltage to tell you the desired resistance.

This is called a load line( it still uses ohm's law ).

So since point C is about 15mA it would be 5/.015 or 333.3?

But what is that value telling me?

Say we have this circuit

Using the LED from the datasheet V

_{f} = 2V

And I want to limit the current to 12.5mA

So formula is V-V

_{f} / .0125 = R

R=240

So I would need a 240Ω resistor in the circuit above.

According to LarryD my total circuit resistance is V

_{tot}/I

_{tot}=R

_{tot} or 5/.0125 = 400Ω

So 400 minus the value of the resistor of 240 leaves an equivalent resistance of 160Ω for the LED.

None of those numbers is 333.3

Is the load line telling me the R

_{tot} =333?

How does that show what current limiting resistor should be should be?

Or is it showing what the current limiting resistor should be so my R

_{tot} is still 400Ω?

One can draw a new line for each resistor size and see

where it crosses the LED's line.

How? Where did resistor value have influence where I drew the load line?

Or is this all not making sense to me due to the poor quality of my drawing?