How do I calculate voltage at an LED in parallel?

I have this circuit: If D2 was a resistance, I could just combine it with R2 to get some composite resistance that I could put in series with R1. That way, I could calculate the resistance of the entire circuit and, thus, the total current.

Now, this is not the case here but I know a few things here:

• whatever voltage drops at D2, also drops at R2
• the current through R1 is the same as through R2+D2

But I do not know the current from the beginning, so I cannot calculate the voltage drop at R1. And the diode is non-ohmic, so I cannot find out anything about it.

In this way, I do not see how I could apply Kirchhoff's laws or any other methods because I seem to be missing information... where do I start here?

• Although the diode is not ohmic, it still has a characteristics describing its voltage-current relation. Apr 28 '19 at 22:12

One good place is to start is LTL-307EE datasheet. From the datasheet take look at current profile for the device. Below is the forward current and voltage characteristics.

Also take look at absolute maximum rating for the device. Now you have to size R1 resistor such that no more than in this case 30mA of current flows through the LED. R1 is called a current limiting resistor. I would size the R1 to make ensure 25mA of current, with will provide Relative Luminous Intensity of around 3.0. Like wise R2 can be used to manage current LED current flow.