How do I calculate voltage using the complete diode method?
Im not sure if I understood your question correctly. But, when using the complete diode model, you assume that diode has two different equivalent circuit. One when it is directly polarized:







simulate this circuit &ndash; Schematic created using CircuitLab

and another one when it is reversely polarized







simulate this circuit

In this way, to solve your circuit, you need to suppose a diode initial condition, substitute it with the corresponding circuit and assert that initial condition is valid.

For example. Suppose that diode is reversely polarized (it is clear, from your circuit, that this suppostion is false, but let us see how this work).

Your circuit can now be described by this circuit (below) when Vd &lt; Vt (your Vt is zero, and Vd is the voltage measured across the diode as it was directly polarized):







simulate this circuit

Solving this circuit, you will find that Vd is approximately equal to voltage across R3, i.e., Vd = V3 = 4.7k * 30 / (1k + 1.5k + 4.7k) = 19.5V

Our initial supposition was that Vd &lt; 0 and we found a positive voltage, which indicate that this suppostion was false.

In this way, we need to redo our analisys supposing now that diode is directly polarized and computing again the voltage across Vd and verifying that it is positive.

I'll let this analisys for you.