Finding Thevenin eq of this circuit

[The_Lobster]

There's a problem in my book where it asks me to find the Thevenin and Norton equivalent circuit of the attached circuit, as viewed from AB. I believe I know the answer, but just had to check to be sure this is not a trick question?

Isn't the Thevenin eq. circuit as seen from AB exactly the one drawn, without R1 and R2? And the Norton one just a source transformation from the one we see?

Thanks in advance!

 

[UR_Correct]

A thevenin equivalent circuit is always a voltage source in series with a resistance. To get the thevenin resistance, you set all independent sources to zero (short voltage source, open current source), and find the equivalent resistance in reference to the terminals. So in reference to the terminals AB, we have R_th = R_s // R_1 // R2 = (R_s*R_1 + R_s*R_2 + R_1*R_2)/(R_s + R_1+R_2). To get thevenin voltage, we can easily find by finding the voltage across whatever is in parallel with the terminals, which can be either R_1 or R_2. Doing a simple node equation at A, calling A V_th, and calling B ground, (V_th - 15)/R_s + V_th/R_1 + V_th/R_2 = 0. Solve for V_th.

EDIT: For Norton equivalent, you simply do a source transformation. Norton equivalent is a current source in parallel with a resister. R_n = R_th. I_n = V_th / R_th