Resistors R1 = 6 kΩ and R2 = 4 kΩ are connected in series across a 10 V DC voltage source (zero internal resistance), with the output terminals across R2. Which is the correct Thevenin equivalent (Vth and Rth) seen from the output terminals?

Answer: B. Vth = 4 V, Rth = 2.4 kΩ

Vth is the open-circuit output voltage: 10 V × 4/(6 + 4) = 4 V. Rth is the resistance seen from the output with the voltage source shorted; R1 and R2 then appear in parallel, giving 6 × 4/(6 + 4) = 2.4 kΩ. With this equivalent, the current into a load RL is simply 4 V/(2.4 kΩ + RL). In circuits containing current sources, those are opened when finding Rth.

• A. Vth = 6 V is the voltage across R1, not at the output terminals.
• B. Correct. Vth = 4 V and Rth = 6 kΩ ∥ 4 kΩ = 2.4 kΩ.
• C. Rth = 10 kΩ adds R1 and R2 in series. With the voltage source shorted, they are in parallel as seen from the output.