When calculating R_th by deactivating independent sources, which actions are performed?

• A. Independent voltage sources are replaced by a short, independent current sources are opened; dependent sources remain.
• B. All sources are left on; resistors are recalculated.
• C. Independent sources are doubled.
• D. Dependent sources are turned off.

Answer: (B)

When finding the Thevenin resistance, you “kill” only the independent sources, not everything. Replace independent voltage sources with a short circuit and replace independent current sources with an open circuit, while leaving dependent sources in place because they depend on the circuit’s variables and can influence the resistance you measure. If the circuit contains dependent sources, you may need to apply a test source to determine the resulting voltage and current and thus compute R_th. The other options misstate the process: leaving all sources on doesn’t deactivate them; doubling an independent source isn’t the method used; and turning off dependent sources would ignore their effect on the network’s resistance.