Using Thevenin's theorem on Wheatstone Bridge

[V8meathead]

What's the correct way to use Thevenin's theorem to find the current going through Rs?  I keep getting 5mA and it should be 3.33mA.  I can't figure out where my problem is.

View attachment 39579

 

[indulis]

Does it have to be with Thevenin's? Try it with a "delta-Y" transformation..........

 

[V8meathead]

V8meathead, I know what you're doing wrong:

When using Thevenin to determine the current through Rs, you need to find an equivalent thevenin source for the rest of the circuit (the red block), this consists of Vth & Rth. For that you need to figure out the open circuit voltage & short circuit current at the terminals of Rs. 5mA is the SC current; it's not your final answer! Find the thevenin parameters as above & replace the red block with the thevenin equivalent source, then calculate the current through Rs, it is 3.3mA.

Ref: Nilsson & Riedel, Electric Circuits 6th edition 2001

Ah ha!  Thanks, I appreciate it.