Thevenin resistance and Thevenin voltage confusion

[theycallmevirgo]

Homework Statement

Use Thevenin Equivalent Circuit to find current through R_L

Relevant Equations

E_Th=open circuit voltage

R_Th=R_t looking into the circucit from the load

 

[gneill]

Your numerical expression for $R_{Th}$ is not correct. $1000 \Omega \cdot 690 \Omega$ is not $690 k\Omega$. Ohms times Ohms is Ohms².

 

[theycallmevirgo]

Ok, but that should only throw me off by a power of 10. The simulation results aren't even close.

 

[gneill]

Well, your Thevenin resistance should be greater than 1000 Ω(since R₄ is already 1000 Ω), right? So check your math.

 

[gneill]

@theycallmevirgo has your confusion been resolved?

 

[DaveE]

A slightly different definition (with the same answers); the "black box" approach:
1) Measure (or calculate) the open circuit voltage on the output terminals (i.e. R_L = ∞ in your schematic). This is the Thevenin voltage.
2) Measure (or calculate) the short circuit current from the output terminals (i.e. R_L = 0 in your schematic). Divide the open circuit voltage by the short circuit current to get the Thevenin resistance.

Of course this only works for circuits that are linear and passive.