# How to find the equivalent Thevenin's resistance?

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1. Jan 20, 2017

### doktorwho

1. The problem statement, all variables and given/known data

Find the Thevenin's equivalent with respect to points A and B

2. Relevant equations
3. The attempt at a solution

The resistance i found to be
$R_t=(R_2+R_1)||R_3+R_4$ and that is the correct expression but for the voltage source the book gives
$\frac{R_3}{R_1+R_2+R_3}*(V_1+V_2)-V_4$
I understand that the first part is a voltage divider but the second i dont get? Dont we have a voltage drop along $R_4$ as well? How could we ignore it and just subtract $V_4$? Could you explain?

2. Jan 20, 2017

### Staff: Mentor

The Thevenin voltage is given by the open-circuit voltage at the output. If that is the case, what must be the current through $R_4$ in that circumstance?

3. Jan 20, 2017

### doktorwho

The same current that flows into A is the current through B but still cant see why its not included... That would be the case if the current is 0 but it cant be zero right?

4. Jan 20, 2017

### Staff: Mentor

When AB is an open circuit, the current is zero. The Thevenin voltage is obtained when the output is an open circuit.

Note that the Thevenin resistance does include R4. So its effects are included when a load is connected and current flows.