# How to find the equivalent Thevenin's resistance?

## Homework Statement

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

## Homework Equations

3. The Attempt at a Solution [/B]
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?

## Answers and Replies

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gneill
Mentor
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?
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?

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?
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?

gneill
Mentor
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?
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.

doktorwho