Open Circuit Voltage Calculation

Question

I am trying to find out why this system:

should have an Open Circuit Voltage of:

\$U = U0 + I0*R2 + 0*R3\$

\$ = U0 + I0R2\$

I understand why the R3 component doesn't change anything but I fail to understand why R1 isn't contributing anything.

To be clear with all of you. This is an old exam question that I'm using to study for the test but can't find the answer in either the textbook or in the lecture material.

Thanks in advance

Answer 1

Start with KVL,

-U0 - I0R2 + 0*R3 + U = 0

U = U0 + I0R2

Answer 2

Look at the current source and where the current must flow. It goes through \$R_1\$ and \$R_2\$, right? But when looking at the terminals \$A\$ and \$B\$, they only see the voltage of \$U_0\$ plus the drop across \$R_2\$. The drop across \$R_1\$ exists, but it doesn't have an impact on the terminals. It just impacts the potential difference across the current source. And terminals \$A\$ and \$B\$ don't care about that part of it.

Just walk through by summing the voltages around the loop from \$B\$ to \$A\$. No \$R_1\$ in there, right?

Answer 3

• Short out the part with no current flowing ( Open circuit ) like R3

• Short out the R values in series with a current source like R1 because it has no effect on the voltage drop on R2 in that single loop.

• now what do you get?