In order to prepare for an exam in electro technics I have a specific task:
I have the following measurements of an unknown circuit:
\$(1)\quad U_{2(1)} = 4.28V \quad @ I_{(1)}=0.67A \$
\$(2)\quad U_{2(2)} = 1.38V \quad @ I_{(1)}=4.91A \$
Now I have to calculate the internal resistance.
The formula therefor is either \$ R_i = \dfrac{\Delta U}{\Delta I}\$ or \$R_i = \dfrac{U_s}{I_c}\$
where \$U_s\$ is the source potential and \$I_c\$ the short-circuit current.
I don't want to use the delta formula because I need the other two values in a later task anyway.
How do I calculate the source potential just having the both values?
Best Answer
That's unfortunate because that's the formula to use.
\${{4.28\text{V}-1.38\text{V}}\over{4.91\text{A}-0.67\text{A}}} \approx 684 \text{m}\Omega\$
\$4.28\text{V}+0.67\text{A}\cdot684\text{m}\Omega\approx4.74 V\$
\$1.38\text{V}+4.91\text{A}\cdot684\text{m}\Omega\approx4.74 V\$