The values for VT and Rint can either be calculated or measured.
To measure, first find the short circuit current from A to B and call this ISC. Next measure the open circuit voltage (VT). The value of Rint is VT/ISC.
We'll calculate and measure the values for VT and Rint for the circuit below.
We can use Kirchhoff's voltage law to give us the current:
5 - 2.8 = I*(100+33+33) 2.2 = I*166. I = 2.2/166 = 0.013
Now VT is VCD, so:
VT = 2.8 + 0.013*33 = 3.23
For Rint we short all voltage sources (assuming no internal resistance) and this gives the following circuit:
calculation of Rint is quite easy:
Rint = 100 + 1/(1/33 + 1/133) = 100 + 26.4 = 126.4
From this we can calculate the short circuit current:
ISC = 3.23 / 126.4 = 0.0255.
The circuit was built and using a multimeter the value of VT(VCD) was 3.3V.
The short circuit current ISC was measured to be 25.6mA.
From this we can calculate Rint:
Rint = 3.3/0.0256 = 128.9
The table below shows how close the actual measurements were to the theoretical calculations.
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
Copyright © 2014 Barry Watson. All rights reserved.