P 5.9-3 In preparation for lab, your lab partner determined the Thévenin equivalent of the circuit connected to RL in Figure P 5.9-3. She says that the Thévenin resistance is and the open-circuit voltage is  In lab, you built the circuit using R = 110 Ω and RL = 40 Ω and measured that i = 54.5 mA. Is this measurement consistent with the prelab calculations? Justify your answers.

 

c05f141.eps

Figure P 5.9-3

 

So it is proposed that the Thevenin equivalent circuit is

With

 

First check these, so deactivate the power sources and we have

So we just have three resistors in parallel, their effective resistance is

 

So

As suggested!

 

Now to find the open circuit voltage.  Recognize we have a set of voltage sources in series with resistors.  Each can be replaced by a current source in parallel with the resistance and the original circuit can be source transformed into

Now add parallel current sources and we already know the effective resistance from above is Rt.  So the circuit becomes

Now do a final source transformation and we can have our Thevenin circuit hooked up to the load resistance.

Clearly we can see the Thevenin voltage does not agree with the prelab determination.  This is three times the voltage.  Just a final check using the values for R and RL provided we can determine the current.

 

 

So this is not consistent with Pre-Lab calculations!

 

 

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This page last updated on March 9, 2019