ee201scoring

EE 201 (Professor Russell’s’) Exam # 2 - Scoring Rationale

Question #1:

Part a.)
Points were deducted for -

Invalid source transformations.

Solving the circuit not using superposition.
Points were added for -

Setting-up correct superposition models:

Independent current sources removed, then the branch from which they were removed left open.

Independent voltage sources removed, then the branches from which they were removed shorted – thereby shorting parallel resistors.

Accurate circuit analysis applied.

Circuit analysis completed.

Results summed appropriately (because superposition employed).

Recognition of purposeless resistors.

Part b.)
Points were deducted for -

Incorrectly using the P_max equation. It’s only valid when R_th = R_L, therefore; this must first be verified.
Points were added for -

Recognition of the need to find R_th (with respect to R_load).

Usage of the P_max equation, when first finding R_th(although it was unnecessary at that point).

Actually answering the question.

NOTE: The instructions clearly asked IF maximum power was being delivered ?

This simply meant that you were to see if R_th = R_L.

Because the circuit was rather simple (for finding R_th) that meant that independent current sources could be removed, then the branch from which they were removed left open, and independent voltage sources also removed, then those branches from which they were removed then shorted – thereby shorting parallel resistors. Then finding R_th with respect to R_Lwas only a matter of finding the R_eq(with respect to R_L): which meant merely summing three series resistors.

So there was no need, at all, to find v_th for the unnecessary maximum power equation. I know you're getting tired of hearing this, but the very first instructions given with the examination – were "Read and follow instructions".

Question #2:

Extra credit (only given if ³ 1 incorrect answer occurred):

Additional accurate illustrative circuit drawings for relevant time periods.

Explicitly stating that v_capacitor(t) and/or i_inductor(t) cannot change instantaneously.

Question #3:
Deductions were taken for -

Not using the correct equation: That is, using the ‘Natural Response’ equation, rather than the ‘Step Response’ equation.

Setting up (or attempting to) the ‘Step Response’ equation for voltages directly from (guesses at) the inductors’ initial and final voltage values.
Points were added for -

Using the correct t :

Using L_eq.

Using R_th with respect to L_eq.

Finding i_L(0).

Finding correct i_L(0).

Finding i_L(¥ ).

Finding correct i_L(¥ ).

Finding i_L(t), then finding the inductor voltage by: v_o(t) = L di_L(t)/dt.

Using the correct L (80 m H) for v_o(t) = L di_L(t)/dt.

Question #4:

Part a.)
Deductions were taken for -

Not even attempting to develop the circuit model. In particular: using the exact equation directly from crib sheets - ‘Integrator’ model, without even attempting to substitute given (or derived) values into the (identically copied from the book - it was pretty obvious) equation.

The incorrect (equivalent source) resistor value (frequently 24 kW was used): 30 kW was the correct value.

Order of magnitude errors.

Sign errors (v_o(t) should have been less than zero).

Writing the equation for i_c only, then integrating it from 0 to t (showing why the voltage across the capacitor is derived by integrating the capacitor current [ + initial value]). It’s an interesting relationship, but did not explain v_o(t).

For no apparent reason ‘tacking’ on an exponential, with a meaningless t . Intuitively it might have seemed logical, but if you developed the circuit model, you would have found why the circuit doesn’t work that way.

Setting up the equation for v_o(t), but not integrating it. I know time was tight, but you were only integrating a constant from 0 to t (that = constant * t).

Showing no work whatsoever other than the books equation, the final result from part b.) and then scribbling "Used calculator".

Part b.)
Deductions were taken for -

Equating v_o(t) to the wrong saturation limit (I used the v_o(t) from your part a.) result to dictate which limit that would be compared for grading - in other words, you didn’t get penalized for the same mistake twice.)
The majority of points were added for -

Knowing how to find the time at which the op-amp would saturate.