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I'am trying to solve this problem using superposition but can't seem to get the right answer.

The problem is to look for the voltage in R3 in the following circuit:

  • The V1 graph is green
  • The R3 voltage graph is red.
  • (The red graph, R3's voltage, goes up to about 11V and stays there until the .5ms and then goes down to at about -71V)

I used LTSpice to find the voltage in R3 and it came out to be this:

Could someone tell me what technique to use and how to start please.

Edit 1: It was hard to see the R3 voltage graph colored blue, so changed it to red.

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    \$\begingroup\$ Superposition does not hold true since that's not a linear circuit. Could be used to Theveninize everything but the diode, but in this case is not convenient since you would loose output quantity Vout. \$\endgroup\$ – carloc Sep 5 '16 at 6:28
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Depending whether the diode is in the conducting or non-conducting state, your circuit could have two different equivalent circuits:

schematic

simulate this circuit – Schematic created using CircuitLab

A is the equivalent circuit when the diode is conductiong. B is the equivalent circuit when the diode is not conducting.

You should be able to see that solving Vout is trivial in case A.

In case B you can solve using mesh analysis, or using modified node analysis. Superposition could be applied, but is not needed.

Then you must examine the solution for case B and find any situations where the diode would not be biased correctly to be non-conducting, and replace the solution at those points in time with the solution from case A.

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  • \$\begingroup\$ Could you explain me again what you meant in the last paragraph? In circuit A the voltage in R3 is 10V, right? \$\endgroup\$ – laco13 Sep 5 '16 at 16:14
  • \$\begingroup\$ Yes. Now solve circuit B. Anywhere where R30 has more than 10 V across it, that's a case where the preconditions for circuit B to model the nonlinear circuit are not valid, so you need to use circuit A as the model instead. \$\endgroup\$ – The Photon Sep 5 '16 at 16:21
  • \$\begingroup\$ What do you mean to use circuit A as a model for the circuit B? Sorry if I'm not understanding it, but I'm just starting to use diodes. \$\endgroup\$ – laco13 Sep 5 '16 at 16:55
  • \$\begingroup\$ Circuit A and circuit B are two models for your original nonlinear circuit. \$\endgroup\$ – The Photon Sep 5 '16 at 16:56
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Let's call the node the anode of the diode is connected to "node A" and the node the cathode is connected to "node C" and the voltages of those nodes \$v_A\$ and \$v_C\$ respectively.

Now look at the circuit without the diode and find out what V1 needs to be for

  • case1: \$v_A > v_C\$
  • case2: \$v_A < v_C\$

Now do circuit analysis for each of the two cases where in

  • case1: diode is replaced by a short between node A and node C
  • case2: diode is removed (no connection between node A and node C)

Case1 is the part where the red graph is flat.
Case2 is the part where the red graph is curved.

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Solve for threshold Vt of V1, when I(D1)=V(D1)=0 then loops are isolated.

When V1 > Vt is when diode conducts, then Vout=V2

and for V1 < Vt diode is not conducting and solve for Vout.

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  • \$\begingroup\$ What do you mean to solve for the threshold? When I tried to solve for Vout when the diode is not conducting, I tried using mesh analysis but couldn't get the answer. \$\endgroup\$ – laco13 Sep 5 '16 at 5:00
  • \$\begingroup\$ ... cut D1 out of the loop and can you solve the Vout for Vin=-200? Then compute V1 when Vout =V2 using same ratio for currents in each loop \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 5 '16 at 5:42

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