ENGI 241 Course Project

Computer Simulation
Revised 15December 2002
by Professor Andrew H. Andersen, Jr.

ENGI 241 – Computer Simulation Project

Computer Simulation of Thevenin’s Theorem

We wish to use a computer simulation to develop the Thevenin Equivalent circuit at terminals AB in the schematic shown above. Previously we learned that we cannot directly calculate RTH when we have a dependent source. Instead we short the load terminals and calculate the short circuit current ISC. This should provide us with the insight we need to perform this assignment.

If we use PSpice or MicroCAP to draw the circuit above, we should save the circuit with a name like THEV1.CIR

Neither PSpice nor MicroCAP will run a simulation without a ground. Therefore, we should assign ground to a convenient location. Since we will be calculating VTH, it would be advantageous to ground the terminal labeled B.

Procedure

  1. VTH is the voltage at node A with respect to ground (at Node B). However, we should not leave the node open. To simulate an open, use a resistor that is very large compared to the other resistors in the circuit. Since the largest resistor is 7, let’s connect a resistor between node A and B of at least 10M.
  2. Save the circuit as thev_open.cir
  3. For PSpice, perform a Bias simulation. Return to the schematic editor and read the node voltage for VTH. When finished, open the file named thev_open.out which is a standard ASCII text file.
  4. For MicroCAP, perform DC analysis stepping VS1 from 19 to 20 in steps of 0.1 and create numeric output. Plot V(A) as a function of the input. When finished, open the file named thev_open.dno which is a standard ASCII text file.
  5. If we determine ISC and divide VTH by ISC, we have RTH. To obtain a current between terminals A and B, we need a resistance, but it should be very small compared to all other resistance. Change the value of the resistor between nodes A and B to a very small value like 10-6
  6. Save the circuit as thev_short.cir
  7. For PSpice, perform a Bias simulation. PSpice will default to print the current through the 2V source. Since this is in series with our “short,” you can read the current from the out file. Open the output file thev_short.out, and read ISC.
  8. For MicroCAP, perform DC analysis stepping VS1 from 19 to 20 in steps of 0.1 and create numeric output. Plot I(R) as a function of the input. When finished, open the file named thev short.dno.
  9. Read VTH and ISC. Calculate RTH for the two simulations.
  10. Print the open and short schematics and output files.
  11. Perform the calculation for VTH, ISC, and RTH by hand.
  12. Write a brief report on the project and compare the simulation with your calculations. Submit the report with a lab cover sheet and hard copies to your instructor.

NOTE

When entering grounds in PSpice, make sure the Ground Name is 0 (zero) or you will get a floating nodes error message when you run a simulation.

Dependent Sources in PSpice

NOTES:
When using Dependent Sources with PSpice:
Voltage Controlled Voltage Sources must begin with E.
Current Controlled Current Sources must begin with F.
Voltage Controlled Current Sources must begin with G.
Current Controlled Voltage Sources must begin with H.

Selecting Dependent Sources

From the list of components, select the components using the following:

Component / PSpice / MicroCAP
Voltage Controlled Voltage Sources. / E / VOFV
Current Controlled Current Sources / F / IOFI
Voltage Controlled Current Sources / G / IOFV
Current Controlled Voltage Sources / H / VOFI

The following graphics show actual circuit connections. The dependent symbols at the top are not entered into the schematic. They are presented merely to show the relationship between the symbols used in the text with the symbols and connections required by the software. Also we do not add the arrow labeled ID.

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ENGI 241 – Computer Simulation Project – Entering Dependent Sources in PSpice

Voltage Controlled Voltage Source / Current Controlled Current Source
Voltage Controlled Current Source / Current Controlled Voltage Source

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ENGI 241 – Computer Simulation Project – Entering Dependent Sources in MicroCAP

Voltage Controlled Voltage Source / Current Controlled Current Source
Voltage Controlled Current Source / Current Controlled Voltage Source

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ENGI 241 Computer Simulation II

Second Circuit for the Course Project

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ENGI 241 Computer Simulation II

PROJECT REQUIREMENTS

1.Use PSpice to solve the circuit on the previous page.. Perform two separate runs, one for VS2 swept from 110V to 130V in 2V increments, and another for VS3 swept from 70V to 90V in 2V increments. Place voltage difference probes at these points of interest and print the probe output. Have PSpice print the probe output values. Use two different names for the VDC sweep, one for VS2 sweep, and another for VS3 sweep. Print the file with the out extension for each.

2.Use either loop analysis or nodal analysis to solve for the DC parameters by hand. To accomplish this, it may make the circuit easier to solve by hand if you do the following:

a:Convert R1, R2, and R3 to a single resistor, and convert 2VR2 to 2VA/3 (use the voltage divider theorem to get this.)

b: Convert R5, R6, and R7 to an equivalent (single) resistor, and convert 10I2 to 10IS1/6. (Use the Current Divider to verify this)

Submit the package to your classroom instructor.

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