Week 4Experiments 8 and 9

Same Circuit: Experiment 8 and 9You should use the +5 V and +9 V supplies on the ANDY board.

You should use red wire to bring the +5 V and +9 V from the block terminals to the breadboard, but place these on to two different red buses A bus is one of the horizontal rows that are used to distribute ground and voltages from the 4-pin plugs to the breadboard.

Note that the + side of the +9 V supply is connected to R4.

Added InstructionsDetermine the Thévenin equivalent circuit theoretically.Determine the Thévenin equivalent circuit

experimentally.Construct the Thévenin equivalent circuit.Demonstrate that the current and voltage across the load

resistor R5 is the same in the Thévenin equivalent circuit as in the original circuit.Within the constraints of the component tolerances, and

measurement accuracy of the DMM.

Shared and Distributed Nodes

Since there is a node (dot) where the horizontal wire crosses the vertical wire between the +5 V supply and R3, the wires are connected.

X and Y are distributed nodes. The voltage is the same at all points along the wires.

Redrawing the Circuit

Labeling NodesLeft click on the wire attached to the node that you want to label.

Go to Edit/Label and enter the name of the node in the Set Attribute Value pop-up window and click OK.

Alternatively, just double click on the wire and the Set Attribute Value window will open.

Set Resistor Tolerance on SchematicsDouble click on the resistorIn the Part Name pop-up

window, click on the part attribute called Tolerance.In the window below Value,

enter 5% Click Save Attr and then click

OK. 5% will appear next to the resistor

symbol.

When to Set the Tolerance• If you make the change to the TOLERANCE on the first

resistor that you place in the circuit and then copy the resistor to place the other resistors into the circuit, you only need to modify the TOLERANCE of the resistors once.

• If you place all of the resistors into the circuit before changing the properties, you will need to click on each resistor and enter ‘5%’ into TOLERANCE.

How PSpice has Labeled Nodes • Open a Netlist

– This is created automatically when you run a simulation. You can generate it before running a simulation by:• Select Analysis/Create Netlist• Then select Analysis/Examine Netlist

– If an error message is displayed– Verify that the ground has been placed in the circuit and that all

parts are wired in place.

Netlist • Listed are all of the components in the circuit

• The strings starting with an N are the number of the nodes that the component is attached to. – For example, R3 is

connected to nodes N00043 and 0 (ground). The node N00043 is shared with R4 and R5.

– Note that the list also shows which of the resistors have had the tolerance changed.

Thévenin Equivalent Resistance• There are several methods that can be used to determine

the Thevenin equivalent resistance, RTH, of a circuit.

1. Find the open circuit voltage and short circuit current: RTH = VOC/ISC

Also used in calculating the Load Line of a circuit, which will be used frequently in ECE 2204 and 3204.

2. Source transformationNorton/Thévenin source conversions , which is emphasized in the lecture portion of the course.

Determine VOC

Replace R5 with an open circuit and determine the voltage across the nodes where R5 had been attached (Nodes X and Y).

The polarity of VOC is not important when calculating RTH. However, the way you orient the +/_ signs for VOC will determine the direction of the short circuit current, which will be calculated next.

The value of VOC is the final Thevenin equivalent voltage source for the circuit.

Determine ISC

Replace R5 with a short circuit and determine the current that flows through the short circuit between the nodes where R5 had been attached (Nodes X and Y).

The short circuit current should flow out of the – side of the open circuit voltage.

RTH = VOC /ISC

The value of ISC is the final Norton equivalent current source for circuit.

Source Transformations

By choosing appropriate values for R1 and R2, the Thévenin resistor and one of the power supplies on the ANDY board, Rth in the final transformation will have the same value as Rth calculated for the circuit in Experiment 8 and 9.

Creating VthOne technique is to use a voltage divider.

Thevenin Equivalent CircuitFor this week’s experiment, I would prefer that you will use the function generator with +DC or –DC selected on the Velleman arbitrary function geneator to create a d.c. voltage for Vth, which is equal to the open circuit voltage Voc.

Rth is:(a) the equivalent resistance of the circuit

without the 680 W resistor in it and with the two voltage supplies replaced with short circuits.

(b) equal to Voc/ Isc.