Thevenin Voltage Equivalents w Thevenin invented a model

The Function Generator Model Recall that the function generator has an internal impedance of

Note: w We might also see a circuit with no load resistor, like this

Thevenin Method A B w Find Vth (open circuit voltage) • Remove load if

Find Vth w Let Vo=12, R 1=2 k, R 2=4 k, R 3=3 k,

Find Rth w Short out the voltage source (turn it off) & redraw the

Find Rth w First find the parallel combinations of R 1 & R 2

Redraw Circuit as a Thevenin Source w Then add any load and treat it

Thevenin Method Tricks w Note • When a short goes across a resistor, that

Thevenin Applet (see webpage) w 12 Test your Thevenin skills using this applet from

Does this really work? To confirm that the Thevenin method works, add a load

Thevenin Method Example w Checking the answer with PSpice w Note the identical voltages

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Thevenin Voltage Equivalents w Thevenin invented a model called a Thevenin Source for representing a complex circuit using • A single “pseudo” source • A single “pseudo” resistance Thevenin source, Vth, with its associated resistance, Rth, “looks” to the load on the circuit like the actual complex combination of resistances and sources. w This model can be used interchangeably with the more complex circuit when doing analysis. w 1

The Function Generator Model Recall that the function generator has an internal impedance of 50 Ohms. w Could the internal circuitry of the function generator contain only a single source and one resistor? w This is actually the Thevenin equivalent model for the circuit inside the function generator w 2

Thevenin Model Load Resistor 3

Note: w We might also see a circuit with no load resistor, like this voltage divider. 4

Thevenin Method A B w Find Vth (open circuit voltage) • Remove load if there is one so that load is open • Find voltage across the open load w Find Rth (Thevenin resistance) • Set voltage sources to zero (current sources to open) – in effect, shut off the sources • Find equivalent resistance from A to B 5

Find Vth w Remove Load A B A 6 B

Find Vth w Let Vo=12, R 1=2 k, R 2=4 k, R 3=3 k, R 4=1 k 7

Find Rth w Short out the voltage source (turn it off) & redraw the circuit for clarity. A A B B 8

Find Rth w First find the parallel combinations of R 1 & R 2 and R 3 & R 4. w Then find the series combination of the results. 9

Redraw Circuit as a Thevenin Source w Then add any load and treat it as a voltage divider. 10

Thevenin Method Tricks w Note • When a short goes across a resistor, that resistor is replaced by a short. • When a resistor connects to nothing, there will be no current through it and, thus, no voltage across it. 11

Thevenin Applet (see webpage) w 12 Test your Thevenin skills using this applet from the links for Exp 3

Does this really work? To confirm that the Thevenin method works, add a load and check the voltage across and current through the load to see that the answers agree whether the original circuit is used or its Thevenin equivalent. w If you know the Thevenin equivalent, the circuit analysis becomes much simpler. w 13

Thevenin Method Example w Checking the answer with PSpice w Note the identical voltages across the load. • 7. 4 - 3. 3 = 4. 1 (only two significant digits in Rth) 14