P5.4-18 Consider the circuit shown in Figure P5.4-18. Determine

a) The value of that occurs when R = 9 W.

b) The value of R that causes

c) The value of R that causes .

Figure P 5.4-18

First find the Thevenin equivalent circuit for the part to the left of . Do a source transformation on the 9 V source and the 20 Ω resistor and the circuit becomes:

We can add the two current sources to get a 750 mA source and we can add the 20 Ω and the 30 Ω resistors in parallel.

So the circuit now looks like

Do a source transformation converting the 750 mA source to a 9 V source and we get the circuit

Finally combine the 12 Ω and the 6 Ω resistor and our full circuit with the resistor R looks like

a) The value of that occurs when R = 9 W.

is the voltage across R so we find current from Ohm’s law

Now we calculate

b) The value of R that causes

We can use voltage division

c) The value of R that causes .

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This page last updated on March 8, 2019