THEVENIN’S THEOREM
Thevenin’s theorem permits the
reduction of a two-terminal dc
network with any number of
resistors and sources (Complex
Circuit) to one Equivalent circuit
having only one source and one
internal resistance in a series
configuration shown belw:
Original and Thevenin Equivalent Circuit
Measuring & Calculating
Vth by MultiSim:
• Circuit above shows how to measure the
Thevenin Voltage (Vth). That is measured 4V.
• The Circuit below shows how to measure
Thevinin’s Resistance (Rth):
• Conclusion: To measure Vth, remove the load
resistance (unknown), then measure the voltage
across open circuit terminal
• (Vo.c. = Vth).
• The Circuit below, shows how to measure and
calculate the Thevinin Resistance (Rth)
Original and circuit to measure Rth
Measured and Calculated Rth
• To measure Rth, with load resistance removed,
also remove and replace source voltage by a
short (jumper wire), then measure and calculate
the Thevenin resistance zacross the Open Circuit
terminals:
• Rth is measured 6 Ohms.
• Considering the jumper wire, 6 and 3 Ohms
resistors are in parallel, and result is in series with
4 Ohm resistor.
• Rth (Cal.)= [(6 x 3)/(6+3) + 4] =2 +4 = 6 Ohms
Original & Thevenin Equivalent Circuit:
Using Ohm’s Law to Calculate
Unknown Current
• As you can see from this simple Tutorial Example, the
load current for original and the Thevenin Equivalent
Circuit are Identical.
• This verifies our solution and demonstrate how to
measure and calculate the unknown element current by
using Ohm’s Law for Thevnin Equivalent Circuit:
• IL = (Vth)/(Rth + RL) = (4V) / (6+6)
• IL = 4/12 = 0.333A => Calculated!
• Compare this with measured, they are exactly equal!
Now you can apply this to other Problems.