The Maximum Power Transfer Theorem is another useful Circuit Analysis method to ensure that the maximum amount of power will be dissipated in the load resistance when the value of the load resistance is exactly equal to the resistance of the power source. The relationship between the load impedance and the internal impedance of the energy source will give the power in the load. Consider the circuit below.

Thevenins Equivalent Circuit.

 

In our Thevenin equivalent circuit above, the maximum power transfer theorem states that “the maximum amount of power will be dissipated in the load resistance if it is equal in value to the Thevenin or Norton source resistance of the network supplying the power“.

In other words, the load resistance resulting in greatest power dissipation must be equal in value to the equivalent Thevenin source resistance, then RL = RS but if the load resistance is lower or higher in value than the Thevenin source resistance of the network, its dissipated power will be less than maximum.

For example, find the value of the load resistance, RL that will give the maximum power transfer in the following circuit.

Back to our circuit! To use the power rule (P = I × V), we need to know both the current through the resistor, and the voltage across the resistor.

First, we use Ohm’s law ( V = I × R ), to find the current through the resistor.
• The voltage across the resistor is V = 9 V.
• The resistance of the resistor is R = 100 Ω.

Therefore, the current through the resistor is:

I = V / R = 9 V / 100 Ω = 90 mA

Then, we can use the power rule ( P = I × V ), to find the power dissipated by the resistor.
• The current through the resistor is I = 90 mA.
• The voltage across the resistor is V = 9 V.

Therefore, the power dissipated in the resistor is:

P = I × V = 90 mA × 9 V = 0.81 W

 

We can now complete the following table to determine the current and power in the circuit for different values of load resistance.

Power Rule: P = I × VIf a current I flows through through a given element in your circuit, losing voltage V in the process, then the power dissipated by that circuit element is the product of that current and voltage: P = I × V.

And more Tutorial Video related lesson : http://www.studyyaar.com/index.php/module-video/279-maximum-power-transfer-theorem

with DJ, Matildo