Maximum Power Transfer Theorem Basic Explanation

In many practical situations, a circuit is designed to provide power to a load. There are applications in areas such as communications where it is desirable to maximize the power delivered to a load. This method is called maximum power transfer theorem.

We now address the problem of delivering the maximum power to a load when given a system with known internal losses. It should be noted that this will result in significant internal losses greater than or equal to the power delivered to the load.

Make sure to read what is electric circuit first.

These circuit analysis theorems are classified as:

  1. Superposition theorem
  2. Source transformation
  3. Thevenin theorem
  4. Norton theorem
  5. Maximum power transfer

Maximum Power Transfer Theorem

The Thevenin equivalent is useful in finding the maximum power a linear circuit can deliver to a load. We assume that we can adjust the load resistance RL.

maximum power transfer theorem
Figure 1

If the entire circuit is replaced by its Thevenin equivalent except for the load, as shown in Figure.(1), the power delivered to the load is

maximum power transfer theorem

For a given circuit, VTh and RTh are fixed. By varying the load resistance RL, the power delivered to the load varies as sketched in Figure.(2). We notice from Figure.(2) that the power is small for small or large values of RL but maximum for some value of RL between 0 and ∞.

maximum power transfer theorem
Figure 2

Let me show that this maximum power occurs when RL is equal to RTh. This is known as the maximum power theorem.

Maximum power is transferred to the load when the load resistance equals the Thevenin resistance as seen from the load (RL = RTh)

To prove the maximum power transfer theorem, we differentiate p in Equation.(1) with the respect to RL and set the result equal to zero. We obtain

maximum power transfer theorem

which yields

maximum power transfer theorem

showing that the maximum power transfer takes place when the load resistance RL equals the Thevenin resistance RTh. We can readily confirm that Equation.(3) gives the maximum power by showing that d2p/dRL2<0.

The maximum power transferred is obtained by substituting Equations.(3) to (1), for

maximum power transfer theorem

Equation.(4) applies only when R= RTh. When R≠ RTh, we compute the power delivered to the load using Equation.(1).

Maximum Power Transfer Theorem Example

For better understanding, let us review the example below :

1. Find the value of RL for maximum power transfer in the circuit of Figure.(3)

maximum power transfer theorem
Figure 3

Solution :

We need to find the Thevenin resistance RTh and the Thevenin voltage VTh across the terminals a-b. To get RTh, we use the circuit in Figure.(4a) and obtain
maximum power transfer theorem
maximum power transfer theorem
Figure 4

To get VTh, we consider the circuit in Figure.(4b). Applying mesh analysis gives

maximum power transfer theorem

Solving for i1, we get i1 = -2/3. Applying KVL around the outer loop to get VTh across terminals a-b, we obtain

maximum power transfer theorem

For maximum power transfer,

maximum power transfer theorem

and the maximum power is

maximum power transfer theorem

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