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 the maximum power transfer theorem.
What is the 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.
Maximum power transfer theorem is one of the popular electric circuit analysis. This theorem will help us to simplify a complex electrical circuit into a maximum external power generated with an internal resistance.
That is the basic idea of it.
Maximum Power Transfer Theorem Statement
The maximum power transfer theorem states that:
In order to generate the maximum external power with a finite internal resistance, this finite internal resistance has to be equal to the resistance of the available sources in the same circuit.
This statement shows that the said resistance is equal to the Thevenin equivalent resistance.
Maximum Power Transfer Theorem Formula
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.
If the entire circuit is replaced by its Thevenin equivalent except for the load, as shown above, the power delivered to the load is
For a given circuit, VTh and RTh are fixed. By varying the load resistance RL, the power delivered to the load varies as sketched below. We notice that the power is small for small or large values of RL but maximum for some value of RL between 0 and ∞.
Below is the maximum power transfer theorem graph based on our discussion above.
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
which yields
Maximum Power Transfer Theorem Formula
Just as stated above, the maximum power transfer theorem formula will focus on an electrical circuit with a source and a variable load resistance RL acts as a finite internal resistance.
Observe the circuit below.
The maximum power transferred is obtained by substituting Equations.(3) to (1), for
Equation.(4) applies only when RL = RTh. When RL ≠ RTh, we compute the power delivered to the load using Equation.(1).
Maximum Power Transfer Theorem Solved Problems
For better understanding, let us review the example below :
1. Find the value of RL for maximum power transfer in the circuit below
Answer:
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 above and obtain
To get VTh, we consider the circuit below
Applying mesh analysis gives
Solving for i1, we get i1 = -2/3. Applying KVL around the outer loop to get VTh across terminals a-b, we obtain
For maximum power transfer,
and the maximum power is
