Maximum Power Transfer

Maximum Power Transfer
The Maximum Power Transfer Theorem is not so much a means of analysis as it is an aid to system design. Simply stated, the maximum amount of power will be dissipated by a load resistance when that load resistance is equal to the Thevenin/Norton resistance of the network supplying the power. If the load resistance is lower or higher than the Thevenin/Norton resistance of the source network, its dissipated power will be less than maximum.

Here’s how to arrange for the maximum power transfer.

1. Find the internal resistance, R_I. This is the resistance one finds by looking back into the two load terminals of the source with no load connected. As we have shown in the Thevenin's Theorem and Norton's Theorem chapters, the easiest method is to replace voltage sources by short circuits and current sources by open circuits, then find the total resistance between the two load terminals.

2. Find the open circuit voltage (U_T) or the short circuit current (I_N) of the source between the two load terminals, with no load connected.

Once we have found R_I, we know the optimal load resistance
(R_Lopt = R_I). Finally, the maximum power can be found

In addition to the maximum power, we might want to know another important quantity: the efficiency. Efficiency is defined by the ratio of the power received by the load to the total power supplied by the source. For the Thevenin equivalent:

 

Sample Problem