BASIC ELECTRONICS - OD1633 - LESSON 1/TASK 2
e. Power Transfer and Efficiency.
Maximum power is transferred from
the source to the load when the resistance of the load is equal to the
and the graph of figure 41 (on the previous page). When the load resistance
is 5 Ohms, matching the source resistance, the maximum power of 500 watts is
developed in the load.
The efficiency of power transfer (ratio of output power to input power) from
the source to the load increases as the load resistance is increased. The
efficiency approaches 100 percent as the load resistance approaches a
relatively large value compared with that of the source, since less power is
lost in the source. The efficiency of power transfer is only 50 percent at
the maximum power transfer point (when the load resistance equals the
internal resistance of the source).
The efficiency of power transfer
approaches zero efficiency when the load resistance is relatively small
the chart of figure 41 on the previous page.
The problem of a desire for both high efficiency and maximum power transfer
is resolved by a compromise between maximum power transfer and high
Where the amounts of power involved are large and the
efficiency is important, the load resistance is made large relative to the
source resistance so that the losses are kept small.
In this case, the
efficiency is high.
Where the problem of matching a source to a load is
important than a high percentage of efficiency.
In such cases, the
efficiency of power transfer should be only about 50 percent; however, the
power transfer would be the maximum which the source is capable of
You should now understand the basic concepts of series circuits.
principles which have been presented are of lasting importance.
equipped with a firm understanding of series circuits, you hold the key to